What is a normal boil-off rate?

Most homebrew systems lose 3-6 litres per hour. Gas burners typically have higher boil-off than electric elements. A boil-off rate of 10-15% per hour of the total volume is common.

How do I measure my boil-off rate?

Mark your kettle at known volume intervals. Measure the volume before and after a timed boil (e.g. 60 minutes). The difference divided by the time in hours gives your boil-off rate in L/hr.

Does kettle diameter affect boil-off?

Yes, a wider kettle has more surface area exposed to air, increasing evaporation. This is why boil-off rate varies between systems even with similar heat input.

Should I aim for a specific boil-off percentage?

A boil-off of 10-15% per hour is typical and provides adequate DMS removal and hot break formation. Less than 8% may not drive off enough DMS from pilsner malt. More than 20% wastes energy.

How do I reduce my boil-off rate?

Partially cover the kettle (leave a gap for DMS escape), reduce heat to a gentle rolling boil rather than a vigorous boil, or switch to electric heating which is more controllable. A 10-15% per hour boil-off rate is sufficient for good hot break and DMS removal.

BrewCalcs

Boil-Off Calculator — Brewing Equipment

Calculate your boil-off rate and total evaporation for your system.

Pre-Boil Volume

Volume at start of boil

Post-Boil Volume

Volume at end of boil

Boil Time

Kettle Diameter

Internal diameter of kettle

How We Calculate This

Boil-Off Calculation

Boil-Off Rate = (Pre-Boil Volume - Post-Boil Volume) / Boil Time in Hours. This gives your system-specific evaporation rate which you can use to plan future brews and calculate pre-boil volumes.

Evaporation Rate by Surface Area

Dividing the boil-off rate by the kettle surface area gives litres per hour per square metre, which helps compare performance between different kettles and heat sources.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify values and adjust based on your equipment and ingredients.