Why check pre-boil gravity?

Taking a pre-boil gravity reading lets you verify your mash efficiency before committing to the boil. If gravity is too low, you can extend the boil or add extract. If too high, you can add water.

How are gravity points conserved?

Sugar dissolved in wort is not lost during evaporation — only water leaves. So the total gravity points (volume x gravity points) remain constant throughout the boil. This principle lets us calculate backwards.

What if my pre-boil gravity is off target?

If low: extend the boil (more evaporation concentrates sugars), or add DME/LME. If high: add water to dilute. Always measure before boiling so you can make adjustments.

What if my pre-boil gravity is too low?

Options include: extending the boil to concentrate the wort (but this reduces volume), adding dry malt extract to boost gravity, or accepting a lower OG beer. For future brews, investigate whether the cause is crush, efficiency, or water volume related.

What if my pre-boil gravity is too high?

Add treated water to dilute down to your target. This also increases your batch size. Alternatively, reduce boil time to retain more volume while accepting slightly higher gravity. Taking accurate pre-boil measurements lets you adjust before committing to the boil.

BrewCalcs

Pre-Boil Gravity Calculator — Recipe Design

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

Target Post-Boil Gravity

Your target OG

Post-Boil Volume

Pre-Boil Volume

How We Calculate This

Gravity Point Conservation

The key principle is: Pre-Boil Volume x Pre-Boil Gravity Points = Post-Boil Volume x Post-Boil Gravity Points. Gravity points = (SG - 1) x 1000. Since boiling only removes water, the total dissolved sugars remain constant.

Using This for Brew Day

Calculate your expected pre-boil gravity before brew day. Then measure your actual pre-boil gravity to verify mash efficiency. This is the most important quality control step in all-grain brewing.

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.