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Wort Boil Gravity Calculator

The calculator starts with the pre‑boil specific gravity and wort volume. It estimates how much wort evaporates during the boil based on a user‑specified evaporation rate per hour, subtracting that loss to obtain the final volume. Since the dissolved sugars remain unchanged, the total gravity points (points × L) stay constant; dividing by the new volume yields the post‑boil gravity. If an optional target gravity is supplied, the tool then determines how many litres of water must be added after the boil to reach that value, providing brewers with a precise dilution plan for consistent fermentables and alcohol levels.

The specific gravity of the wort after mash but before boiling, expressed as 1.xxx (e.g., 1.050).
Total wort volume after mash, before boil, in litres.
Length of the boil in minutes.
Estimated wort loss as a percentage per hour during boiling (typical 5–15%).
Optional gravity you want to achieve after adding water (e.g., 1.040).

What it is

The Wort Boil Gravity Calculator predicts how a brewer’s wort gravity changes through the boiling phase when evaporation occurs. It takes the pre‑boil specific gravity and volume, applies an evaporation rate to estimate lost water volume, then keeps the total number of gravity points constant while reducing volume—tightening sugars and increasing gravity. By providing the post‑boil gravity and optional dilution recommendation, brewers can plan fermentable concentrations, calculate needed water additions, and maintain consistent batch performance across recipes. Existing tools often omit evaporation or give only rough estimates; this calculator delivers precise numbers for exact recipe scaling.

How to use it

First enter your pre‑boil OG as a decimal (e.g., 1.050) and the volume in litres after mash. Input how long your boil will run in minutes and estimate the evaporation rate as a percent per hour. Optionally type a target gravity you want to reach once all water is added. The tool immediately shows: 1) how many litres evaporated, 2) final wort volume, 3) expected post‑boil gravity, and 4) litres of water required to reach the target. Use these figures when setting mash schedules, adding kettle water or planning bottling concentrations.

Worked example

The pre‑boil wort is 20 L at OG 1.050. With an evaporation rate of 10 % per hour over a 60‑minute boil, the loss equals 20 L × 0.10 × (60/60) = 2 L. The final volume is 18 L. Total gravity points are (1.050‑1)×1000×20 = 1000; dividing by 18 L gives 55.56 points per litre, i.e., OG ≈ 1.055556 after the boil. Since the target gravity is also 1.050, we need to dilute the 18 L of post‑boil wort back to 20 L (the original volume) by adding 2 L of water—exactly what the calculator reports.

Inputs

  • Pre‑boil Original Gravity (OG): 1.05
  • Pre‑boil Volume: 20
  • Boil Time: 60
  • Evaporation Rate: 10
  • Target Gravity After Dilution: 1.05

Result

  • Volume Lost During Boil: 2
  • Final Volume After Evaporation: 18
  • Wort Gravity After Boil: 1.055556
  • Water to Add Post‑Boil (if Needed): 2

Frequently asked questions

Why does evaporation raise my wort gravity?

Evaporation removes water but leaves dissolved sugars and proteins intact. The same mass of solids occupies a smaller volume, so their concentration—and thus the specific gravity—increases.

Do I always need to set an evacuation rate?

Setting an evaporation rate allows the calculator to account for kettle size, boil vigor, steam lift and ambient conditions. If you’re unsure, start with 10 % per hour—typical for a standard 5‑gal pot at moderate heat.

What if my wort ends up more concentrated than my target gravity?

The tool indicates how many litres of water must be added post‑boil to reach the desired gravity. Add the calculated amount of kettle or brew‑stand water gradually while stirring, then recheck the OG before proceeding.

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