Kettle Size Calculator
What size brew kettle you need: pre-boil volume from your batch, boil-off and losses, plus a headspace margin for foam.
Your batch, boil-off and boil time are remembered across the burner, element and kettle-volume tools. Sizes are guidance - round up to the next real kettle and leave extra room if your worts foam hard.
How it works
The binding constraint on kettle size is the moment the boil starts: that is when the pot holds the most liquid. From then on evaporation only lowers the level. So we work out the pre-boil volume and add a headspace margin for the foam of the hot break.
boil-off = boil-off rate × (boil time ÷ 60)
pre-boil = batch + losses + boil-off
recommended kettle = pre-boil × (1 + headspace%)
Evaporation varies a lot with your burner, kettle width and how hard you boil - measure your own boil-off rate once and reuse it. This is a planning figure, not a precision spec.
Sources: standard brew-day volume accounting (same pre-boil model as our water & sparge volume calculator); headspace is a rule-of-thumb margin.
Frequently asked questions
- What size kettle do I actually need?
- Your kettle has to hold the pre-boil volume - the most liquid in the pot, right at the start of the boil - plus headroom so it does not boil over. That pre-boil figure is your batch size into the fermenter, plus everything that leaves during the boil (evaporation) and any trub or kettle loss. This tool adds those up and applies a headspace margin (25% by default) for a recommended size.
- How much headspace should I leave?
- A hot break foams up hard in the first few minutes of the boil, so 20-30% above the pre-boil volume is a sensible margin - more if you brew high-gravity worts that foam aggressively or you have been caught by boil-overs before. A bigger kettle is rarely a problem; a kettle that only just holds the pre-boil volume will fight you.
- Does the grain go in the kettle?
- Not for a normal three-vessel mash - the grain sits in a separate mash tun, so only wort is in the boil kettle. For full-volume Brew In A Bag the grain is in the kettle during the mash, so use the BIAB calculator to check the mash-plus-grain volume fits as well.