Free BAC Calculator (+ Guide)

What is a BAC calculator?

A BAC calculator estimates how much alcohol is in your blood, in grams per deciliter, from what you drank, your body and the time that's passed.

This one uses Widmark's formula, the one forensic scientists use, and shows the result as a curve over the hours.

How it works

The calculator works it out on your device, and your numbers stay there.

What counts as a drink

NIAAA defines a US standard drink as about 14 g, or 0.6 fl oz, of pure alcohol.

• 12 fl oz of regular beer at 5%
• 5 fl oz of wine at 12%
• 1.5 fl oz of distilled spirits at 40%

A 12 oz craft beer at 10% is 2 standard drinks, so add strong drinks under "Your own" with their real size and percentage.

The formula

Widmark's formula spreads the alcohol through a share of your body weight, then takes off a steady amount each hour.

• The peak: grams of alcohol ÷ (your Widmark factor × your weight in grams) × 100 = g/dL
• The factor: about 0.68 for men and 0.55 for women, the averages researchers use, since women have less body water for their weight
• The burn-off: the calculator uses 0.015 g/dL an hour; Jones and Andersson measured an average of 0.0133 in 108 men, and many studies use 0.016

Their study also found that a single BAC estimates the alcohol someone drank only to within about 20%, so treat every number here as a rough guide.

The legal limits

NHTSA says crash risk rises sharply at a BAC of .08 g/dL, and it's illegal to drive at .08 or higher in all 50 states, DC and Puerto Rico, except Utah, where the limit is .05.

The limit isn't a safe line. In 2024, 2,028 people were killed in alcohol-related crashes where a driver had a BAC of .01 to .07, and 11,904 died in crashes with a BAC of .08 or higher.

Why the estimate can be wrong

Widmark's model assumes the alcohol is absorbed at once, and NIAAA warns that BAC can keep rising after someone stops drinking, as alcohol in the stomach and intestine keeps entering the blood.

• Food slows absorption and changes the curve
• Your own body water and burn-off differ from the averages
• Drinks poured at home are often bigger than a standard drink

So the curve is the shape to expect, not your number.

A worked example

Here's the example the calculator loads: a man of 80 kg, three beers and one glass of wine, two hours after his first drink.

That's 4 standard drinks, or 56 g of alcohol. 56 ÷ (0.68 × 80,000) × 100 = 0.103 g/dL at the peak.

Two hours at 0.015 an hour takes off 0.030, so he's at about 0.073 now, and it takes about 4.9 more hours to reach zero.

Mistakes to avoid

These are the slips that come up most.

• Using a BAC estimate to decide whether to drive
• Counting a large glass of wine or a strong beer as one drink
• Forgetting that BAC can still be rising for a while after the last drink

Who is the BAC calculator for?

It's for anyone who wants to understand how drinks add up and how long alcohol takes to leave the body, and these tools help around it.

• Water intake calculator – for a day's water
• Sleep calculator – for when to go to bed
• Calorie calculator – for a day's calories

If you've been drinking, get a ride, and let the curve be the reason you're glad you did.