Tool
Roast loss and green-to-roasted yield calculator
Two weighings give the roast loss as a percentage, the span your scale's tolerance allows around it, and the green coffee needed for a target roasted weight.
Roast loss
15.2%
If each weighing could be off by 1 g either way, the loss lies between 14.5% and 15.9%, a span of 1.4 points. Two batches whose figures differ by less than that have not been shown to differ.
Inside the 14 to 20 per cent that a thesis on roasting reports for the literature. That span describes what roasters have measured; it is not a target, and a figure inside it says nothing about how the coffee tastes.
How this was worked out
Weight lost = green weight − roasted weight
250 − 212 = 38 g
Roast loss = weight lost ÷ green weight
38 ÷ 250 = 15.2%
The roasted coffee is 84.8% of the green weight.
Green needed = roasted wanted ÷ (1 − loss)
1000 ÷ (1 − 15.2%) = 1179.2 g
Dividing, not adding the percentage on: adding it would leave the batch short.
- What it is good for
- Checking that you repeated a roast of the same green coffee. Two batches of one coffee with the same loss ended in a similar place.
- What it cannot tell you
- How the coffee tastes, or how one coffee compares with another. Most of the loss is water, so a green coffee that started wetter loses more for the same roast.
- Keep the same
- The scale, the container, and when you weigh: roasted coffee weighed warm is still losing moisture and gas.
Exact arithmetic on your numbers
What to do with the result
- You want the figure kept with the rest of the roast: Logging a roast
- You want to know what the lost weight was: Coffee roasting chemistry
- The roast is done and you are deciding when to brew it: Resting roasted coffee
- The roast tastes grassy or flat: Home roast tastes grassy or flat
How it works
Roast loss is the green weight minus the roasted weight, divided by the green weight. The tool then recomputes it with the green weighing off one way and the roasted weighing off the other, by an allowance you set, and reports the lowest and highest results. For planning it divides the roasted weight you want by one minus the loss. The formulas are exact; the weighings are not, which is why the span is shown. Worked by hand: 250 g of green coffee that comes out at 212 g has lost 38 g, and 38 ÷ 250 is 15.2%. With each weighing good to a gram, that lies between 14.5% and 15.9%.
What the inputs mean
- Green weight
- The green coffee loaded into the roaster, weighed without its container.
- Roasted weight
- The roasted coffee after cooling. Beans that fall through with the chaff belong in this weighing; the chaff itself does not.
- Scale allowance
- How far each weighing may be off, in grams, either way. It is your judgement of your own scale and not a property the tool can know. Zero treats both weighings as exact.
- Roasted weight wanted
- Optional. The tool divides it by one minus the loss to give the green coffee to load, using the loss just worked out.
Worked examples
A home batch
- Green 250 g
- Roasted 212 g
- Scale good to 1 g
Result: 38 g lost: a 15.2% loss, between 14.5% and 15.9% once the scale is allowed for.
The same loss on a sample roast
- Green 100 g
- Roasted 84.8 g
- Scale good to 1 g
Result: 15.2% again, but between 13.3% and 17.0%: the same scale on a smaller batch.
A scale reading to a tenth of a gram narrows this to about a third of a point.
Planning a kilogram of roasted coffee
- Roasted wanted 1,000 g
- Loss 16%
Result: 1,190.5 g of green coffee to load.
Common mistakes
Adding the percentage on to plan a batch
To end with 1,000 g at a 16% loss you need 1,000 ÷ 0.84, about 1,190 g. Adding 16% gives 1,160 g, which roasts down to about 974 g.
Comparing the loss between two different coffees
Most of the loss is water, so a green coffee that started wetter loses more for the same roast. The figure compares batches of one coffee.
Reading a small difference on a small batch
On 100 g weighed to the gram the span is nearly four points wide. Two batches that differ by one point have not been shown to differ.
Weighing the roasted coffee while it is still warm
It is still giving up moisture and gas, so the weight is still falling. Weigh at the same point after cooling each time.
What the result does not mean
- Roast loss is not a measure of quality or of flavour. Two roasts with the same loss can have been run in different ways and taste different.
- It is not a degree of roast that transfers between coffees. A thesis on roasting says the loss may serve as an indicator of the degree of roast for a given raw material, and the last four words carry the meaning.
- The span does not include a scale that reads consistently high or low. An error of that kind largely cancels in the percentage and does not cancel in the weights.
What this does not account for
- The span is a worst case in which the two weighings are off in opposite directions by the full allowance. It is not a statistical confidence interval.
- The planning figure assumes the next batch loses the same share as this one. That holds best for the same green coffee roasted the same way; a different coffee, or the same coffee after months of storage, starts with a different amount of water.
- The comparison with 14 to 20 per cent is a description of what a doctoral thesis on roasting reports for the literature before it. It is not a target and the tool names no degree of roast.
Where to go next
Learn the technique
Where this is taught
Become a Coffee Expert · Level 7
What heat does to a green seed, stage by stage, and what a roaster actually controls.