A BAC calculator that shows the estimate and its uncertainty
This BAC calculator estimates your blood alcohol concentration from what you drank, your body weight, your sex and the time since your first drink, using the Widmark formula that forensic toxicologists still start from. It shows the result in the three ways limits are written — percent (g/100 mL), mg per 100 mL and grams per litre (‰) — and draws a curve of how the estimate falls over time.
Unlike a single number, it also shows a range. People clear alcohol at different speeds, so the shaded band covers the elimination rates found in most adults. The real figure for any one person can still fall outside it.
Please read this first: a calculator cannot measure your blood. It is for learning how alcohol builds up and wears off, not for deciding whether you can drive, work or look after someone. Not medical or legal advice.
How to use the BAC calculator
- Choose Male or Female; this sets the Widmark factor r (you can type your own between 0.40 and 0.80).
- Enter your body weight in kg or lb and the hours since your first drink.
- Under Your drinks, pick each drink from the US, UK or India sizes, set how many, and adjust the size or % ABV to match the label. Use Add a drink for more.
- Read the estimate, the range for slow to fast elimination, and the chart; change the hours to see the curve at another time.
BAC Calculator features
Widmark formula, shown in full
Your grams of alcohol, r, weight and elimination rate appear in the working, so you can check every step.
Three units
Percent BAC (US), mg per 100 mL (UK and India) and g/L or ‰ (much of Europe).
Over-time chart
The estimate and its slow–fast band from the first drink until it reaches zero.
Real drink sizes
US 12 fl oz beer and 5 fl oz wine, UK pints, 175 mL glasses and 25 mL singles, Indian 650 mL bottles and 30/60 mL pegs — all editable.
How the Widmark estimate works
First the tool works out the pure alcohol: volume × ABV × 0.789 g/mL. A 12 fl oz (355 mL) beer at 5% holds 14.0 g, which is one US standard drink. The peak estimate is A ÷ (r × body weight × 10) in g/100 mL, where r is the share of body weight alcohol spreads into. The body then removes alcohol at a roughly steady amount per hour, β, so the estimate t hours later is peak − β × t.
Worked example: four 12 fl oz beers (56.0 g) for an 80 kg man (r 0.68) give a peak of 56.0 ÷ 544 = 0.103%. After two hours at 0.015 per hour the estimate is 0.073%, or 73 mg/100 mL. At the slowest common rate, 0.010 per hour, it would take about 10 hours to reach zero.
The ranges come from the forensic toxicology guideline OSAC 2020-S-0003 (Guidelines for Performing Alcohol Calculations in Forensic Toxicology, April 2022): r between 0.40 and 0.80 L/kg, and elimination between 0.010 and 0.025 g/dL per hour, which it says covers most people regardless of age, sex or drinking experience. Widmark’s classic averages of 0.68 for men and 0.55 for women are the defaults. Checked 24 October 2026.
Drink-driving limits (for reference, not a target)
Limits are set in law and change; the figures below were checked on 24 October 2026. Being under a limit does not mean you are safe or unimpaired, and you can be prosecuted for impaired driving below it.
| Where | Blood limit | Source |
|---|---|---|
| England, Wales, Northern Ireland | 80 mg/100 mL (0.08%); breath 35 µg/100 mL | GOV.UK, The drink drive limit |
| Scotland | 50 mg/100 mL (0.05%); breath 22 µg/100 mL | GOV.UK, as above |
| United States (drivers 21+) | 0.08 g/dL in every state except Utah, 0.05 g/dL since 30 Dec 2018; 0.04 for commercial drivers | NTSB testimony, 24 Mar 2026; 49 CFR 382.201 |
| India | 30 mg/100 mL (0.03%) | Motor Vehicles Act, 1988, section 185 (as amended in 2019) |
Why your real BAC can differ
- Absorption: food slows it, and complete absorption can take up to about 2 hours after the last drink; this model assumes instant absorption, so early estimates run high.
- Body composition: r depends on body water, which varies with fat, muscle, age and pregnancy.
- Elimination: liver health, medicines and regular heavy drinking change β. Coffee, cold showers and sleep do not speed it up.
- Drink strength: craft beers, home pours and cocktails are often stronger than the presets — check the label. The alcohol unit calculator counts units and standard drinks.
BAC Calculator FAQ
How accurate is a BAC calculator?
Only roughly. The body-water factor varies between people from about 0.40 to 0.80 and the elimination rate from 0.010 to 0.025 per hour, so two people with the same weight and drinks can differ a lot. That is why this page shows a range and why no calculator should be used to decide whether to drive.
How do I use a BAC calculator by weight?
Enter your weight in kg or lb: the same drinks give a higher estimate for a lighter person because the alcohol is spread through less body water.
How long does it take for BAC to go down?
Most adults remove about 0.010 to 0.025 g/dL per hour, so 0.08% takes roughly 3 to 8 hours to clear. Nothing speeds it up.
What does 0.08 BAC mean in the UK?
0.08% is 80 mg of alcohol per 100 mL of blood — the limit in England, Wales and Northern Ireland. Scotland’s is 50 mg/100 mL.
Why is the estimate higher for women?
On average women have less body water for the same weight, so the Widmark factor is lower (0.55 versus 0.68) and the same alcohol gives a higher concentration.
Can this BAC calculator tell me if I am safe to drive?
No. It is an estimate for information only. Impairment starts at low levels and varies between people; if you will drive, don’t drink.