What rule does this calculator use?

It conserves gravity points. Gravity points are (SG − 1) × 1000, and the total dissolved extract is gravity points × volume. Adding water increases volume without adding extract, so points fall; boiling off reduces volume, so points rise. Points × volume stays constant.

How do I dilute wort to a target gravity?

Enter your current gravity and volume, choose 'add water', and enter the target gravity. The tool returns the final volume and exactly how much water to add. Because extract is fixed, the new volume equals old points × old volume ÷ new points.

Does boiling longer always raise gravity?

Yes — boiling evaporates water but not sugar, so the remaining wort becomes more concentrated and its gravity rises. Enter your boil-off target volume to see the resulting gravity, or enter a target gravity to see how much volume you must evaporate.

Why doesn't this account for temperature?

Gravity-point conservation is independent of temperature, but a hydrometer reading still needs temperature correction before you use it here. Correct your reading to the hydrometer's calibration temperature first, then apply this dilution or boil-off math.

Can I use it for top-up water in the fermenter?

Yes. If you brewed a concentrated boil and plan to top up with water in the fermenter, enter the concentrated gravity and volume, choose 'add water', and enter the final fermenter volume to see the diluted starting gravity.

Wort Dilution & Boil-Off Calculator

The Wort Dilution & Boil-Off Calculator tells you the gravity you will end up with after adding water or after boiling off volume — and works backwards to give the exact volume change needed to hit a target gravity. It is the fast way to rescue a batch that came out too strong or too weak.

How it works

The tool conserves gravity points. Gravity points are:

points = (SG − 1) × 1000

The total dissolved extract in the kettle is points multiplied by volume. Adding water raises volume but adds no extract, and boiling off lowers volume without removing extract — so the product stays constant:

points₁ × volume₁ = points₂ × volume₂

This is the brewing form of the C₁V₁ = C₂V₂ relationship. From it you can solve for any unknown. To find the new gravity after changing volume:

points₂ = points₁ × volume₁ ÷ volume₂

To find the volume needed for a target gravity:

volume₂ = points₁ × volume₁ ÷ points₂

Example — diluting

You have 18 L of wort at 1.060 (60 points) but wanted 1.050. Target points are 50, so the final volume must be 60 × 18 ÷ 50 = 21.6 L, meaning you add 3.6 L of water.

Example — boiling off

You have 24 L at 1.040 (40 points) and want 1.048 (48 points). The final volume must be 40 × 24 ÷ 48 = 20 L, so you need to boil off 4 L.

Notes

Always temperature-correct your hydrometer reading before using it here.
The relationship assumes water and wort mix ideally, which is accurate at homebrew scale.
Volumes are unit-agnostic — just keep litres or gallons consistent throughout.