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Dilution and boil-off calculator

Overshot your gravity? Find how much water brings it down. Planning a brew day? Enter your post-boil target, boil-off rate and losses to get the pre-boil volume and gravity.

Golden wort with hot-break foam in a stainless pot
Photo: PierreSelim, CC BY 3.0, via Wikimedia Commons
Units
Dilute to a target gravity
Pre-boil volume from boil-off
Water to add—
Final volume—
Pre-boil volume—
Boiled off—
Pre-boil gravity for that OG—
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How it’s calculated

  1. Gravity points × volume stays constant: V2 = V1 × (SG1 − 1) / (SG2 − 1)
  2. Water to add = V2 − V1
  3. Pre-boil volume = post-boil volume + boil-off rate × hours + losses

Assumptions

  • Boil-off rate is yours to measure: electric all-in-ones often run about 0.5–1 gal per hour.
  • Volumes measured at the same temperature.

Worked example: 5 gal at 1.065 diluted to 1.050 → 6.5 gal (add 1.5 gal). 5.5 gal post-boil, 1 gal/hr for 60 min, 0.5 gal losses → 7.0 gal pre-boil.

Sources: Craft Beer & Brewing: dilution tool

For guidance only: verify with your own readings, your equipment’s manual and the product labels.

Measure your own boil-off

Boil-off depends on the kettle, the element or burner and the weather. Measure the volume before and after a 60-minute boil of plain water once and use that number; electric all-in-ones often lose about 0.5–1 gallon per hour.

Questions

How do I lower my original gravity?
Add clean, cool water. Gravity points times volume stay constant, so 5 gallons at 1.065 becomes 6.5 gallons at 1.050.

Sources

  1. Craft Beer & Brewing: tools