What is thermal shrinkage?

Wort contracts by approximately 4% when cooling from boiling to room temperature. Hot wort at 100°C measures ~4% more than the same wort at 20°C. This is called thermal contraction or shrinkage.

Should I measure volume hot or cold?

For accuracy, measure at a consistent temperature. If your volume markings were calibrated with cold water, add 4% when measuring hot wort. Many brewers measure cold post-boil volume after chilling.

How do I mark my kettle for accurate volumes?

Add measured amounts of water (e.g. 5L at a time) and mark the levels on your kettle with a permanent marker or calibrated dipstick. Do this with cold water for a cold-volume reference.

Why is pre-boil volume important?

Knowing your required pre-boil volume ensures you collect enough wort from the mash to hit your target post-boil volume and gravity. If you start with too little, your beer will be smaller than planned but higher gravity.

How does boil-off rate vary between systems?

Gas burners typically boil off 4-6 L/hr, electric elements 3-5 L/hr. Wider kettles lose more due to greater surface area. Measure your own boil-off rate by noting volume before and after a timed boil. Use our Boil-Off Calculator to determine your rate.

BrewCalcs

Boil Volume Calculator — Recipe Design

Calculate required pre-boil volume for your target batch size.

Target Post-Boil Volume

Cold volume into fermenter

Boil-Off Rate

Boil Time

Trub Loss

Thermal Shrinkage

~4% from boiling to 20°C

How We Calculate This

Pre-Boil Volume Formula

Pre-Boil Volume = (Post-Boil Volume x Shrinkage Factor) + Boil-Off + Trub Loss. The shrinkage factor accounts for the ~4% thermal contraction when wort cools from boiling to room temperature.

Why This Matters

Starting with the correct pre-boil volume ensures your post-boil gravity and volume hit their targets. Too much volume dilutes the wort; too little concentrates it and changes your recipe.

Frequently Asked Questions

Related Calculators

Recipe Scaling Calculator

Scale a recipe up or down to a different batch size.

Style Guidelines Calculator

Check if your recipe parameters fall within BJCP style guidelines.

Batch Size Calculator

Calculate final batch size accounting for all system losses.

Pre-Boil Gravity Calculator

Calculate expected pre-boil gravity from post-boil targets.

Post-Boil Volume Calculator

Calculate post-boil volume from pre-boil volume and boil-off rate.

Last updated: February 2026

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