Blood Alcohol Calculator (BAC)

Estimate blood alcohol concentration from drinks, body weight and time, with the Widmark formula.

Widmark formula: BAC (%) = alcohol in grams ÷ (body weight in grams × r) × 100 − 0.015 × hours, where r is about 0.68 for men and 0.55 for women. Four standard drinks (56 g alcohol) for an 80 kg man after 2 hours gives about 0.073%.

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How to use it

1. Choose sex and enter body weight.
2. Add each drink type and how many you had.
3. Enter hours since your first drink to see estimated BAC now.

What you enter

• Sex
• Body weight
• Drinks
• Hours since first drink

What you get

• Estimated BAC
• Hours until BAC reaches zero

Formula

BAC = [A ÷ (W × r)] × 100 − 0.015 × t

A is grams of alcohol, W is body weight in grams, r is the Widmark factor (0.68 men, 0.55 women) and t is hours since drinking started.

Worked example

Input: Man, 80 kg, four 12 oz beers (56 g alcohol), 2 hours

Result: BAC ≈ 0.073%

Below 0.08% but clearly impaired; about 5 more hours to reach zero.

Questions

How accurate is a BAC calculator?

It is a rough estimate. Food, drinking speed, medications, liver health and body composition all change real BAC. Never use it to decide whether you can drive.

How fast does BAC go down?

On average by about 0.015% per hour, roughly one standard drink an hour. Coffee, water, cold showers and sleep do not speed it up.

What is a standard drink?

In the US, 14 g of pure alcohol: 12 oz of 5% beer, 5 oz of 12% wine or 1.5 oz of 40% spirits.

What is the legal driving limit?

0.08% in most US states, 0.05% in Utah and much of Europe, and lower or zero for new and commercial drivers. Impairment starts well below these limits.

Limitations

• Estimates only; not a breathalyser or legal evidence.
• Does not model food in the stomach or individual metabolism.

References

• National Institute on Alcohol Abuse and Alcoholism (NIAAA).
• Posey D, Mozayani A (2007) Forensic Science, Medicine and Pathology 3:33-39.

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