Boil-Off Calculator
Water boils off but sugar stays, so gravity climbs as volume drops.
During the boil, water evaporates but sugar stays. That concentrates your wort — gravity goes up as volume goes down. The calculator above tells you how much volume you'll end up with, or how long to boil to hit your target gravity.
How We Calculate It
Same conservation principle as dilution, in reverse. Water leaves, sugar stays, gravity concentrates.
If you know your boil-off rate (liters per hour), you can also calculate the boil time needed to reach your target. Typical boil-off rates are 3–5 L/hr depending on your kettle and burner. Need to go the other way and bring gravity down? Add water with the dilution calculator.
How do I calculate post-boil gravity?
Gravity points are conserved during the boil, so post-boil gravity rises as volume falls. Concentrating 27 L of 1.040 down to about 21.6 L gives roughly 1.050.
What is a typical boil-off rate?
Most homebrew kettles evaporate 3–5 L/hr (about 1–1.5 gal/hr) depending on burner power, kettle width, and how vigorous the boil is. Measure yours once and reuse it.
How do I hit a target OG if I overshoot or undershoot?
Boil longer or harder to raise gravity, or add water to lower it. This calculator gives the post-boil volume for your target; the dilution calculator handles adding water.
Do hops and trub affect my volume?
Slightly — break material and hops trap some wort. Account for that as kettle and hop losses when planning your pre-boil volume; this calculator works on the wort volume itself.