Brew Kettle Size

BrewCo Brew Tools

Use this free brew kettle size calculator to work out the minimum kettle size needed for your batch, accounting for boil-off, trub loss, and headspace.

How to use it

  1. Enter your target batch size — the volume you want in the fermenter.
  2. Enter your planned boil time and your kettle's typical boil-off rate.
  3. Add your usual trub and deadspace losses.
  4. Read off the minimum kettle size needed, with headspace included.
Typical: 3–4L/hour on a standard vigorous boil.
— L
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Why add headspace on top of your pre-boil volume?

Wort foams up vigorously when it comes to the boil — without extra headspace above your pre-boil volume, you risk a boil-over that's messy and wastes valuable wort. 20% headspace is a safe standard margin for most home brew setups.

More free home brew calculators

Browse our range of brewing kettles and urns at BrewCo UK, Sowerby Bridge.

How to work out the right kettle size

What it does: this calculator tells you the smallest boiler or kettle that can handle your batch, so you don't buy one that's too small or end up with a boil-over.

How to use it: enter the volume you want in the fermenter, your boil time, your boil-off rate (3–4 litres an hour is typical for a rolling boil) and the wort you usually leave behind as trub and deadspace.

The formula in plain words: boil-off is your rate per hour multiplied by the boil time in hours. Your pre-boil volume is the batch size plus trub loss plus boil-off. The calculator then adds 20% headspace, because wort foams up as it comes to the boil.

Worked example: a 23-litre batch, a 60-minute boil, 3 litres an hour of boil-off and 1 litre of trub. Boil-off is 3 litres, so the pre-boil volume is 23 + 1 + 3 = 27 litres. Add 20% headspace and you need a kettle of at least 32.4 litres, so a 35-litre or 40-litre boiler is a comfortable choice.

Planning an all-grain brew? Check your tun with the mash size calculator too.

Frequently Asked Questions
How do I find my boil-off rate?
Measure the volume in your kettle at the start of the boil and again at the end, then divide the difference by the boil time in hours. Most home setups boil off 3–4 litres an hour.
Why add 20% headspace?
Wort foams up hard just before and during the first minutes of the boil. Without space above the liquid, it can boil over, wasting wort and making a mess.
Does a longer boil need a bigger kettle?
Yes. A 90-minute boil loses more to evaporation than a 60-minute one, so you need to start with more wort to end up with the same batch size.