Why are hot and cold post-boil volumes different?

Wort contracts by approximately 4% when cooling from boiling (100°C) to room temperature (20°C). So 24L of hot wort becomes about 23L when cold. This thermal contraction must be accounted for.

How do I measure my boil-off rate?

Boil water in your kettle for a measured time (e.g. 30 minutes). Measure the volume before and after. Double the loss for a per-hour rate. Do this with your normal heat setting and lid arrangement.

Can I reduce boil-off?

Partially covering the kettle reduces evaporation but may trap DMS. Using a lower boil intensity saves water but reduces hop utilisation. A vigorous rolling boil is preferred for most styles.

Does post-boil volume affect gravity?

Yes, directly. Less post-boil volume means higher gravity (more concentrated). If you boil off more than expected, your OG will be higher but you will have less beer. You can dilute with sterile water to hit both your target volume and gravity.

Should I measure hot or cold volume?

Wort contracts about 4% as it cools from boiling to 20°C. Hot-side measurements will read about 4% higher than the actual cold volume. For accuracy, either measure cold or apply the 4% correction.

BrewCalcs

Post-Boil Volume Calculator — Recipe Design

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

Pre-Boil Volume

Boil-Off Rate

Boil Time

Thermal Shrinkage

~4% from boiling to 20°C

How We Calculate This

Post-Boil Volume Calculation

Post-Boil (hot) = Pre-Boil Volume - (Boil-Off Rate x Boil Time). The cold post-boil volume accounts for thermal shrinkage: Post-Boil (cold) = Post-Boil (hot) x (1 - Shrinkage %).

Practical Use

Knowing your expected post-boil volume helps verify that your brew day is on track. Compare your actual post-boil volume to the calculated value to confirm your boil-off rate is consistent.

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.