Cask ale carbonation levels explained

Last updated: 29 June 2026

Most pub operators think carbonation is something the brewery handles — and then wonder why their cask ale tastes flat or over-fizzy, or why the first three pints pour with a head the size of a pillow and the last three are all foam. Carbonation in cask ale is not set and forgotten. It changes with cellar temperature, how long a cask has been on, and how clean your lines are. Getting it right means better-tasting beer, less waste, and customers who actually come back for a second pint instead of switching to lager. This guide explains what cask ale carbonation levels should be, why they matter to your bottom line, and what you can actually control in your cellar.

What is cask ale carbonation?

Cask ale carbonation is measured in volumes of CO₂ — a number representing how much carbon dioxide gas is dissolved in the beer. One volume of CO₂ means there is one litre of CO₂ gas dissolved in one litre of liquid beer. Most British cask ales sit between 1.5 and 2.5 volumes, which gives them their gentle fizz and soft, creamy head — nothing like the aggressive carbonation in a lager or a keg cola.

Unlike kegged beer or bottled beer, cask ale is not force-carbonated at the brewery. The carbonation comes from two sources: residual CO₂ left over from fermentation, and secondary fermentation that happens inside the cask during storage and transport. When a cask arrives at your pub, it is still a living thing — yeast is still dormant in the sediment at the bottom, and the beer is still changing subtly. This is why cask ale tastes different at day two than it does at day ten, and why your cellar temperature matters so much.

The brewery calculates the target carbonation based on the beer style, the yeast strain, and how long they expect the cask to condition in your cellar. A session bitter might be 1.8 volumes. A stronger ale or a fruit ale might be 2.2 volumes. These are targets — not guarantees — and your cellar conditions will either help the cask hit that target or miss it entirely.

Why carbonation levels matter to your pub

If you have never measured the impact of carbonation on your bar, here is what you are probably missing: overcarbonateded cask ale creates excessive foam, which means you pour a pint and half of it is head. That half-inch of liquid is money you gave away. Undercarbonataded ale tastes flat and unappetising, so customers order something else instead. Both scenarios hit your profit.

The most direct link between carbonation and your pub’s gross profit is waste and perceived quality. A 1% stock loss on wet sales quietly costs a typical pub £3,000–£5,000 a year. A huge chunk of that loss is not theft — it is over-pouring, spillage, and excessive foam going down the drain. Poor cellar temperature management that lets carbonation go haywire is one of the easiest and most overlooked causes of that loss.

Customers also notice carbonation immediately. The first thing they taste is the fizz and the head. If your bitter is flat compared to the one at the pub down the road, they remember it. Consistency matters in cask ale more than in any other drink — it is not a commodity. People drink their regular bitter because it tastes a certain way at your bar. If it is flat one week and over-fizzy the next, you lose trust and you lose sales.

Ideal carbonation levels for UK cask ale

The Brewers Association and the Institute of Brewing both recommend cask ales be served at between 1.5 and 2.5 volumes of CO₂. But that is a range, and where your beer should sit within that range depends on the style.

Session bitters and ales (lower ABV, hop-forward) typically target 1.5–1.9 volumes for a dry finish and delicate head. Brown ales and milds aim for 1.8–2.1 volumes to support a creamier mouthfeel. Stronger ales, IPAs, and fruit ales often sit at 2.0–2.5 volumes to balance their complexity and give them presence on the palate.

Your brewery will tell you the target for each beer if you ask. Most will put it on the cask label or the delivery note. If you do not see it, ask your rep or check the brewery website — it is not a secret. Write it down. Use it as a baseline for what your cellar should be delivering.

The real carbonation level your beer actually has when it reaches the customer’s glass depends on three things: what the brewery put in, what your cellar temperature did to it, and how clean your lines are. Most pubs can control two of those three things, and most do not.

How cellar conditions affect carbonation

Temperature is the biggest lever you have. Warmer cellars cause dissolved CO₂ to escape from the beer, making it flatter. Colder cellars trap CO₂ in solution, which can actually push carbonation slightly higher than the brewery intended. The relationship is not linear — a 2°C rise in cellar temperature does not cause a tiny loss of fizz; it causes a noticeable one. This is why breweries always specify a serving temperature range (usually 52–55°F or 11–13°C for British ales) and expect your cellar to stay within it.

If your cellar is running at 16°C in summer, your cask ale will carbonate lower than intended. If it swings from 10°C at night to 15°C during the day, the beer will release CO₂ during the warm hours and reabsorb it during the cold hours — and that cycling weakens the bubble structure, leading to a flatter beer by midweek. Most pub cellars are not temperature-controlled well enough to avoid this, and most licensees do not check a thermometer once a week.

Line cleaning also affects perceived carbonation. Dirty lines trap sediment and beer oils that degrade the foam structure, so even if the carbonation is correct, the head will be thin and unstable. A 50-pint line that has not been cleaned in two weeks will pour flat-looking ale even if the cask carbonation is perfect. This is one of those invisible money leaks — customers think your beer is flat, but the real problem is your line hygiene.

Cask age matters too. A cask that has been on your bar for three weeks will have slightly lower carbonation than one that has been on for three days. The gas does not all escape — cask breathers (the one-way valves on top of the cask) prevent that — but there is a slow, gradual loss over time. By day twenty-one, most casks will be a touch flatter than they were at day three. This is normal and expected; breweries account for it. But if you are seeing a dramatic drop in carbonation by day ten, your cellar temperature is too high.

What you can actually control

You cannot add carbonation to a cask after it arrives. You cannot inject CO₂ into real ale (it would no longer be real ale under CAMRA rules). What you can do is preserve the carbonation the brewery put in.

Three actions will protect cask ale carbonation and reduce foam waste:

• Keep cellar temperature stable between 11–13°C. Use a simple wall thermometer or a wireless probe. Check it every Monday morning as part of a routine. If it is creeping above 14°C, investigate the cause (faulty cooler, blocked vents, too many casks too close together). If it swings by more than 2°C day-to-day, your cellar insulation needs work.
• Clean your draught lines with hot water and alkali cleaner once a week, every week. Most pubs that move from a messy spreadsheet to a disciplined weekly routine claw back 1–2 GP points on draught within a couple of months. Line cleaning is one of the biggest ROI actions you can take. Use the same day and time every week — Tuesday morning, for example — and log it. That consistency matters.
• Rotate casks properly and use them before they are too old. A cask is at its best from day three to day fourteen. After day twenty-one, most will show noticeable carbonation loss. If you have a slow-moving ale, either drop the number of casks on or rotate the stock faster. Dead casks on your bar cost money twice — in waste and in opportunity cost.

The other factor you can influence is line pressure. If your font pump is set too high, it will force CO₂ out of solution as the beer travels up the line, creating excessive foam at the tap. Too low, and you get a slow, sluggish pour that looks unprofessional. Most hand pumps do not have adjustable pressure (which is fine — they work on gravity), but if you use electric pumps, a pressure gauge and regular checks will help. Most breweries specify the ideal pump pressure on the cask.

Common carbonation mistakes pub operators make

I made most of these myself in my first few years, so you are not alone if you recognise one.

Mistake 1: Assuming the brewery got it wrong. You get a new cask of bitter and it tastes flat. Instead of checking your cellar temperature, you blame the brewery. You ring them and complain. Meanwhile, your cellar is 16°C and has been for a week. The ale is fine; your conditions are not. Ring the brewery only after you have ruled out your own cellar.

Mistake 2: Not cleaning lines for weeks. You think foam is a sign of over-carbonation, so you do nothing. Actually, dirty lines are causing it. The fix is not to ignore it — it is to clean the lines and then see how the beer pours. This should be non-negotiable weekly work, like cashing up.

Mistake 3: Leaving casks on too long. You have a slow-moving ale that has been on for five weeks because you are trying to get through the whole cask before opening a new one. The carbonation is gone, it tastes flat, so you sell fewer pints and the cask ultimately gets pulled part-empty. It would have been better to drop a cask earlier, rotate faster, and capture more sales at full quality. Do not let sunk cost fallacy drive your cellar decisions.

Mistake 4: Ignoring temperature swings. Summer hits and you think, “Well, the cooler is trying its best.” It is not about trying — it is about numbers. If your cellar hits 16°C in July, your ales will carbonate differently than they do in December. This is not optional. Either you control the temperature or your customers drink flat ale.

I was running stock on a tangle of spreadsheets and partial kegs and still losing track of variance until I built a simple count routine around a dip stick, temperature checks, and a set of scales. The weekly variance went from guesswork to a number I could trust within a fortnight. That same discipline applies to carbonation — you need a routine, not good intentions.

Monitoring carbonation in practice

You do not need laboratory equipment to tell if your carbonation is where it should be. You need your senses and a routine.

Pour a pint at the start of the week and at the end of the week from the same cask. Look at the head. Feel the fizz on your palate. Does it feel flat? Correct? Over-fizzy? Write it down. After a few weeks, you will have a reference for what “right” feels like for each of your regular ales. When a cask deviates from that, you will notice it immediately.

Temperature is the easiest thing to track and the thing that matters most. A simple wireless thermometer in your cellar costs £15 and will tell you if your conditions are drifting. Check it every Monday. If it is running above 14°C or swinging by more than 2°C day-to-day, that is your explanation for flat ale or excessive foam.

Line cleaning is not subjective. Either you cleaned the lines this week or you did not. Use StockTap pub stock app or a paper log to record when cleaning happened and by whom. Do not skip it or shuffle it to next week. It is as important to your stock accuracy as it is to your beer quality.

For more information, visit SmartPubTools.