Rough estimate only. Never use this to decide whether you are safe to drive. If you have been drinking, do not drive.
About this tool
This blood alcohol content (BAC) calculator estimates your BAC from your weight, sex, the number of standard drinks and the time since you started drinking. It uses the Widmark formula, a long-standing way to estimate BAC. It runs in your browser and nothing is stored. It is an educational estimate, not a measurement.
How to use it
Enter your weight, choose sex, and enter the number of standard drinks and the hours since the first one. The result shows an estimated BAC now, roughly when it would reach 0.08% and 0.00%, and a label for the likely level of impairment.
What counts as a standard drink
In the US a standard drink holds about 14 grams of pure alcohol. That is about 12 oz of regular beer (5%), 5 oz of wine (12%) or 1.5 oz of spirits (40%). Craft beers, large pours and mixed drinks often contain more than one standard drink, so count carefully.
The formula
BAC (%) = alcohol in grams / (body weight in grams x r) x 100, minus about 0.015 per hour. The constant r is about 0.68 for men and 0.55 for women, reflecting differences in body water. The body clears alcohol at roughly 0.015% per hour on average, and nothing speeds that up: coffee, cold showers and food do not.
BAC levels and effects
- 0.02% to 0.05%: relaxed, mild loss of judgment and slightly slower reaction.
- 0.06% to 0.09%: reduced coordination and judgment. The legal driving limit in most US states is 0.08%.
- 0.10% to 0.15%: clear impairment, slurred speech and poor balance.
- 0.20% and above: severe impairment and risk of vomiting and blackout. 0.30% or more can be life threatening.
Legal limits
In most of the US the limit for drivers 21 and older is 0.08%, and Utah uses 0.05%. Canada has a 0.08% criminal limit, and many provinces apply roadside penalties from 0.05%. Drivers under 21 and commercial drivers face lower limits, often 0.00% to 0.04%. You can be charged for impaired driving below the legal limit.
Things to watch out for
Real BAC varies with food, medication, body fat, how fast you drank, hydration and genetics. Two people of the same weight can differ a lot. This estimate can be too low as well as too high, so treat it as a guide and never as permission to drive. If someone shows confusion, vomiting, slow breathing or cannot be woken, call emergency services.
Frequently asked questions
The Widmark formula divides the grams of alcohol by body weight times a body-water constant (about 0.68 for men and 0.55 for women), then subtracts about 0.015% for each hour.
The body clears about 0.015% BAC per hour on average. Coffee, food and cold showers do not speed this up. Only time does.
No. The result is a rough estimate and impairment begins well below the legal limit. If you have been drinking, do not drive.