Why your keg and cask calibration is probably wrong, and how to fix it

Right, mate, quick gut check before you read on: when was the last time you actually checked your dip stick against a known volume, rather than just trusting the numbers stamped on the side? If the answer’s “never” or “when I bought it,” this is for you.

Here’s the conclusion first, because I know you’ve got a cellar drop to do. Most keg and cask “calibration” in pubs isn’t calibration at all — it’s a guess dressed up as a measurement. The stick’s fine, the scale’s fine, but the reading is wrong, and it’s wrong in the same three or four ways in nearly every pub I’ve ever heard from. Fix those and your variance sheet stops lying to you.

The sizes everyone gets almost right

Let’s nail the basics first, because half the arguments about “missing” stock start here.

A standard UK keg is 50 litres, which is 88 pints. The smaller keg — increasingly common for guest ales and low-and-slow sellers — is 30 litres, which comes out at just under 53 pints (52.8, if you want to be pedantic about it, and for stocktaking you should be).

Cask sizes are imperial and always have been: a pin is 4.5 gallons, a firkin is 9 gallons (72 pints), and a kilderkin is 18 gallons (144 pints). The conversion constant behind all of it is that one imperial gallon equals 4.546 litres — actually 4.54609 litres if you’re going to the decimal place, which is the legal definition under the Weights and Measures Act 1985.

None of that is controversial. Where it goes wrong is what happens after the keg or cask is broached.

Reason one: your dip stick was calibrated for a cask, not a stillage

Cask dip sticks are graduated assuming the cask is lying on its side on a stillage at a specific angle and a specific level of chock. Tip the cask slightly more or less than it was calibrated for — which happens constantly when staff re-rack casks or when your cellar floor isn’t dead level — and every reading on that stick is off, sometimes by half a gallon or more on a firkin. That’s not “close enough,” that’s nearly 4% of the whole cask, before you’ve poured a drop.

Fix: check your stillage is level (a spirit level, not a squint), and dip the same cask before and after moving it to see how much the reading shifts. If it shifts more than a pint or so, your stillage geometry is the problem, not the beer.

Reason two: keg “weight” calibration assumes the keg is empty when it isn’t

If you’re weighing kegs to estimate contents — which most gas-driven cellars end up doing at some point — you need the true empty (tare) weight of that specific keg, not a generic figure off a spec sheet. Steel kegs vary by supplier and by wear, and I’ve seen tare weights on “identical” 50L kegs vary by close to a kilo, which on the maths is roughly two pints’ worth of apparent stock that isn’t real. Weigh a genuinely empty one from each of your suppliers and write the number on it in marker. Don’t trust the number in your head from three suppliers ago.

Reason three: line contents aren’t in your dip or your weight

Whatever’s sitting in your beer lines between the cellar and the tap isn’t in the cask or keg anymore, but it’s also not through the till yet. On a typical run, that’s not nothing — line cleaning alone accounts for real, documented wastage. Trade press reporting in 2019 put UK pub beer wastage at around 40 million pints a year industry-wide, working out to something like 20 pints a week per average pub, mostly down to inefficient line-cleaning practice. Whatever your actual number is, if you’re not accounting for line volume and clean-related loss separately from “cellar stock,” your variance sheet is comparing two different things and calling it one number.

Fix: measure your line run length once (get your cellar tech to do it when the lines are next changed) and treat it as a fixed, known deduction — not a mystery that gets blamed on bar staff.

Reason four: you’re applying spirits-measure logic to draught, and it doesn’t transfer

This one’s less about kit and more about a mental model. Bottled spirits go through an optic that’s individually tested and tightly toleranced — government guidance on measuring instruments gives an example tolerance of about ±0.7ml on a 35ml measure, roughly 2%. Draught beer and cider dispense equipment is explicitly treated differently under the same regulations, because line pressure and the condition of the cask or keg itself affect what actually comes out the tap — there’s no equivalent fixed, certified tolerance for a pint pulled through a hand pump the way there is for a vodka optic. That’s not an excuse to shrug off variance. It’s the reason you can’t calibrate a cellar once and assume it holds — pressure changes, temperature changes, and cask condition all move the number, so it needs rechecking, not just trusting.

None of this is about the legal serving measure, by the way — the rule that draught beer, lager and cider has to be sold in a third, a half, two-thirds, or multiples of a half pint is a separate bit of law (the Weights and Measures Act 1985 and the 1988 Intoxicating Liquor Order) covering what goes in the customer’s glass, not what you’re counting in the cellar. Don’t let anyone conflate the two when they’re trying to explain away a stock gap.

Where this actually saves you money

Get the four points above right and your variance report stops being noise. That matters because a wrong calibration doesn’t just cost you at stocktake — it costs you every time you make a buying or pricing decision off numbers that were never right to begin with. If you want to see how variance actually breaks down by product rather than guessing, our guide on stock variance and till buttons walks through that side of it. And if you’re still doing any of this by hand with a pad and a calculator, it’s worth seeing what pub stocktaking software actually compares on price and features before you commit to anything.

Common questions