Boil Off Rate Calculator

Work out boil off rate instantly with clear inputs, formula shown and shareable results.

About Boil Off Rate Calculator

Boil-off rate is volume evaporated per hour and is a fixed property of your kettle geometry, burner and lid arrangement rather than a percentage of batch size. Since sugar is conserved, gravity concentrates in inverse proportion to volume: post-boil points equal pre-boil points times the volume ratio.

Formulas

Evaporation rate

Rate (L/h) = (pre-boil - post-boil volume) x 60 / boil minutes

Gravity concentration

Post-boil points = pre-boil points x pre-boil volume / post-boil volume

Worked example

With the default values below, Boil Off Rate Calculator returns Evaporation Rate of 4.0000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Pre-Boil Volumeminimum 0 L: 25 L
Post-Boil Volumeminimum 0 L: 21 L
Boil Timeminimum 0 minutes: 60 minutes
Pre-Boil Gravityaccepts 0.9–1.2: 1.046

Results

Evaporation Rate: 4.0000
Evaporation Rate As Percentage: 16.000%
Volume Evaporated: 4.000
Post-Boil Gravity: 1.05476

Step by step

1Values used
2Evaporation rate
3Gravity concentration
4Evaporation Rate= 4.0000
5Evaporation Rate As Percentage= 16.000
6Volume Evaporated= 4.000
7Post-Boil Gravity= 1.05476

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Is evaporation a fixed percentage or a fixed volume?

A fixed volume for a given kettle and vigour, because it depends on surface area and heat input, not on how much wort is underneath. Scaling recipes by percentage boil-off is a common error.

Why is a minimum boil-off needed?

To drive off dimethyl sulphide precursors and to promote hot break protein coagulation. A vigorous uncovered boil of 60-90 minutes is standard for pale malt beers.