Boil Off Calculator

Calculate the pre-boil volume and gravity you need from your post-boil volume, boil time and boil-off rate, and the volume left after the wort cools.

Pre-boil volume is the post-boil volume plus the boil-off rate times the boil time. To finish a 60-minute boil with 23 L at 1.050 in a kettle that boils off 4 L per hour, collect 27 L at about 1.043. The hot wort then shrinks about 4% as it cools, to about 22 L.

Reviewed by Diego Santiago, Professional Brewer

MetricUSImperial (UK)

Post-boil volume (L)

Hot wort in the kettle at the end of the boil

Boil time (min)

Length of the boil

Boil-off rate per hour (L)

Volume your kettle evaporates in one hour

Post-boil gravity (SG)

Optional: target gravity after the boil

Cooling shrinkage (%)

Wort shrinks about 4% as it cools from boiling

Pre-boil volume27.00LPre-boil gravity1.043SGEvaporated4.00L (14.8%)Volume after cooling22.08L

Results are estimates provided "as is," for informational and planning purposes only, without warranties of any kind. Check them before you rely on them: you are responsible for decisions about your beer and equipment, including pressure and carbonation levels, the pressure ratings of your kegs and fittings, and any alcohol content you declare. Use of this calculator is subject to our Terms and Conditions.

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How boil-off is calculated

During the boil, water evaporates at a roughly constant rate, so the volume to collect before the boil is:

V_{pre} = V_{post} + r \times t

where $V_{post}$ is the volume at the end of the boil, $r$ the boil-off rate per hour and $t$ the boil time in hours. Boiling removes water but not sugar, so the gravity points scale with the volume:

G_{pre} = G_{post} \times \frac{V_{post}}{V_{pre}}

where $G$ is the gravity in points (1.050 is 50 points).

For example, to end a 60-minute boil with 23 L at 1.050 in a kettle that boils off 4 L per hour, collect 27 L at about 1.043. After chilling, the 23 L of hot wort shrinks to about 22.1 L.

Inside Foambyte, mash and boil profiles keep your evaporation rate and losses, so every recipe starts from your own system.

Reference tables

Pre-boil volume (L) for 23 L at the end of the boil, by boil time (rows) and boil-off rate (columns)
Boil \ rate	2 L/h	3 L/h	4 L/h	5 L/h	6 L/h
30 min	24.0	24.5	25.0	25.5	26.0
60 min	25.0	26.0	27.0	28.0	29.0
75 min	25.5	26.8	28.0	29.3	30.5
90 min	26.0	27.5	29.0	30.5	32.0
120 min	27.0	29.0	31.0	33.0	35.0

Pre-boil volume (US gal) for 6 gal at the end of the boil, by boil time (rows) and boil-off rate (columns)
Boil \ rate	0.50 gal/h	0.75 gal/h	1.00 gal/h	1.25 gal/h	1.50 gal/h
30 min	6.25	6.37	6.50	6.63	6.75
60 min	6.50	6.75	7.00	7.25	7.50
75 min	6.63	6.94	7.25	7.56	7.87
90 min	6.75	7.12	7.50	7.87	8.25
120 min	7.00	7.50	8.00	8.50	9.00

Frequently asked questions

How do I find my boil-off rate?

Boil a known volume of water, or wort, for an hour with the lid off at your usual heat, then measure what is left. The difference is your boil-off rate per hour. It depends on your kettle's diameter, your burner and the weather, so measure it on your own system.

Does a longer boil change the boil-off rate?

Not much. The rate per hour stays about the same, so a 90-minute boil evaporates about half again as much as a 60-minute boil. Plan the extra volume, or the gravity will come out higher than intended.

Why does the volume drop after the boil?

Hot wort takes more space than cold wort. Water is about 4% less dense at boiling than at room temperature, so wort measured hot in the kettle shrinks by about that much once it is chilled. Some wort also stays behind with the hops and trub.

Why calculate the pre-boil gravity?

Measuring the gravity before the boil tells you early whether you will hit your original gravity. If the pre-boil reading is low, you can boil a little longer or add extract; if it is high, you can add water.

Does boiling change the amount of sugar?

No. Boiling removes water, not sugar, so the total gravity points stay the same and the gravity rises as the volume falls.