Dilution & Boil-Off Gravity
Water to add to lower a wort’s gravity, or how long to boil to raise it, and the gravity after a boil — by conserving gravity points.
The volume that gives your target gravity, whether that means adding water or boiling it off and for how long at your boil-off rate, or the volume and gravity after boiling for a set time.
Example: 5.5 gal at 1.062 takes 0.7 gal of water to reach 1.055; 6.5 gal at 1.044 reaches 1.050 at 5.72 gal after 46.8 minutes at 1 gal an hour.
Same sugar,
new volume.
Why gravity points times volume stays constant, how that gives dilution and boil-off, and how accurate it is.
Points are conserved
Adding water or boiling it away does not change the sugar in the kettle. Gravity points — (gravity − 1) × 1000 — times volume therefore stay the same: 5.5 gallons at 62 points hold 341 point-gallons, which at 55 points fill 6.2 gallons.
Dilution and boil-off
For a target gravity the new volume is the old one × current points ÷ target points. More volume means topping up with water; less means boiling off the difference, which at your kettle’s boil-off rate takes difference ÷ rate hours. Or, the other way round, a boil of a given length leaves volume − rate × time at a correspondingly higher gravity.
Accuracy
Gravity points are not exactly proportional to sugar, so the rule drifts slightly for large changes in strong worts; for the adjustments of a brew day it is well within what a hydrometer can read.
What the numbers do not settle
Boil-off rates depend on the kettle, the burner and the weather; measure yours. Any volume unit works, as long as it is the same throughout. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.