HesapHanem

Boil-Off Gravity Calculator

Calculate post-boil gravity as volume evaporates.

Result

1.050
50

How it is calculated

The tool assumes that boiling removes water but no sugar, so the total sugar, measured as volume times gravity points, is constant. post-boil points = pre-boil points × pre-boil volume ÷ post-boil volume, and post-boil gravity = 1 + points ÷ 1000. Points are the digits after the decimal point of the specific gravity, so 1.042 is 42 points. Both volumes are in gallons. The points are rounded to a whole number, while the gravity is shown to three decimals and is calculated from the unrounded points. Volumes must be above zero and the points cannot be negative. The tool does not predict evaporation: you supply the expected post-boil volume yourself, for example from your kettle's measured boil-off rate.

Worked example

The wort measures 42 points before the boil, with a pre-boil volume of 6.5 gallons. You expect 5.5 gallons after the boil. Total sugar: 6.5 × 42 = 273. Divide by the new volume: 273 ÷ 5.5 = 49.6, which rounds to 50 points. Gravity is 1 + 49.6 ÷ 1000 = 1.0496, shown as 1.050. So a loss of one gallon raises the gravity by about 8 points.

Try this example →

Common mistakes

  • Typing the full gravity, such as 1.042, into the points field. The tool then reports a final gravity of about 1.001, as if there were almost no sugar.
  • Entering the volume that is lost to evaporation as the post-boil volume. The field is the volume left in the kettle, so 6.5 gal minus 1 gal lost is 5.5.
  • Forgetting the volume lost to hop material and trub when you plan to measure what goes into the fermenter. The tool counts only the volume you enter.
  • Ignoring that hot wort has a lower gravity reading than the same wort measured cooled. Take the pre-boil reading at the hydrometer's calibration temperature.
  • Entering litres. Both volume fields are in gallons, so 25 litres must be converted to about 6.6 gallons.

About this calculator

During a brewing boil, water evaporates while sugars stay behind, so the wort's gravity rises as its volume shrinks — a straightforward mass-balance calculation: pre-boil volume × pre-boil gravity points = post-boil volume × post-boil gravity points, since the total dissolved sugar (in gravity points) doesn't change even though the water it's dissolved in does.

Homebrewers and professional brewers use this to plan a recipe's boil: knowing your kettle's typical boil-off rate (often 1-1.5 gallons per hour, depending on kettle size and boil vigor) lets you predict what gravity you'll land on for a given boil time, or work backward to figure out how long to boil to hit a target original gravity before pitching yeast. It's also useful mid-boil if a gravity reading comes in off-target and you need to decide whether to extend or shorten the boil.

As wort boils down the same sugar sits in less liquid; enter the pre-boil volume and gravity with the expected post-boil volume to see the gravity you will end up with.

Was this helpful?

You can pick up to 3 reactions.

Comments

  • Be the first to comment.

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators