A boil-off calculator helps homebrewers predict how gravity changes when wort is boiled down (concentrated) or diluted with water. Boiling evaporates water but leaves dissolved sugars behind, raising the specific gravity. Conversely, adding water lowers the gravity. Knowing the math lets you hit your target original gravity consistently on every brew day.
Boil-Off Calculator
Boil-Off Results
Pre-Boil to Post-Boil Calculator
Enter your pre-boil measurements and boil time to predict your post-boil volume and gravity.
Typical Boil-Off Rates by Kettle Size
These are rough estimates. Always measure your own boil-off rate for best accuracy.
| Kettle Size | Typical Boil-Off (gal/hr) | Typical Boil-Off (L/hr) | Notes |
|---|---|---|---|
| 5 gal (19 L) | 0.5 - 0.75 | 1.9 - 2.8 | Small stovetop kettle |
| 8 gal (30 L) | 0.75 - 1.0 | 2.8 - 3.8 | Standard homebrew kettle |
| 10 gal (38 L) | 1.0 - 1.5 | 3.8 - 5.7 | Large homebrew or small pilot system |
| 15 gal (57 L) | 1.5 - 2.0 | 5.7 - 7.6 | Large-batch or pilot system |
| 20+ gal (76+ L) | 2.0 - 3.0 | 7.6 - 11.4 | Commercial pilot or large homebrew system |
How to Use the Boil-Off / Dilution Calculator
Hitting your target original gravity is one of the most important steps on brew day. If your pre-boil gravity reading is too low, you need to boil longer to concentrate the wort. If it is too high, you can dilute with water. This free boil-off calculator takes the guesswork out of both scenarios so you can dial in your numbers with confidence.
Step 1: Choose your mode
Select Boil-Off if you want to concentrate your wort by evaporating water. Select Dilution if your gravity is too high and you need to add water to bring it down. Each mode shows the inputs that are relevant to that calculation.
Step 2: Enter your current measurements
In boil-off mode, enter your current wort volume, current specific gravity, and the target volume you want to reach. The calculator uses gravity points — the digits after 1.000 multiplied by 1000 — to determine the new gravity. For example, a gravity of 1.048 has 48 points. In dilution mode, enter your current volume, current gravity, and the target gravity you want to achieve.
Step 3: Set your boil-off rate
For boil-off calculations, enter your system's evaporation rate. Most homebrew setups lose about 1 gallon (3.8 liters) per hour. A vigorous boil in a wide kettle may evaporate closer to 1.5 gallons per hour, while a gentle boil in a narrow pot may only lose 0.5 gallons. If you have not measured your rate, use the reference table at the bottom of the page as a starting point.
Step 4: Read the results
The calculator displays the new gravity, volume change, and estimated boil time. In boil-off mode you will see how long to boil at your evaporation rate to reach the target volume. In dilution mode you will see exactly how much water to add. The pre-boil to post-boil section lets you enter a planned boil time and see what your post-boil volume and gravity will be, which is useful for recipe planning before brew day.
Tips for accurate results
Always take gravity readings at the correct temperature or use a hydrometer correction calculator. Measure volume at the same temperature each time, as hot wort expands roughly 4% compared to room temperature. Track your boil-off rate over several brews and update the input accordingly. These small refinements compound to make your brewing more consistent and repeatable over time.
All calculations run entirely in your browser. No data is stored or sent anywhere, so your brewing notes remain completely private. Bookmark this page so it is ready on your next brew day.
FAQ
Is this boil-off calculator really free?
Yes, the boil-off and dilution calculator is completely free with no limits and no account required. All calculations run locally in your browser. Nothing is sent to any server or stored anywhere.
Is my data safe and private?
Absolutely. Every calculation happens entirely in your browser using JavaScript. No brewing data is transmitted to any server, stored in any database, or shared with anyone.
How does boil-off affect wort gravity?
Boiling evaporates water but leaves sugars behind, so the wort becomes more concentrated. The gravity increases proportionally to the volume reduction. For example, boiling 7 gallons of 1.040 wort down to 5 gallons raises the gravity to approximately 1.056.
What is a typical boil-off rate for homebrewing?
Most homebrew kettles lose about 1 to 1.5 gallons per hour. The exact rate depends on kettle diameter, heat intensity, and whether the lid is on or off. A wider kettle with a vigorous boil will evaporate faster. Measure your own rate by noting volume before and after a timed boil.
When should I dilute my wort instead of boiling longer?
Dilute when your post-boil gravity is higher than your target original gravity. This often happens with extract brewing or a very efficient mash. Simply add pre-boiled and cooled water to bring the gravity down to your desired level without extending the boil.
Does boil-off rate change during the boil?
The rate is generally consistent once you reach a steady rolling boil, but it can decrease slightly as volume drops because there is less surface area exposed to heat. Environmental factors like humidity, altitude, and draft also play a role. Tracking your own system over several brews gives the most accurate number.
How do I measure my boil-off rate at home?
Fill your kettle with a known volume of water, bring it to a rolling boil, and maintain it for exactly 60 minutes. Measure the remaining volume and subtract from the starting volume. That difference is your hourly boil-off rate. Repeat a few times and average the results for a reliable figure.
Can I use this calculator for all-grain and extract brewing?
Yes. The gravity-point math works the same whether you mash grain or dissolve extract. Enter your current volume and gravity, and the calculator handles the rest. Extract brewers often top up with water after the boil, making the dilution mode especially useful.