Brew day
Boil-off rate
Your kettle loses 3 litres an hour? That is the number to write down once and for all.
Result
Fill in the values and run the calculation, or try the example.
This is the calculation Krausi uses. Create an account to save it in a recipe or a brew.
Create my accountWhy measure your boil-off rate?
Every system boils off in its own way. A wide kettle on a powerful burner loses twice as much water as a narrow vessel on an electric plate. The default values in software are averages: they match nobody.
Measuring your rate once makes all later water volume calculations right. All you need is the volume at the start and at the end of the boil.
How is the boil-off rate calculated?
The calculation compares the two volumes:
boil-off (L/h) = (volume before − volume after) / time (h)
boil-off (%/h) = boil-off (L/h) / volume before × 100Measure both volumes at the same temperature, or bring them back to 20 °C: boiling wort takes up 4% more space.
The rate in litres per hour is the most useful: it changes little when the batch size changes.
Worked example
Values entered
- Volume before the boil in litres: 25
- Volume after the boil in litres: 22
- Boil time in hours: 1
Result from the tool
- Boil-off in L/h
- 3 h
- Boil-off in % of the starting volume per hour
- 12 %
What the tool does not do
- The rate varies with the vigour of the boil, and with wind and humidity if you brew outdoors.
- A measurement on a boil that is too short is imprecise: take it on a full brew.
- The tool assumes steady evaporation throughout.
Frequently asked questions
What boil-off rate is normal?
Between 8 and 15% per hour, which is 2 to 4 L per hour for a 20 to 25 L brew. Above that, turn the heat down: a rolling boil is enough, no need for a violent one.
Do I need a lid?
No, not during the boil: the steam has to carry off some unwanted compounds, such as DMS, which gives a cooked-corn taste. A lid left ajar can help reach the boil, then you take it off.
And with the assistant?
Give Krausi your two volumes at the end of the boil: he keeps the boil-off rate in your equipment settings.
I had 25 L before the boil and 22 L after an hour.
Your kettle boils off 3 L per hour (12% per hour). I'm saving it in your equipment settings: your water volumes will be calculated with this rate.