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.
When to use dilution vs. boil-off
Dilution is your tool when gravity comes in too high — common in all-grain brewing when efficiency exceeds expectations, or when you brewed a concentrated boil and plan to top up the fermenter. You always have clean water available, so dilution is instant and reversible (up to a point).
Boil-off is your tool when gravity comes in too low. A longer or more vigorous boil concentrates the wort, but it also darkens it, increases bitterness from hop isomerisation, and cannot be undone. That makes boil-off a correction method to use sparingly — it is better to get the grain bill right than to routinely salvage low gravity with extended boiling.
Common mistakes and edge cases
Not temperature-correcting the hydrometer first. Hydrometers read correctly only at their calibration temperature (usually 60°F/15°C). A reading taken from hot wort is falsely low, leading you to think you need to add water when you do not. Always cool the sample or apply a temperature correction before entering the gravity here.
Mixing units mid-calculation. The formula is unit-agnostic, but only if you stay consistent. Mixing litres and gallons in the same calculation gives a nonsense result.
Ignoring the impact on hop bitterness. Adding water after the boil is complete (fermenter top-up) does not change bitterness — the hops are already out. But adding water mid-boil reduces contact time with hops, lowering bitterness extraction. If you dilute in-kettle, recalculate your hop additions too.
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.