The alcohol unit formula
UK units = volume in ml × ABV (%) ÷ 1,000. Enter the percentage printed on the label as a whole percentage: use 5 for a 5% drink, not 0.05. One UK unit is 10 ml of pure alcohol. The formula measures alcohol amount, not how intoxicated a person will feel.
For a 500 ml beer at 5%, the calculation is 500 × 5 ÷ 1,000 = 2.5 UK units. Two identical servings contain 5 units. If you drank only half the bottle, use the amount consumed, not the full package size. Keeping those two quantities separate avoids a common source of inflated totals.
Use the calculator for a quick result, then keep the underlying serving information. Rounding every individual entry too aggressively can distort a longer total. Calculate from the original amounts and round the displayed total when you review it.
Common serving examples
These are worked examples, not standard serving recommendations. Actual glass sizes, recipes and strengths vary. Check the label and the pour rather than assuming every beer or glass of wine matches the table.
| Serving | ABV | Calculation | UK units |
|---|---|---|---|
| 330 ml beer | 5% | 330 × 5 ÷ 1,000 | 1.65 |
| 500 ml beer | 5% | 500 × 5 ÷ 1,000 | 2.50 |
| 175 ml wine | 12% | 175 × 12 ÷ 1,000 | 2.10 |
| 250 ml wine | 13% | 250 × 13 ÷ 1,000 | 3.25 |
| 25 ml spirit | 40% | 25 × 40 ÷ 1,000 | 1.00 |
| 750 ml wine bottle | 12% | 750 × 12 ÷ 1,000 | 9.00 |
A unit is not the same as a standard drink
The United States defines a standard drink as 14 grams of pure alcohol. Australia uses 10 grams. The UK unit is defined by volume, 10 ml of pure alcohol, approximately 8 grams. The words sound similar, but the numbers are different. A serving is simply what was poured; it is not automatically one unit or one standard drink.
For comparison, the 500 ml, 5% beer contains 25 ml of pure alcohol. Using an approximate ethanol density of 0.789 g/ml gives 19.725 grams: about 1.41 US standard drinks or 1.97 Australian standard drinks, alongside 2.5 UK units. These are three ways of expressing the same alcohol amount, not three different drinks.
Choose one system for your main record and label it explicitly. When comparing a total with information from another country, convert the measurement first. The definitions do not themselves establish an individual safe allowance.
Build a record you can actually maintain
Start with the date, drink name, volume and ABV. Add the number of identical servings if your tracker supports it, or enter each serving separately. Price is optional. A few reliable fields are more useful than a detailed entry you never finish. For repeat drinks, save a template and check whether the serving or strength has changed before reusing it.
If a pour is uncertain, record a reasonable estimate and keep the uncertainty visible. Do not turn an unknown into an exact-looking figure just because the app displays two decimal places. If you cannot reconstruct an evening, leave it marked incomplete rather than filling the gap with zero.
The free diary gives you a blank weekly structure. It works on paper or as a CSV opened in a spreadsheet, so you can try the routine before choosing an app.
Review totals without losing the pattern
Imagine a fictional log with one 330 ml, 5% beer on Friday and two 175 ml, 12% wines on Saturday. Friday contributes 1.65 UK units and Saturday contributes 4.2, for a recorded total of 5.85. That arithmetic says nothing about unlogged days. Only call it a complete weekly total after checking all seven days.
Keep the day-by-day view next to the weekly number. An average can hide concentration on one occasion, and one unusually incomplete week can make the next look like a large increase. Compare periods of equal length, using the same measurement and similar completeness.
This is a measurement tool, not a blood-alcohol estimate or permission to drive. If you have concerns about drinking, symptoms or withdrawal, seek qualified advice rather than relying on a unit total.
Three mistakes worth catching early
First, check the package versus the portion: a 750 ml bottle and a 150 ml glass should not share the same volume. Second, check percentage versus fraction: 12% is entered as 12 in the formula above. Third, check the country definition before comparing two totals. Those errors can matter much more than a small difference in rounding.
For mixed drinks, count each alcoholic ingredient. Adding soda changes the finished drink's volume but does not remove the alcohol already in it. A cocktail name alone is not enough to calculate an accurate amount.
The short version
Keep volume and ABV, use one clearly labelled measurement system, and review complete periods. The arithmetic can be precise even when the original serving is an estimate.
