Last updated: 2026-06-29

Standard Drinks and Alcohol Units

A standard drink is a way to measure alcohol that ignores the glass and looks only at what is actually in it: pure ethanol. Beers, wines and spirits come in wildly different sizes and strengths, so "one drink" tells you very little on its own. Converting each one to a fixed mass of pure alcohol puts them on a common scale — and that scale is what health guidelines, drink-driving limits and personal tracking are all built on. The catch is that countries disagree on how much alcohol counts as one standard drink, so the same beer can be one US standard drink yet nearly two UK units.

How grams of pure alcohol are calculated

Every standard-drink figure starts from the same quantity: the grams of pure ethanol in the glass. That depends on three things — how much liquid there is, how strong it is, and the density of alcohol:

Here $V_{\text{mL}}$ is the volume in millilitres, ABV is the alcohol by volume printed on the label as a percentage, and $0.789$ g/mL is the density of ethanol at room temperature. A 355 mL12 fl oz can of 5% beer therefore holds $355 \times 0.05 \times 0.789 \approx 14$ grams of pure alcohol — which, not by coincidence, is exactly one US standard drink.

Why the count differs by country

Once you have the grams of pure alcohol, a "standard drink" or "unit" is just that number divided by a reference mass. Each health authority chose its own reference, so the divisor — and therefore the count — changes with the country:

None of these is more "correct" than the others; they are simply different rulers for the same litre of beer. This is why it is essential to know which standard a guideline refers to before comparing it to your own intake.

UK units as a second reading

The calculator always shows UK units alongside your chosen standard, because they are a particularly clean shorthand. One UK unit is 10 mL or 8 g of pure ethanol — roughly the amount an average adult metabolises in an hour. The NHS reduces the whole calculation to:

which is just the pure-alcohol formula with the density and the 8-gram divisor folded into a single constant.

Worked examples

A 175 mL6 fl oz glass of 13% wine holds $175 \times 0.13 \times 0.789 \approx 17.9$ g of alcohol — about 1.3 US standard drinks, or 2.3 UK units. A full 750 mL25 fl oz bottle of the same wine is roughly $750 \times 13 \div 1000 = 9.75$ UK units, close to 5.5 US standard drinks (76.9 g of pure alcohol divided by 14). A 568 mL1 imperial pint of 4% cask beer comes to about 2.3 units. And a 44 mL1.5 fl oz shot of 40% spirits is about 13.9 g — essentially one US standard drink in a single small glass.

Low-risk drinking context

Standard drinks exist mainly so that guidelines can be expressed in a portable way. The UK Chief Medical Officers advise drinking no more than 14 units per week, spread over three or more days, with several alcohol-free days. Canada's 2023 guidance reframed the conversation around a continuum of risk that rises with each drink rather than a single safe threshold. Across all of these, the consistent message from health bodies is that no level of alcohol consumption is completely risk-free.

This calculator is an informational tool, not medical advice. To estimate intoxication rather than intake, the Blood Alcohol Content (BAC) Calculator applies the Widmark formula to body weight and time, and the Sober Time Estimator projects how long alcohol takes to clear.