Boil Off Rate Calculator
Every brewer learns the same lesson on batch three: the recipe said five gallons into the fermenter, and four arrived. The missing gallon did not leak anywhere; it went up in steam, exactly as physics intended. The only question is how much steam your system makes per hour, because once you know that number you can start every brew day with the right amount of water.
The Boil Off Rate Calculator below computes your evaporation rate from pre- and post-boil volumes and boil time, in liters or gallons per hour and as a percentage per hour. It includes the 4% hot-volume correction that separates careful measurements from sloppy ones, and a verdict on whether your rate sits in the normal homebrewing band.
Quick Answer
Key takeaways
- Rate = volume lost / hours boiled. Percentage per hour = rate / pre-boil volume x 100.
- Typical homebrew boil-off is 8-12% per hour, roughly 1-1.5 gallons per hour on a keggle.
- Measure volumes at the same temperature or apply the 4% hot correction; mixing hot and cold readings corrupts the rate.
- Your rate is equipment-specific: burner power, kettle geometry, and boil vigor all move it.
The formula, and the temperature trap
The temperature trap is the classic error. Water at a rolling boil occupies about 4% more volume than the same water at room temperature. Measure pre-boil hot and post-boil cold, and you will understate the loss by the expansion difference, which on a 25-liter batch is a full liter of phantom error. Either measure both cold or tick the correction checkbox in the calculator.
Worked example, checked against this exact tool
Enter the pre-filled defaults (25 L pre-boil, 20 L post-boil, 120 minutes, liters, no correction) and click Calculate:
| Figure | Value |
|---|---|
| Boil off rate | 2.5 L/hr ((25 – 20) / 2) |
| In gallons | 0.66 gal/hr |
| Percentage | 10.0%/hr (2.5 / 25 x 100) |
| Verdict | Typical range (8-12%/hr) |
Tick the hot-measurement box and the corrected volumes become 24.04 and 19.23 liters, giving 2.4 L/hr and 10.0%/hr: the percentage barely moves because both volumes shrink together, but the absolute rate drops, which matters when you scale recipes.
Typical boil-off rates by setup
| Setup | Typical rate | Typical %/hr |
|---|---|---|
| Stovetop, 5-gallon pot | 0.5-0.75 gal/hr | 8-12% |
| Propane burner, keggle | 1.0-1.5 gal/hr | 8-12% |
| Electric element, 10-gallon kettle | 0.8-1.2 gal/hr | 8-12% |
| Gentle simmer (sauces, stocks) | 0.3-0.5 gal/hr | 5-8% |
Notice the percentage column barely moves across setups: that is why brewers quote %/hr rather than volume. A percentage travels with batch size; an absolute rate does not.
Measure your own system in one boil
Forum wisdom from experienced brewers converges on one procedure, and it needs no special equipment:
Fill with a known cold volume. Five gallons of cold water in the kettle, measured with a marked stick or a graduated vessel.
Boil for exactly 60 minutes. Use your normal boil vigor: the rolling boil you actually brew with, not a showcase inferno.
Cool and measure again. Let it cool to the starting temperature and measure. The difference is your hourly rate.
Enter it in your brewing software. BeerSmith and similar tools take the %/hr figure directly into the equipment profile, and your strike-water math gets dramatically better.
What competing calculators miss
Volume-only outputs. The top-ranking boil off tools return liters per hour and stop. Brewers think in %/hr because it scales across batch sizes; this calculator leads with both.
No thermal correction. Nobody else offers the 4% hot-volume adjustment, even though the brewing forums have preached it for years. Mixing hot and cold measurements is the most common source of a mysteriously wrong rate.
No unit flexibility. Several tools are liters-only or gallons-only. American homebrewers buy grain by the pound and measure in gallons; the unit toggle here respects that.
No typical-range verdict. A number without context is trivia. The 8-12%/hr band, drawn from years of forum-reported measurements, tells you whether your system is behaving or your thermometer is lying.
Frequently asked questions
Subtract the post-boil volume from the pre-boil volume and divide by the boil time in hours. For 25 liters down to 20 over 2 hours: (25-20)/2 = 2.5 L/hr, or 10%/hr relative to the starting volume.
About 8-12% per hour, which works out to roughly 1-1.5 gallons per hour on a typical keggle setup. Stovetop pots run lower in absolute terms but similar in percentage.
Either, but be consistent. Water expands about 4% at boiling temperature, so divide hot measurements by 1.04 to compare with cold ones. The calculator has a checkbox that applies this correction for you.
Yes, noticeably. A violent boil can push past 12%/hr while a gentle rolling boil sits near 8%. Most brewers aim for a steady rolling boil: enough to drive off unwanted compounds without scorching or excessive loss.
Check the other losses: grain absorption (about 0.12 gallons per pound of grain), dead space in the mash tun and kettle, and trub left behind. Boil-off is only one line in the water budget.
Yes, the same math applies to reducing stocks and sauces. Measure the starting volume, simmer for a timed period, measure again, and you have your stove’s reduction rate for that pot and heat setting.
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References
- Brave Calculator, “Boil Off Rate Calculator” – (initial – final) / time formula with 25L to 20L worked example. Brave Calculator: Boil Off Rate Calculator. Accessed October 19, 2026.
- CalculatorUltra, “Boil Off Rate Calculator and Formula” – BOR formula in mL/min with worked examples. CalculatorUltra: Boil Off Rate Calculator. Accessed October 19, 2026.
- BeerSmith Forum, “Boil off rate” – equipment-profile calibration method with the 4% hot-to-cold expansion factor. BeerSmith Forum: Boil Off Rate. Accessed October 19, 2026.
- BeerSmith Forum, “Evaporation Rates for Keggles” – real-world evaporation measurements around 10%/hr and 1-1.5 gal/hr. BeerSmith Forum: Evaporation Rates for Keggles. Accessed October 19, 2026.
- Homebrewers Association Forum, “Water loss to absorption and evaporation” – ballpark 1.25-1.5 gallons per hour evaporation starting point. Homebrewers Association: Water Loss. Accessed October 19, 2026.