BAC Calculator

BAC calculator at a glance#

A BAC calculator estimates your blood alcohol content (BAC), the percentage of alcohol in your bloodstream, from how much you drank, your body weight, your sex and the time since your first drink. It uses the Widmark formula: BAC = (grams of alcohol / (body weight in grams times r)) times 100, then subtract 0.015 for every hour since drinking started. The factor r is about 0.68 for men and 0.55 for women.

One US standard drink contains about 14 grams of pure alcohol. Each of these counts as one standard drink:

Worked example (approximate): a 180 lb man (about 81.6 kg, roughly 81,600 g) has 3 standard drinks, so about 42 g of alcohol, over 2 hours. The raw figure is 42 / (81,600 times 0.68) times 100, which is about 0.076. Subtract 0.015 times 2 hours, about 0.03, and the estimate is roughly 0.05% BAC. The same drinks for a 140 lb woman (r about 0.55) come out higher, near 0.09%, which shows how much body weight and sex change the result.

In the US, 0.08% is the legal limit for driving in most states, and lower limits apply to commercial and underage drivers. These are estimates only. Food, metabolism, medication, health and your actual drink sizes all shift real BAC, and impairment can start well below any legal limit. Enter your drinks, weight, sex and time in the calculator above for your estimate, and never drink and drive.

How the calculator estimates your BAC#

The calculator above uses the Widmark formula. It converts your drinks into grams of pure alcohol, divides that by your body weight times a distribution factor (about 0.68 for men and 0.55 for women), and then subtracts roughly 0.015 percentage points for every hour since your first drink, since your liver clears alcohol over time. The answer block above works a full example. Inputs that matter most are how much you drank, your weight, your sex and the elapsed time.

What counts as one drink#

A US standard drink is about 14 grams of pure alcohol. That works out to 12 oz of regular beer at 5% ABV, 5 oz of wine at 12% ABV, or 1.5 oz of distilled spirits at 40% ABV. A pint of strong craft beer or a generous home pour can be 1.5 to 2 standard drinks on its own, so count by the alcohol, not by the glass, when you enter your drinks.

How BAC levels affect you#

Effects build as BAC rises, though they vary by person. Around 0.02 to 0.05% most people feel relaxed with mild loss of judgment. By 0.05 to 0.08% reaction time, coordination and steering control are measurably worse. At 0.08% and above, the US legal driving limit in most states, impairment is clear, and higher levels bring loss of balance, vomiting and, past roughly 0.30%, risk of unconsciousness. Impairment can start below any legal limit, so a "legal" reading is not a "safe to drive" reading.

Legal limits#

In the US, 0.08% is the standard limit for drivers 21 and over in nearly all states (Utah is 0.05%). Commercial drivers are limited to 0.04%, and under-21 drivers face zero-tolerance limits, often 0.01 to 0.02%. The only reliable way to stay under a limit is not to drive after drinking.

An estimate, not a breathalyzer#

This tool gives an estimate, not a legal or medical measurement. Food in your stomach slows absorption, while an empty stomach speeds it up. Medication, body composition, hydration, fatigue and how fast you drink all shift your real BAC, and the formula cannot see any of them. Never use the number to decide you are fine to drive. The safe choice after any alcohol is to not drive.

Frequently asked questions#

What is a BAC calculator?#

It estimates your blood alcohol content, the percentage of alcohol in your blood, from your drinks, weight, sex and the time since your first drink. It applies the Widmark formula to return an approximate figure.

How accurate is it?#

It is a rough estimate. Food, metabolism, medication, hydration and genetics all change how your body handles alcohol, none of which the formula captures. Treat the result as a guide, never as proof you are under a limit.

Can I use it to decide if I am safe to drive?#

No. Impairment can occur below the legal limit, and the estimate cannot account for your individual factors. The only safe option after drinking is not to drive.

What is a standard drink?#

About 14 grams of pure alcohol: 12 oz of 5% beer, 5 oz of 12% wine, or 1.5 oz of 40% spirits. Counting in standard drinks is how you enter accurate amounts into the calculator.

What affects my BAC?#

Body weight, sex, how much you drank, how fast you drank it, and the time since your first drink. Food, medication and overall health also change how quickly your body absorbs and clears alcohol.

Do heavy drinkers have lower BAC?#

No. Tolerance can mask the signs of impairment, but it does not lower the alcohol in your blood. Drinking more to feel an effect usually pushes BAC higher, not lower.