Choosing a kettle size for the batch you want to brew

Start from the fermenter, not the shop

Shops sell kettles by headline volume. Brewers should buy by working backwards. Decide how much beer you want in the fermenter, then add back everything you lose on the way: trub and hops left in the kettle, water boiled off and, for all-grain, the water the grain soaks up. The kettle has to hold the biggest of those volumes with room to spare.

For most homebrewers in India the answer comes down to two batch sizes: 10 litres or 20 litres into the fermenter.

The losses you have to add back

Three losses matter on a homebrew scale:

• Boil-off: a vigorous boil on a home kettle typically loses 2 to 4 litres an hour. Wide kettles lose more than tall ones.
• Kettle loss: hops and trub hold back 1 to 2 litres you leave behind.
• Grain absorption: spent grain keeps roughly 1 litre per kilogram, a little less if you squeeze the bag.

Measure your own boil-off on a plain water boil before your first brew. It is the one number that changes most between kettles and stoves.

Headroom stops boil-overs

Wort foams hard just before it reaches a full boil, which brewers call the hot break. A kettle filled to the brim will throw sticky wort over your stove. Leave at least 20% of the kettle empty at the start of the boil, more if you are still learning to watch for the break.

A boil-over on an induction plate is more than a mess. Sugar burns onto the glass and the plate may cut out on its overheat sensor mid-boil. That one detail is reason enough to buy a size up.

Sizing for extract and partial boils

Extract brewing is the most forgiving. You can boil a concentrated wort in a small pot and top up with clean water in the fermenter. A 15 to 20 litre stockpot handles a 10 to 15 litre partial boil for a 20 litre batch. The catch is that hop bitterness extracts less well in strong wort and extract darkens more in a concentrated boil, so full volume is better when your kit allows it. Our guide to getting the best from malt extract covers the trade-offs.

Sizing for brew in a bag

Brew in a bag needs the biggest kettle, because the grain and all the water sit in it together. Here is the working for 20 litres into the fermenter with about 4.5 kg of grain:

• Fermenter 20 L plus kettle loss 2 L gives 22 L after the boil.
• Add 3 L boil-off for 60 minutes: 25 L before the boil.
• Add 4.5 L the grain absorbs: about 30 L of water at mash-in.
• Grain takes up space too, about 0.7 L per kilogram, so the mash fills about 33 L.

That is a 40 litre kettle at the very least and 50 litres if you want headroom for a stronger beer. For a 10 litre batch the same working gives about 17 to 18 litres of mash, so a 25 to 30 litre kettle is right. See our small batch brewing guide if you are choosing between the two sizes.

Check your heat source can boil it

A big kettle is useless if your stove cannot boil it. A typical 2 kW home induction plate struggles to bring much more than 15 to 20 litres to a rolling boil. A weak boil leaves dimethyl sulphide (DMS), the cooked corn flavour, in pale beers. A gas burner copes with more, but a single domestic burner is slow on 25 litres.

For induction, check the base. It must be magnetic stainless steel and roughly the diameter of the plate. A wide 50 litre pot on a small plate heats a ring in the middle and little else.

Buy for the batch you will brew in a year

Material and build matter more than brand. Stainless steel is the safe default: it cleans easily and works on induction if the base is magnetic. Aluminium pots are cheap and heat fast on gas. They are perfectly usable for boiling wort, but they do not work on induction and should never be cleaned with caustic, which attacks the metal. A thick, encapsulated base spreads heat and stops scorching when you heat a thick extract wort. A lid that fits speeds up heating strike water, though the boil itself should run uncovered so DMS can escape.

Most homebrewers move up from 10 to 20 litres within their first year. If you think that is you, buy the larger kettle now and brew half batches in it; a small batch in a big pot works fine, the reverse does not. Spend the rest on a ball valve and a lid that fits.

Your first step: decide your fermenter volume, run the brew in a bag working above with your own boil-off, then measure how many litres your stove can bring to a rolling boil before you buy.

Common questions

What size kettle do I need for a 20 litre batch?

About 30 litres for extract or a full boil and 40 to 50 litres for brew in a bag, where the grain and all the water sit in the kettle together.

What size kettle do I need for a 10 litre batch?

A 20 litre kettle suits extract and a 25 to 30 litre kettle suits brew in a bag.

How much headroom should a kettle have?

Leave at least 20% empty at the start of the boil so the hot break foam does not spill over.

Can I use any steel pot on induction?

Only if the base is magnetic. Check with a fridge magnet. The base should also be close to the diameter of the induction plate.