Measure, understand, improve
Pre-boil to post-boil gravity calculator
If extract is retained while water evaporates, gravity points rise as volume falls. For 25 L at 1.040 reduced to 20 L, the estimate is 50 points, or SG 1.050.
BrewMyBeer editorial · Sources checked 24 September 2026 · Calculated examples, not laboratory measurements. How we create content
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Measure a mixed sample at a known temperature
Mix the wort before sampling. Cool the sample as required by your instrument and use corrected gravity. Reference both volumes to the same temperature; comparing hot kettle volume with a cold fermenter volume introduces a second effect.
The volume after boiling is the kettle volume before transfer losses. Leaving wort in a kettle removes extract and water together. That is different from evaporating water and is not a reason to calculate a higher gravity.
Keep an extract budget
Multiply initial gravity points by initial volume, then divide by final volume. This practical approximation assumes no extract additions or losses. If you add sugar, spill wort or remove a substantial sample, adjust your recipe accounting separately.
Longer boiling can change the beer beyond concentration, including hop utilization and colour. Check the whole recipe before choosing extra boil time as a correction.
Before you use the result
- Final volume cannot exceed initial volume in this evaporation model.
- A predicted SG is only as good as the volume measurements.
- For top-up water, use the separate dilution tool.
A quick learning check
Does leaving 2 L of well-mixed wort behind increase the SG of the transferred wort?
Try a craft decision game →Sources & further reading
These references explain the methods and limitations. The worked examples and learning questions on this page are BrewMyBeer calculations, not claims of hands-on testing.