BAC Calculator
Educational blood alcohol estimate; never use to decide whether to drive.
About the BAC Calculator
Blood alcohol content depends on your weight, your sex, how much you drank and over how long. This gives a rough estimate and nothing more.
How to use it
- Enter your weight and select sex.
- Enter the number of standard drinks.
- Enter the hours since your first drink.
The formula
This is an estimate for education only. Never use it to decide whether to drive. The Widmark equation is the standard rough model:
BAC = (A × 5.14 ÷ (W × r)) − (0.015 × H)
Where A is fluid ounces of pure alcohol consumed, W is body weight in pounds, r is the distribution ratio — about 0.73 for men and 0.66 for women, reflecting differences in body water — and H is hours since drinking began.
A US standard drink contains about 0.6 fluid ounces of pure alcohol: a 12 oz beer at 5%, a 5 oz glass of wine at 12%, or 1.5 oz of spirits at 40%. The 0.015 per hour is the average elimination rate, and it is roughly constant — which is why nothing but time lowers it.
Worked examples
| Drinks | Weight | Ratio | Hours | Estimated BAC |
|---|---|---|---|---|
| 2 | 180 lb | 0.73 (male) | 1 | 0.032 |
| 3 | 180 lb | 0.73 (male) | 2 | 0.040 |
| 3 | 140 lb | 0.66 (female) | 2 | 0.070 |
| 5 | 180 lb | 0.73 (male) | 3 | 0.072 |
Rows two and three are the same drinks over the same time producing nearly double the concentration, because weight and body composition both changed. That is the whole reason a shared rule of thumb like 'one drink an hour' fails: the same quantity of alcohol does very different things to different people. And because elimination runs at a fixed rate, someone at 0.072 needs roughly five hours to approach zero — coffee, food and a cold shower do nothing at all.
Common mistakes
- Using an estimate to decide whether to drive. This model ignores food, medication, illness, fatigue, drinking pace and individual metabolism, and real measurements vary widely from it. Impairment also begins well below any legal limit. If you have been drinking, do not drive.
- Miscounting standard drinks. A pint of strong craft beer can be two standard drinks, and a generous home pour of spirits is often two. Counting glasses rather than alcohol content understates intake badly.
- Believing anything speeds up sobering. Elimination runs at roughly 0.015 per hour regardless. Coffee, cold air, exercise and food after the fact change how awake you feel and not how much alcohol is in your blood.
- Assuming BAC peaks immediately. It typically peaks 30 to 90 minutes after the last drink. Someone who feels fine on leaving can be measurably more impaired shortly afterwards.
Terms explained
- BAC
- Blood alcohol concentration, expressed as a percentage by volume. 0.08 means 0.08 grams of alcohol per 100 millilitres of blood.
- Standard drink
- In the US, about 0.6 fluid ounces of pure alcohol — a 12 oz beer at 5%, 5 oz of wine at 12%, or 1.5 oz of spirits at 40%.
- Widmark factor
- The r in the equation, approximating the fraction of body mass that is water. Roughly 0.73 for men and 0.66 for women, with wide individual variation.
- Elimination rate
- How fast the body clears alcohol, averaging about 0.015 BAC per hour. Largely fixed and not accelerable.
- Absorption
- The passage of alcohol into the bloodstream. Food in the stomach slows it, which lowers the peak without changing the total.
- Legal limit
- 0.08% for most drivers in the United States, lower for commercial and young drivers, and lower still in many other countries. Impairment starts below every one of these.
Common questions
- Can I use this to decide if I am safe to drive?
- No. It is an educational estimate that omits food, medication, fatigue, tolerance and individual metabolism, and impairment begins well below any legal threshold. If you have been drinking, arrange another way home.
- What counts as one standard drink?
- About 0.6 fluid ounces of pure alcohol: a 12 oz beer at 5%, a 5 oz glass of wine at 12%, or 1.5 oz of spirits at 40%. Many real servings are larger than one.
- How long does alcohol take to leave my system?
- Roughly 0.015 BAC per hour. From 0.06 that is about four hours to approach zero, and nothing shortens it.
- Does coffee or a cold shower help?
- No. They may make someone feel more alert while leaving blood alcohol unchanged, which is arguably more dangerous than feeling tired.
- Why do men and women get different results?
- The equation uses a different distribution ratio, reflecting average differences in body water. The same drinks in the same time typically produce a higher concentration in a smaller person with less body water.
- Does eating first make a difference?
- Food slows absorption, so the peak is lower and later. It does not reduce the total alcohol consumed or speed up elimination.
- Why might a breathalyser disagree with this?
- Because this is a population average and you are not the average. Drinking pace, stomach contents, medication, illness and genetics all shift the real figure, sometimes substantially.
- Is there a safe BAC for driving?
- Measurable impairment of reaction time and judgement appears well below 0.08, and some countries set limits at 0.05 or 0.02. The only concentration with no added risk is zero.