Why ‘units’ exist at all
The number on a bottle is ABV (Alcohol By Volume) — a concentration. Two drinks with the same ABV but different volumes contain wildly different amounts of alcohol. A unit fixes that by combining both into a single number, so a 25 ml shot, a 125 ml wine, and a 568 ml pint can all be compared on the same scale.
The formula
units = (ABV% × volume_ml) / 1000This drops out of definitions:
- ABV is the percentage of the drink that is ethanol, by volume.
- So
ABV% × volume_ml / 100= ml of pure ethanol in the drink. - 1 unit = 10 ml of pure ethanol, so divide by another 10 → divide total by 1000.
Worked examples through the Alcohol Units Calculator:
| Drink | ABV | Volume | Units |
|---|---|---|---|
| Pint of lager | 4% | 568 ml | 2.27 |
| Pint of strong lager | 5.2% | 568 ml | 2.95 |
| Small glass of wine | 12% | 125 ml | 1.50 |
| Large glass of wine | 12% | 250 ml | 3.00 |
| Single spirit (shot) | 40% | 25 ml | 1.00 |
| Bottle of wine | 13% | 750 ml | 9.75 |
The shot-of-spirit row is where the 25-ml-pour standard comes from in UK pubs — it’s the volume that makes a single shot exactly 1 unit at the standard 40% ABV.
A pint of normal lager is 2.3 units, not 2 — because the UK pint is 568 ml, not the round 500 ml people sometimes assume. This is the most common reason people underestimate their weekly intake.
Grams of ethanol — the biology version
Units are a UK convention. The thing your liver actually deals with is mass of ethanol:
1 UK unit = 10 ml ethanol × 0.789 g/ml (density) ≈ 7.89 gThe NHS rounds this to 8 g. So a “14 unit week” is roughly 112 g of ethanol — about a quarter-pound, distributed across whatever drinks got you there.
- Ethanol per UK unit (volume)
- Ethanol per UK unit (mass)
- Total ethanol in 14 units
Other countries use mass directly:
- US standard drink = 14 g ethanol ≈ 1.75 UK units
- Australia = 10 g ≈ 1.25 UK units
- Japan = 20 g ≈ 2.5 UK units
So a “1 drink” labelled on a US menu is closer to 1.75 UK pints-worth of alcohol than 1.
Where 14 a week comes from
The UK Chief Medical Officers (CMOs) revised the low-risk drinking guideline in January 2016. The headline number is 14 units per week — applied equally to men and women, with three additional pieces of guidance:
- Spread drinks across 3 or more days.
- Include 2 or more alcohol-free days per week.
- If you’re pregnant or trying to be, don’t drink at all.
The 14-unit number came from a Department of Health expert review that mapped weekly intake against long-term mortality from alcohol-attributable causes (mostly cancer, liver, cardiovascular). The team chose the level at which the lifetime risk of dying from alcohol-related causes was about 1%, which is what the CMOs decided counted as “low risk”.
The 14-unit guideline doesn’t mean drinks under 14 are safe — it means the population-level risk is low. The Lancet 2018 Global Burden of Disease study concluded that no level of alcohol is fully risk-free for total mortality. The right framing: every unit nudges your risk up a little; below 14/week the slope is gentle, above it the slope steepens.
Why the calculator can’t tell you if you’re drunk
Units measure intake, not blood alcohol concentration (BAC). BAC depends on:
- Body weight — same drink, lighter body → higher BAC
- Sex — women generally have less body water → higher BAC for the same dose
- Stomach contents — food slows absorption
- Time — your liver clears ~1 unit/hour on average; sober up over the same number of hours
So “I had 4 units” tells you about long-term health risk, but not about whether you’re legally over the driving limit (which in England/Wales/NI is 80 mg/100 ml of BAC — Scotland is lower at 50 mg). For that, use a BAC Calculator and a wide safety margin. The honest version: don’t drive after drinking.
How to use the calculator well
A few patterns we’ve seen people get value from:
- End-of-week recap. Add every drink from Mon–Sun, see the weekly total. Comparing against 14 turns “I had a few” into a number.
- Pre-event budget. Add the drinks you expect to have at a wedding/party first, see the total. If one event eats your whole weekly allowance, adjust before, not after.
- ‘Healthy’ drink check. A 250 ml glass of wine is 3 units. A pint of strong lager is 2.95. They’re functionally identical for unit-counting purposes — the calculator surfaces that quickly.
The Alcohol Units Calculator does all of the above in one screen, with no account, no cloud, no history. Open it, add some rows, see the math.