What Size Conical Fermenter Do You Need?

The rule

Batch size plus 30 to 40 percent

The working rule for any fermenter, conical or not, is to buy one rated at roughly 30 to 40 percent above your batch volume. For a 5 gallon batch that lands at 7 to 8 gallons, which is why nearly every homebrew-scale conical on the market is rated 7 or 8 gallons for a 5 gallon batch. The industry converged on that ratio because it is the one that works.

Two things consume the difference. The first is krausen: the foam cap of yeast, protein and hop matter that rises during active fermentation. How high it climbs depends on the yeast strain, the gravity and the temperature, and a hefeweizen or a vigorous Belgian can double the height a clean American ale produces. The second is the cone itself, which holds real volume below the straight wall and is not available for beer.

Get this wrong and the failure is not subtle. Krausen reaches the lid, pushes into the blowoff or the airlock, and you spend the next week cleaning a tube and wondering how much yeast you lost. Under pressure the same event blocks a spunding valve, which is worse.

Where the volume goes

A 7 gallon conical holding a 5 gallon batch

1. 5 gallons of beer in the straight-walled section, which is the part you actually sell yourself on.
2. Roughly a gallon of krausen headspace above the liquid. This is not spare room; it is working volume that is in use for three or four days.
3. Most of a gallon in the cone. The cone fills with trub and yeast and is the reason you bought a conical, but it is volume the beer occupies without being usefully in the batch.

That accounting is why a conical rated at 6.5 gallons is a tight 5 gallon fermenter rather than a comfortable one, and why anyone who brews 5.5 or 6 gallon batches should be shopping at 8 gallons and up.

The constraint nobody measures

Height, not volume

Volume is the number on the listing. Height is the number that decides whether the purchase works. A conical is tall by design: a straight wall plus a cone plus a stand to raise the dump valve above a bucket, plus whatever sits on the lid. Add those together and a 5 gallon conical can easily be taller than the inside depth of a 7 cubic foot chest freezer.

Measure three things before you order. The inside depth of the chamber you intend to use. The vessel height including the stand. And the clearance you need above the lid for a blowoff, a thermowell or a pressure fitting. If the vessel does not fit, you are choosing between a shorter vessel and integrated cooling -- and integrated cooling is the expensive option.

This is the single most common reason a conical purchase goes wrong, and it is entirely avoidable with a tape measure. A short plastic conical that fits your freezer will out-perform a beautiful welded vessel sitting at ambient garage temperature, every time.

Going bigger

When 10 gallons is the right call and when it is not

Doubling batch size is the most efficient upgrade in homebrewing: the same brew day, the same cleaning, twice the beer. The fermenter is the part that makes it possible, and a 10 gallon batch wants 13 to 14 gallons of rated capacity, or a half barrel vessel.

• Check the brewing system first. A 10 gallon fermenter is useless if the kettle tops out at 8 gallons into the fermenter. The systems that genuinely deliver 10 gallons are covered in best 10 gallon brewing systems.
• Check the chamber again. A half barrel vessel will not fit any chest freezer. At this size integrated cooling stops being a luxury and becomes the only practical option.
• Check whether you drink it. Ten gallons is roughly two full corny kegs. If a keg currently lasts you two months, the honest answer may be to brew more varieties rather than bigger batches.

How we sized these

The basis for this

Capacity figures come from published manufacturer specifications. The headspace ratio reflects long-standing homebrew practice and the observable fact that homebrew-scale conicals are almost universally rated 7 to 8 gallons for a 5 gallon batch. We have not measured krausen height across yeast strains and do not claim to. See how we pick.