Blood Alcohol Content (BAC) Calculator — Widmark Formula
This calculator estimates your blood alcohol content (BAC) by entering your drink type, quantity, weight, and time elapsed. It uses the Widmark formula — the same method used by law enforcement and medical professionals worldwide. In the US, the legal driving limit is 0.08% BAC (0.8 g/L) for adults 21+. Never drive after drinking.
Blood alcohol content (BAC) is calculated with the Widmark formula: BAC = grams of alcohol ÷ (weight in kg × r) − (0.15 × hours elapsed), where r = 0.68 for males and 0.55 for females. Your body eliminates alcohol at ~0.15 g/L per hour. One standard beer (330ml, 5% ABV) produces roughly 0.25 g/L BAC in a 70 kg person and takes about 1.7 hours to clear. In the US, the legal driving limit is 0.08% BAC (0.8 g/L).
When to use this calculator
- You're leaving a party and want to estimate if you're safe to drive.
- You want to know how long alcohol stays in your system.
- You're comparing how different drinks affect your BAC.
- You're educating friends or family about responsible drinking.
- You work in hospitality and want to guide customers about drink content.
Example: 80 kg male, 2 pint beers (500ml), 1 hour after drinking
- Alcohol grams: 2 × 20g = 40g of alcohol.
- Initial BAC (Widmark): 40 ÷ (80 × 0.68) = 0.74 g/L (0.074%).
- After 1 hour: 0.74 − 0.15 = 0.59 g/L (0.059%).
- Time to zero: 0.59 ÷ 0.15 = 3.9 hours remaining.
How it works
2 min readAlcohol Grams by Drink Type
| Drink | Volume | ABV | Grams of alcohol |
|---|---|---|---|
| Beer can | 330 ml | 5% | 13 g |
| Beer pint | 500 ml | 5% | 20 g |
| Wine glass | 150 ml | 13% | 15.4 g |
| Spirits shot | 50 ml | 39% | 15.6 g |
| Whisky / Vodka | 45 ml | 40% | 14.2 g |
| Champagne glass | 150 ml | 12% | 14.2 g |
| Gin / Tequila | 45 ml | 40% | 14.2 g |
Widmark Formula
BAC = (grams_alcohol ÷ (weight_kg × r)) − (0.15 × hours)Where r = 0.68 for males and 0.55 for females. The r factor (Widmark's distribution factor) reflects the proportion of body water where alcohol distributes — women have less body water relative to mass, so the same amount of alcohol produces a higher BAC.
Hours Until BAC Reaches Zero (Reference Table)
| Drinks (330ml beer) | Male 70 kg | Female 60 kg |
|---|---|---|
| 1 beer | ~1.7 h | ~2.5 h |
| 2 beers | ~3.4 h | ~5.0 h |
| 1 wine glass | ~2.0 h | ~2.7 h |
| 2 wine glasses | ~4.0 h | ~5.4 h |
| 2 whisky shots | ~3.7 h | ~5.1 h |
| 4 beers | ~6.8 h | ~10.0 h |
Assuming 0 hours elapsed since drinking. Actual elimination rate varies ±20%.
BAC Percentage vs. g/L Conversion
| g/L | % BAC | Legal status (US) |
|---|---|---|
| 0.2 g/L | 0.02% | Under limit (some impairment) |
| 0.4 g/L | 0.04% | Under limit (commercial drivers: over limit) |
| 0.8 g/L | 0.08% | US legal driving limit |
| 1.0 g/L | 0.10% | Illegal in all US states |
| 1.5 g/L | 0.15% | Marked intoxication |
Legal BAC Limits by Country
| Country | Legal limit |
|---|---|
| USA (21+) | 0.08% (0.8 g/L) |
| UK (England/Wales) | 0.08% (0.8 g/L) |
| UK (Scotland) | 0.05% (0.5 g/L) |
| Canada | 0.08% (0.8 g/L) |
| Australia | 0.05% (0.5 g/L) |
| Germany | 0.05% (0.5 g/L) |
| Argentina | 0.00% (zero tolerance since 2023) |
| Brazil | 0.00% (zero tolerance) |
Effects by BAC Level
| BAC (g/L) | % BAC | Typical effects |
|---|---|---|
| 0.0–0.3 | 0.0–0.03% | Mild relaxation, subtle disinhibition |
| 0.3–0.5 | 0.03–0.05% | Impaired judgment and concentration |
| 0.5–0.8 | 0.05–0.08% | Reduced coordination, slower reflexes |
| 0.8–1.5 | 0.08–0.15% | Marked intoxication, poor balance |
| 1.5–3.0 | 0.15–0.30% | Confusion, nausea, possible blackout |
| >3.0 | >0.30% | Risk of coma or death |
Common Myths About Sobering Up
Safe Alternatives to Driving
Frequently asked questions
How long does it take for one beer to leave your system?
A standard 330ml beer (13g alcohol) in a 70 kg person produces ~0.25 g/L BAC. At the elimination rate of 0.15 g/L per hour, your body clears it in roughly 1.7 hours. A 500ml pint takes about 2.7 hours for the same person.
How long does alcohol stay in your bloodstream?
Your liver eliminates alcohol at a constant rate of approximately 0.15 g/L per hour (range: 0.10–0.20 g/L/h). After 4 standard drinks, most people take 6–8 hours to reach zero BAC. Only time works — coffee, water, and exercise do not speed the process.
What is a standard drink in the US?
One US standard drink contains 14 grams of pure alcohol, equivalent to: 355ml beer (5% ABV), 150ml wine (12% ABV), or 45ml distilled spirits (40% ABV). Our calculator uses similar volumes with accurate ABV-based gram calculations.
Can you fail a breathalyzer after just one beer?
Yes, it's possible. One beer can produce a BAC of 0.02–0.04% (0.2–0.4 g/L), which is detectable by a breathalyzer. Though it's below the 0.08% legal limit in the US, you could still be charged with DUI if your driving is impaired.
Why do women have a higher BAC than men from the same amount of alcohol?
Women have a lower Widmark factor (r = 0.55 vs. 0.68 for men), reflecting lower body water content, less of the stomach enzyme alcohol dehydrogenase (ADH), and typically higher body fat percentage. At the same weight and consumption, women's BAC is typically 20–30% higher than men's.
Does eating before drinking affect your BAC?
Yes, food eaten before or during drinking slows alcohol absorption from the stomach into the bloodstream, delaying and lowering the peak BAC. However, it doesn't reduce the total amount of alcohol processed — the final number of hours to reach zero remains similar.
How many drinks can you have and still legally drive in the US?
For a 70 kg male: roughly 2 standard drinks (28g alcohol) produces ~0.59 g/L (0.059%), which is under the 0.08% limit. But impairment starts at very low BAC levels. The safest answer is zero drinks if you plan to drive — individual metabolism varies widely.
How accurate is this BAC calculator?
The Widmark formula has an estimated margin of ±20% because actual BAC depends on metabolism, genetics, food consumed, hydration, medications, and alcohol tolerance. Never use this calculator to decide whether to drive. If you've been drinking, don't drive.
What's the difference between g/L and % BAC?
They are equivalent measurements. 0.8 g/L = 0.08% BAC (the US legal limit). To convert: divide g/L by 10 to get %. The US and UK typically express BAC as a percentage; other countries use g/L or g/dL.
What should I do if I've been drinking and need to get home?
Call Uber, Lyft, or a local taxi. Designate a sober driver before you start drinking. Many areas have drunk-driving prevention hotlines that offer free rides. A rideshare costs far less than a DUI fine, license suspension, or the risk of harming yourself or others.