BAC Calculator

What is the BAC Calculator?

BAC stands for Blood Alcohol Concentration — the percentage of alcohol in your bloodstream at any given time. This calculator estimates your BAC based on how much you've drunk, your body weight, your sex, and how long you've been drinking. It's used by people who want to make responsible decisions before driving, understand how alcohol affects them personally, or simply satisfy their curiosity about the math. The legal limit for driving in the US is 0.08%. CalciHub's version gives you an estimate, not a breathalyzer reading — but the formula is medically established.

How Does It Work?

The Widmark Formula is the standard method used in forensic and medical contexts to estimate BAC. It accounts for your weight, body composition (which differs by sex), and how much alcohol has already metabolized out of your system since you started drinking.

BAC = (Alcohol Consumed in grams / (Weight in grams x r)) x 100 - (0.015 x Hours)

Alcohol in grams = Number of drinks x 14 (one standard US drink = 14g pure alcohol)

r = body water constant: 0.55 for women, 0.68 for men

0.015 = average BAC metabolized per hour

Hours = time elapsed since you started drinking

How to Use CalciHub's BAC Calculator

1. Enter your body weight and select the unit (lbs or kg).

2. Select your biological sex (this affects the Widmark constant).

3. Enter the number of standard drinks consumed.

4. Input how many hours have passed since you started drinking.

5. Click Calculate — your estimated BAC appears with a clear indicator of how it compares to the legal driving limit.

Tip: One 'standard drink' is 12oz regular beer, 5oz wine, or 1.5oz spirits. Mixed drinks often contain more than one standard drink — factor that in when counting.

A Quick Example

Mike, 180 lbs, has had 3 beers over 2 hours at a backyard party in Denver.

Inputs: Weight = 180 lbs (81.6 kg), Sex = Male, Drinks = 3 standard drinks, Hours = 2

Alcohol in grams = 3 x 14 = 42g

BAC = (42 / (81,600 x 0.68)) x 100 - (0.015 x 2) = 0.076 - 0.030 = 0.046%

Result: Estimated BAC = 0.046% — below the 0.08% legal limit, but not by a massive margin.

Mike decides to wait another hour before driving, just to be safe. That's exactly the kind of decision this tool is built for.

Frequently Asked Questions