How this instrument works
The US defines one standard drink as 14 grams of pure alcohol — roughly 0.6 fluid ounces of ethanol, regardless of what it's dissolved in. To get from a poured volume and a percentage on a label to that fixed unit, the formula multiplies volume in millilitres by the fraction of the drink that's alcohol, ABV ÷ 100, then by ethanol's actual physical density, 0.789 grams per millilitre — because a percentage by volume alone doesn't tell you a weight of alcohol until it's converted through density. Divide the resulting grams by 14 and the answer is how many US standard drinks that specific pour represents.
That 14-gram figure is a US-specific choice, not a universal one, and it's worth knowing before comparing drink-counting advice across borders. The United Kingdom's unit is defined as 8 grams, or 10 mL, of pure alcohol — meaningfully smaller than the US standard, so a British guideline of, say, 14 units a week describes noticeably less total alcohol than 14 US standard drinks would. Other countries set their own figures again. None of these rulers is more correct than another; mixing them up understates or overstates real intake depending on which direction you convert.
This instrument answers one narrow question: how many standard drinks is this specific beverage, given its volume and strength. It doesn't address how many drinks a day or a week fall within commonly cited moderation guidelines — that's a separate question with its own separate numbers, worth checking directly against a health authority's guidance rather than assumed from this result.
- Enter Volume in millilitres — check the can, bottle, or glass for a printed size, or measure a poured drink.
- Enter ABV, the alcohol-by-volume percentage printed on nearly every label.
- Read Standard drinks; 1.0 means the pour contains 14 grams of pure alcohol.
- Repeat for each different beverage — a mixed round of beer, wine, and spirits rarely adds up to a round number of drinks.
Worked example — three drinks, roughly one standard each
A 355 mL (12 oz) can of beer at 5% ABV: 355 × 0.05 = 17.75 grams' worth of alcohol by volume share, × 0.789 density gives 14.005 grams, divided by 14 comes out to 1.0003 standard drinks — essentially exactly the reference point the whole system is calibrated around.
A 148 mL (5 oz) glass of wine at 12% ABV: 148 × 0.12 = 17.76, × 0.789 = 14.013 grams, divided by 14 gives 1.0009 — a different volume and a different strength landing on almost identical total alcohol.
A 44 mL (1.5 oz) shot of 80-proof spirits, 40% ABV: 44 × 0.40 = 17.6, × 0.789 = 13.886 grams, divided by 14 gives 0.9919 — just under one, confirming the familiar 'one shot equals one drink' shorthand.
Questions
Why does the formula multiply by 0.789?
That's ethanol's actual density in grams per millilitre — a physical constant, not something a government agency chose. ABV tells you what fraction of a drink's volume is alcohol, but a fraction of a volume isn't a weight; multiplying by density converts that share into an actual gram figure, which is what the 14-gram standard-drink definition is measured in.
Why is a US standard drink defined as 14 grams specifically?
It's a round, practical figure chosen to roughly match typical US serving sizes — a 12 oz beer, a 5 oz glass of wine, and a 1.5 oz shot of 80-proof spirits all land close to 14 grams of alcohol, as the worked examples above show. It's a policy definition, not a measurement derived from physiology, and the NIAAA states it plainly as the reference point US drinking guidance is built on.
Is a UK unit the same as a US standard drink?
No, and this trips people up constantly. A UK unit is 8 grams, or 10 mL, of pure alcohol, while a US standard drink is 14 grams — nearly twice as much. A UK guideline expressed in units describes meaningfully less total alcohol than the same count expressed as US standard drinks, so treating '1 unit' and '1 standard drink' as interchangeable will understate US consumption or overstate UK consumption.
Does this tell me how many drinks a day is safe?
No — that's a separate question this calculator doesn't answer. It only converts a specific volume and strength into a count of standard drinks. Recommended daily and weekly limits are published separately by health authorities and vary by source; once you know how many standard drinks a beverage represents, you can weigh that figure against whichever guideline you're following.
Why do a beer, a glass of wine, and a shot all count as roughly one drink despite being such different sizes?
Because serving sizes were chosen specifically to equalize alcohol content, not liquid volume. A beer is poured larger because it's weaker; a shot is poured small because it's strong. The worked examples show all three landing within a few percent of 14 grams of alcohol despite ranging from 44 mL to 355 mL — the sizes were picked around the alcohol content, not the other way around.
Does drink strength vary enough in practice to matter?
Yes, considerably. Craft beers commonly run 6-9% ABV rather than the 5% used in the example, and pour sizes at bars or at home often run larger than the reference amounts — a generously poured glass of wine can easily hold 1.5 to 2 standard drinks rather than 1.0. Entering the actual volume and ABV from a label, rather than assuming a 'typical' drink, gives a meaningfully more accurate count.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.