Skip to content
CoffeeHQ

Tool

Water suitability helper

Enter what your test kit or water supplier says and get an interpretation: scale risk, likely effect on taste, and which kind of treatment is worth investigating.

Use the unit on your supplier's report or test kit. Both readings are converted to ppm as calcium carbonate.
Calcium and magnesium, as calcium carbonate. Your water supplier publishes this for your area, or a test kit gives it in a minute.
Also called carbonate hardness or KH. A different property from hardness, and often the more important one for taste.
From a conductivity meter. Useful for spotting change; it cannot tell you what is dissolved.
Does anything heat this water?

Scale forms where water is heated and held.

Moderate scale risk

Moderately hard, Moderate alkalinity

Scale will form gradually. The maker's own descaling guidance should keep ahead of it.

Alkalinity is not at either extreme. In one controlled tasting, a modest amount of bicarbonate made no significant difference to what a trained panel described.

Worth investigating

  • Nothing, on these numbers. Water in these bands is a reasonable starting point and other variables will matter more
What to do first
Nothing. On these numbers water is unlikely to be what is holding the cup back, so look at grind and ratio first.
Keep the same
The coffee, the recipe and the grind. Water is only shown to be the cause when it is the one thing that differs between two cups.
What this cannot tell you
A supplier's figure is an average for an area and can move through the year, and a test strip reads in coarse steps, so treat either as approximate. The band edges are drawn to be readable, not exact. Two waters in the same bands can still behave differently, because these numbers say how much is dissolved and how well it buffers, not which minerals are present.

This interprets numbers; it does not tell you what to add to your water. CoffeeHQ publishes no mineral recipes — water intended for drinking must stay safe to drink, and anything mixed for consumption should follow food-grade product instructions and local drinking-water guidance rather than an internet recipe.

Ranked guidance, not a diagnosis

What to do with the result

  1. You have a bottle label or a water report and need the three numbers: Reading a bottle label
  2. The question is scale in the machine: Maintenance planner: scale
  3. You want to taste what your water does: Exercise: water comparison
  4. You want what was measured, and by whom: Water chemistry and brewing

TDS, hardness and alkalinity are three different numbers

They are often used as if they were one. A cheap meter gives the first; the two that decide anything for coffee are the second and the third, and they come from a water report, a test kit or a bottle’s label.

TDS (total dissolved solids)

What it measures
Everything dissolved in the water, taken together. A hand-held meter does not weigh it: it measures how well the water conducts electricity and estimates a total from that.
Where to find it
A cheap hand-held meter. Some supplier reports give it, or give conductivity, and a bottle may call it dry residue.
What it can tell you
That a supply has changed, that a filter or a purifier is or is not working, and how two waters compare in total.
What it cannot tell you
What is dissolved. Two waters with the same reading can have quite different hardness and alkalinity, and softened water reads much as it did before, because sodium has taken the place of calcium and magnesium.

Hardness (general hardness, GH)

What it measures
Calcium and magnesium, stated as the weight of calcium carbonate with the same effect.
Where to find it
Your supplier's water quality report, often in national degrees; a drop or strip test sold for hardness; on a bottle, the calcium and magnesium lines.
What it can tell you
Together with alkalinity, how readily scale will form where the water is heated. That is a question about the machine and the kettle.
What it cannot tell you
Much about taste on its own: in the one controlled tasting the water guide reports, hardness alone made little difference. A hardness strip also reads softened water as soft while its alkalinity is unchanged.

Alkalinity (carbonate hardness, KH)

What it measures
The water's capacity to neutralise acid, which in drinking water is mostly bicarbonate. It is stated on the same calcium carbonate scale as hardness, and it is not the same thing as pH.
Where to find it
The supplier's report, as alkalinity or as bicarbonate; a drop or strip test sold for carbonate hardness or KH; on a bottle, the bicarbonate line.
What it can tell you
How much of a coffee's acidity the water will neutralise. In the one controlled tasting it was the number that changed the cup. With hardness, it also decides scale.
What it cannot tell you
Whether your machine is safe from scale or corrosion. That is set by the maker's water specification.

On a water report or a label, then, look for two lines: hardness, or calcium and magnesium; and alkalinity, or bicarbonate. The rest of the report is set out in the water guide.

Bottle-label helper

A bottle lists bicarbonate, calcium and magnesium each by its own weight. A tap report states alkalinity and hardness as the weight of calcium carbonate that would have the same effect. This puts a label on the report’s scale so that the two can be compared. It is arithmetic on the weights of the ions: bicarbonate × 0.82 for alkalinity, and calcium × 2.50 plus magnesium × 4.12 for hardness.

The line marked bicarbonate or HCO₃.
The line marked calcium or Ca.
The line marked magnesium or Mg.

Enter the figures from the label. Any one of them gives part of the answer.

The same sum, worked for three labels

Made-up labels, to show the arithmetic. They describe no product and recommend none.

  • A lightly mineralised water

    Label: bicarbonate 30 mg/L, calcium 8 mg/L, magnesium 2 mg/L.

    • Alkalinity: 30 mg/L of bicarbonate × 0.82 = 25 mg/L as calcium carbonate.
    • Hardness: 8 mg/L of calcium × 2.50 + 2 mg/L of magnesium × 4.12 = 28 mg/L as calcium carbonate.

  • A middling water

    Label: bicarbonate 120 mg/L, calcium 40 mg/L, magnesium 8 mg/L.

    • Alkalinity: 120 mg/L of bicarbonate × 0.82 = 98 mg/L as calcium carbonate.
    • Hardness: 40 mg/L of calcium × 2.50 + 8 mg/L of magnesium × 4.12 = 133 mg/L as calcium carbonate.

  • A heavily mineralised water

    Label: bicarbonate 300 mg/L, calcium 90 mg/L, magnesium 25 mg/L.

    • Alkalinity: 300 mg/L of bicarbonate × 0.82 = 246 mg/L as calcium carbonate.
    • Hardness: 90 mg/L of calcium × 2.50 + 25 mg/L of magnesium × 4.12 = 328 mg/L as calcium carbonate.

What this assumes: that all of the water’s alkalinity is bicarbonate, which is the usual case for still drinking water. Label figures are the bottler’s typical analysis and vary a little from batch to batch. The helper converts units; it does not say which water to buy.

Could it be the water?

The helper above reads two numbers. This part starts from what you noticed instead, in the cup, in the kettle or on a label, and says how to find out whether water is behind it before anything is bought or changed.

Each one follows the same order: what it could be, the quickest check, one thing to change first, what to watch for, what to try if that does not help, and why.

The tap water smells or tastes of chlorine, and the coffee carries it

What it could be

  • Chlorine used to disinfect the supply. Likelier when: A glass of cold tap water has a swimming-pool smell.
  • Chloramine, which some suppliers use in its place. Likelier when: Your supplier's water quality report names it.
  • It is not the water: a kettle, a new plastic part or the brewer. Likelier when: Cold tap water tastes clean and the taint appears only after heating or brewing.
  • Cleaning product left in a machine. Likelier when: It began straight after cleaning or descaling.

Quickest check: Smell and taste a glass of cold tap water beside a glass of bottled water. Then look up which disinfectant your supplier uses; it is in the published water quality report.

Change one thing first: Brew the same coffee once with a bottled water, everything else unchanged, and compare the two cups.

Then watch for: Whether the off-note is gone from the second cup.

If that does not help: A carbon filter jug is the usual household answer to a chlorine taste. No test of how well jugs remove chlorine or chloramine was read for CoffeeHQ, so let your own side-by-side cup decide, and change cartridges when the jug's maker says to. If the bottled-water cup has the same taint, the cause is in the equipment.

Why: Tap water is disinfected, usually with chlorine. A United States agency describes chloramines as formed when ammonia is added to chlorine and used because they last longer in pipes. No study of what either does to coffee was read. (Rests on a study a CoffeeHQ guide reports.)

The coffee tastes flat, dull or chalky whatever you do to the recipe

What it could be

  • High alkalinity is neutralising the coffee's acids. Likelier when: Your supplier's report or a test strip shows high alkalinity, and the kettle furs up quickly.
  • The water has been softened. Likelier when: The house has an ion-exchange softener and a hardness strip reads soft. A softener leaves the alkalinity where it was.
  • It is the coffee. Likelier when: The same coffee is just as flat when brewed with a low-bicarbonate bottled water.
  • It is the brew. Likelier when: The cup is thin as well as flat, which is a question of how much coffee went in.

Quickest check: Find the alkalinity figure. A supplier's report lists it as alkalinity or as bicarbonate; a test kit calls it carbonate hardness or KH. A hardness figure alone does not answer this.

Change one thing first: Brew the same coffee once with a bottled water whose label shows less bicarbonate, everything else unchanged.

Then watch for: More acidity and fruit in the second cup. If you cannot tell the cups apart, leave the water alone.

If that does not help: If the cups differ, the options are a water with lower alkalinity: a filter jug may lower it somewhat, though no test of any jug was read, so measure the filtered water with a test kit; a chosen bottled water lowers it by a known amount. If the water goes into a machine, check any change against the maker's water specification first. If the cups do not differ, look at the coffee.

Why: Alkalinity is the water's capacity to neutralise acid. In the one controlled tasting the water guide reports, rising bicarbonate made a coffee less acidic and fruity and more roasted and bitter, while hardness on its own changed little. (Rests on a study a CoffeeHQ guide reports.)

The coffee tastes sharp or sour on very soft or purified water

What it could be

  • Very low alkalinity leaves the coffee's acidity unbuffered. Likelier when: The water is distilled, from reverse osmosis, or a very soft supply.
  • It is the coffee: a light roast, with acidity that is meant to be there. Likelier when: The sharpness tastes of fruit and the cup is otherwise balanced.
  • It is the brew. Likelier when: The cup is strong and short as well as sour. In one panel study sourness rose with strength.

Quickest check: Read the alkalinity of the water you brew with, from the report, a test strip or the bottle's label.

Change one thing first: Brew the same coffee once with a bottled water whose label shows a moderate amount of bicarbonate, everything else unchanged.

Then watch for: A rounder second cup. If the two taste alike, the water was not the cause.

If that does not help: If the cups taste alike, take the sourness to Make my coffee better as a brewing question. If they differ, use the water you preferred. Blending some tap water into purified water raises hardness and alkalinity together in proportion; no particular mix is recommended here.

Why: With little in the water to neutralise them, the coffee's acids reach the cup unbuffered. Older studies reported coffee brewed with distilled water as sharper. The modern controlled tasting the water guide reports did not test this directly, and its panel did not reliably tell hard water from very pure water. (Rests on a study a CoffeeHQ guide reports.)

A white crust in the kettle, or a machine that asks to be descaled often

Safety: What a machine tolerates is its maker's to say. The water specification in the manual or on the maker's support pages governs, for scale and for corrosion alike, and it outranks anything general on this page.

What it could be

  • Hard water with enough alkalinity to form scale when heated. Likelier when: You live in a hard-water area, and the kettle shows a white or grey crust.
  • A filter cartridge that is exhausted. Likelier when: Scale has come back after a spell without it.
  • It is not scale. Likelier when: The deposit is brown and greasy and sits where coffee has been. That is coffee oil, and it is a different job with a different product.

Quickest check: Look up both hardness and alkalinity for your supply, or test for both. Scale needs the two together, so one number does not settle it.

Change one thing first: Read what the maker of your machine or kettle says about water and about descaling, before treating anything.

Then watch for: After any change of water, how quickly a crust returns in the kettle, which is the easy place to see it.

If that does not help: A softening or filtering cartridge slows scale; it does not end the need to descale where the maker asks for it. A softener lowers hardness and leaves alkalinity, so it can protect a machine and still dull the cup. Descale only as the manual permits: some makers ask owners not to.

Why: The World Health Organization's document on hardness, as the water guide reports it, says hard water can deposit scale where water is heated, depending on its acidity and alkalinity. A kettle element and a boiler are such places. (Rests on a study a CoffeeHQ guide reports.)

Worry that soft, filtered or purified water will damage the machine

Safety: What a machine tolerates is its maker's to say. The water specification in the manual or on the maker's support pages governs, for scale and for corrosion alike, and it outranks anything general on this page.

What it could be

  • The water is very low in minerals and nothing has been added back. Likelier when: Distilled or reverse-osmosis water is used neat.
  • A line of the water report is outside what the maker allows. Likelier when: The maker's specification gives a figure, for chloride for instance, and your report exceeds it.
  • There is no problem. Likelier when: The water is an ordinary soft supply and sits inside the maker's stated specification.

Quickest check: Find the water specification in the machine's manual or on the maker's support pages, and set it beside your water report line by line.

Change one thing first: Use water that sits inside the maker's stated specification.

Then watch for: Corrosion is not something the cup will show. A leak, or a discoloured deposit at a fitting, is a reason to ask the maker or a service agent.

If that does not help: If the maker publishes no specification, ask them. Several makers also warn that a machine's sensors may not detect very pure water, which is a separate reason not to run one on it.

Why: The same World Health Organization document says soft water that has not been stabilised tends to corrode metal, and a paper the water guide reports, citing coffee-industry guidance, says low alkalinity may lead to corrosion in coffee machines. Neither gives a threshold. (Rests on a study a CoffeeHQ guide reports.)

A TDS meter gave a number, and it is not clear what it means

What it could be

  • The reading is being taken as hardness. Likelier when: A high number is being read as hard water, or a low one as soft.
  • A low or middling reading is being taken as safe for the machine or good for the cup. Likelier when: The water has been softened: a softener swaps calcium and magnesium for sodium, so the meter still finds plenty dissolved.
  • The reading has changed, and that is real information. Likelier when: The same meter on the same tap now reads differently, or filtered water reads like unfiltered.
  • The unit is not what it seems. Likelier when: The figure is far higher than expected: some meters show conductivity and not an estimate of dissolved solids.

Quickest check: Set the meter's number beside the hardness and the alkalinity on your supplier's report. They are three different lines, and the meter gives none of the other two.

Change one thing first: Get the two numbers the meter cannot give, hardness and alkalinity, from the supplier's report or a test kit for each.

Then watch for: Use the meter for what it is good at: noticing that a supply or a filter has changed.

If that does not help: If you only have the meter, treat any decision about scale or taste as unmade. The section on three numbers, below, sets out what each one measures.

Why: A hand-held TDS meter estimates everything dissolved from how well the water conducts electricity. It cannot say how much of that is hardness and how much is alkalinity, and two waters with the same reading can differ in both. (Reasoned from how the equipment works.)

Brewing with distilled or reverse-osmosis water

Safety: What a machine tolerates is its maker's to say. The water specification in the manual or on the maker's support pages governs, for scale and for corrosion alike, and it outranks anything general on this page.

What it could be

  • The cup is fine. Likelier when: You like it. In the one controlled tasting, very pure water was the reference and hard waters were not reliably told from it.
  • The cup is sharp. Likelier when: The coffee tastes more acidic than it does on tap water. See the flow on very low alkalinity.
  • The machine, not the cup, is the question. Likelier when: The water goes into a boiler or a machine with level sensors.

Quickest check: Decide where the water is going. For a kettle and a manual brewer the cup is the only test. For a machine, the maker's water specification comes first.

Change one thing first: If the water goes into a machine, check it against the maker's water specification before using it.

Then watch for: For a manual brew, a side-by-side cup against your tap or a bottled water.

If that does not help: If the maker's specification rules very pure water out, use a water inside it. If the cup is the only question and you prefer some mineral content, see the flow on adding minerals back.

Why: Reverse osmosis and distillation remove nearly everything, hardness and alkalinity alike. Such water leaves no scale. What it does to a machine is the maker's question, and what it does to taste was only partly tested. (Rests on a study a CoffeeHQ guide reports.)

Thinking of adding minerals back to purified water

Safety: Water you drink has to stay safe to drink. Use only products sold for drinking water, follow their instructions, and do not improvise with chemicals. CoffeeHQ gives no amounts.

What it could be

  • The aim is to protect a machine. Likelier when: The maker's specification asks for some mineral content.
  • The aim is taste. Likelier when: A side-by-side cup has already shown you prefer water with something in it.
  • A recipe from elsewhere is being followed. Likelier when: It names salts and weights. There is no one recipe that suits every coffee or machine, and CoffeeHQ publishes none.

Quickest check: Decide which question is being answered, the machine's or the cup's, because they have different answers.

Change one thing first: Start with a ready-made water: a bottled water whose label fits, or a food-grade remineralising product used exactly as its maker directs.

Then watch for: A side-by-side cup against the purified water alone, with nothing else changed.

If that does not help: Mixing some tap water into purified water lowers the tap water's hardness and alkalinity together, in proportion to the mix. Beyond that, the water guide sets out what is and is not known.

Why: In the one controlled tasting read, alkalinity changed the cup and hardness alone hardly did, and no treatment of any kind was tested. That is too little to build a recipe on. (Rests on a study a CoffeeHQ guide reports.)

Choosing a bottled water from its label

What it could be

  • The label gives bicarbonate. Likelier when: There is a line for bicarbonate or HCO₃. That is the alkalinity, in a different unit from a tap report.
  • The label gives calcium and magnesium as separate lines. Likelier when: Together they are the hardness, again in a different unit.
  • The label gives only a total. Likelier when: It shows dry residue or total dissolved solids and nothing else. That cannot tell you hardness or alkalinity.

Quickest check: Find three lines on the label: bicarbonate, calcium and magnesium.

Change one thing first: Put those three figures through the bottle-label helper on this page, so that they are on the same scale as your tap report.

Then watch for: Two waters you can now compare like for like, before you buy either.

If that does not help: If the label has no bicarbonate line, the bottler's website often publishes a full analysis. A water sold as mineral water can be very high in both figures.

Why: Tap reports state hardness and alkalinity as the weight of calcium carbonate that would have the same effect, while labels list each ion by its own weight. Converting between them is arithmetic on the weights of the ions, which the helper does and shows. (Reasoned from how the equipment works.)

What this cannot know: These flows do not know your water or your machine. They give no mineral recipe and no amount of anything to add, and they set no threshold at which scale or corrosion begins, because none was read. The one controlled tasting they lean on used one coffee, brewed in a French press, and says nothing about espresso.

How it works

Hardness and alkalinity are placed into bands, and the bands are interpreted separately because they do different things. Hardness mostly governs scale; alkalinity governs how much of the coffee's acidity is neutralised. The taste note follows one controlled tasting of a single coffee, in which hardness alone made no marked difference and rising bicarbonate made the coffee less acidic and flatter. The combination then points at a treatment category, never at a formula. Below the helper the page sets TDS, hardness and alkalinity side by side, because they are three different numbers and a cheap meter gives only the first. A small bottle-label helper puts a label's bicarbonate, calcium and magnesium on the same scale as a tap report: bicarbonate times 0.82 for alkalinity, and calcium times 2.50 plus magnesium times 4.12 for hardness, which is arithmetic on the weights of the ions. Then nine flows start from what you noticed, such as a chlorine taste, a flat cup, a furred kettle or a worry about purified water, and say how to find out whether water is the cause.

What the inputs mean

Unit
Suppliers and test kits report in different units. Choose ppm, German degrees, French degrees or grains per US gallon; the conversion to ppm as calcium carbonate is a fixed definition of each unit and is shown with the result.
General hardness
Calcium and magnesium, usually reported in ppm as calcium carbonate or in degrees. Your water supplier publishes this for your area.
Alkalinity
Also called carbonate hardness or KH. It measures the water's capacity to neutralise acid, which is a different property from hardness and often the more important one for taste.
TDS
Total dissolved solids, from a conductivity meter. Useful for spotting change and comparing waters; it does not tell you what the dissolved material is.
Bicarbonate, calcium and magnesium (bottle-label helper)
The three lines of a bottled water's label, each in milligrams per litre of that ion. The helper converts bicarbonate to alkalinity and calcium with magnesium to hardness, both as calcium carbonate, and shows the sum it did.

Worked examples

  • Reading a bottled water's label against a tap report

    • Bicarbonate 120 mg/L
    • Calcium 40 mg/L
    • Magnesium 8 mg/L

    Result: Alkalinity 98 mg/L and hardness 133 mg/L, both as calcium carbonate: 120 × 0.82, and 40 × 2.50 plus 8 × 4.12. Those two figures can be set beside a tap report or entered in the helper as ppm.

    A made-up label, to show the arithmetic. The helper recommends no water.

  • A hard-water area in the south-east of England

    • Hardness 280 ppm
    • Alkalinity 220 ppm

    Result: High scale risk and a strong flattening effect on acidity. Worth investigating filtration or a softening cartridge, and what the machine's maker says about hard water and descaling.

Common mistakes

  • Treating TDS as hardness

    Two waters reading the same TDS can have completely different hardness and alkalinity, and will behave differently in both the machine and the cup.

  • Softening water as far as possible

    Several machine makers warn against water stripped of nearly all minerals, because sensors and electronics may not work with it, and it has nothing to buffer the coffee's acids. The aim is treatment, not purity.

  • Assuming a filter jug removes the need to descale

    It slows scale formation. Descaling is still needed wherever the maker asks for it.

What the result does not mean

  • This interprets numbers. It does not tell you what to add to your water, and CoffeeHQ publishes no mineral recipes — home-formulated drinking water has real safety boundaries.
  • A water in a comfortable band does not guarantee good coffee, and a water outside one does not prevent it.

What this does not account for

  • Bands are broad and interpretive. Two waters in the same band can behave differently depending on what the minerals actually are.
  • Scale risk is about heated equipment, not about whether the water is pleasant to drink.
  • Your supplier's published figure is for the supply area and may not match what comes out of your tap after the building's own plumbing.
  • The bottle-label helper assumes all of a water's alkalinity is bicarbonate, which is the usual case for still drinking water. Label figures are the bottler's typical analysis.
  • Nothing here sets a limit for your machine. For scale and for corrosion the maker's stated water specification governs.

Where to go next

Learn the technique

Background reading

Equipment involved

When it goes wrong

Other tools

Practise this in a lesson

Related guides