BAC Calculator: How It Works
This estimator applies the Widmark formula to give an approximate blood alcohol concentration. It is provided for education only. It cannot tell you whether you are safe or legal to drive, and it should never be used to make that decision — the honest answer to "can I drive?" after drinking is to not drive.
Read this first
The Widmark formula has an error range of roughly ±25% even under ideal conditions. Individual variation in metabolism, stomach contents, medication, hydration, fatigue and drink strength all shift the result. A calculator cannot measure your blood, and impairment begins well below any legal threshold. If you have been drinking, arrange a taxi, a rideshare, public transport, or a sober driver. Nothing on this page is a basis for deciding otherwise.
The formula
BAC % = [A ÷ (W × r)] × 100 − (β × t)
- A — grams of pure alcohol consumed.
- W — body weight in grams.
- r — the distribution ratio: approximately 0.68 for men and 0.55 for women, reflecting differences in body water content.
- β — elimination rate, roughly 0.015% per hour, varying between about 0.010 and 0.020.
- t — hours since drinking began.
A standard drink
Definitions differ by country, which is a frequent source of error.
| Country | Grams of alcohol per standard drink |
|---|---|
| United States | 14 g |
| United Kingdom (1 unit) | 8 g |
| Australia | 10 g |
| India | 10 g |
To compute grams directly: volume (ml) × ABV% × 0.789 ÷ 100. A 330 ml beer at 5% contains 13 g. A 175 ml glass of 13% wine contains 18 g — more than a US standard drink, and more than most people assume.
What the levels mean
| BAC | Typical effects |
|---|---|
| 0.02% | Mild relaxation; measurable decline in visual tracking and divided attention |
| 0.05% | Reduced coordination, slower responses, impaired judgement of risk |
| 0.08% | Poor muscle control, impaired reasoning, substantially reduced concentration |
| 0.15% | Major impairment of balance and control; vomiting common |
| 0.30%+ | Risk of loss of consciousness, respiratory depression, and death |
Driving impairment is measurable from around 0.02%, well below every legal limit. Legal limits vary widely — commonly 0.08% in the US, 0.05% across much of Europe and Australia, 0.03% in India, and zero for novice and professional drivers in many jurisdictions.
Only time reduces BAC
The liver clears alcohol at a near-constant rate of roughly one standard drink per hour, and nothing changes it. Coffee, cold showers, food eaten afterwards, exercise, energy drinks and sleep do not accelerate elimination. Coffee is the most dangerous of these because it increases alertness without reducing impairment, producing someone who feels capable and is not.
Food eaten before drinking does matter — it slows absorption and lowers the peak — but it does not reduce the total alcohol or speed its removal.
Why one number cannot cover people
Body composition, sex, age, liver function, genetics, medication, illness and tolerance all affect both BAC and impairment. Tolerance is particularly deceptive: a regular drinker may feel less affected at a given BAC while their reaction times are impaired just as much. Feeling fine is not evidence of being unimpaired.
This page is general information for educational purposes only. It is not medical or legal advice and must not be used to decide whether to drive or operate machinery. If you are concerned about your own or someone else's drinking, a doctor or a local alcohol support service can help.