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BAC Decline Over Time Calculator

Enter your current BAC and the hours that have passed.

How it is calculated

The tool assumes alcohol leaves the blood at a constant 0.015 percentage points per hour. Future BAC is max(0, current − 0.015 × hours), the amount eliminated is current − future, and the time to reach zero is current / 0.015 hours. Enter the current BAC as a percent (0.08 for 0.08%) and the hours from now as zero or more; negative values return nothing, so the tool only looks forward. BAC values are shown with three decimals, and time with one. Real rates vary between people and with food, health and other factors, and the tool does not model any further absorption if you are still drinking.

Worked example

Your BAC reading is 0.08% and you want to know the value 3 hours later. Eliminated: 0.015 × 3 = 0.045%. Future BAC: 0.08 − 0.045 = 0.035%. Time to reach zero: 0.08 / 0.015 = 5.33, shown as 5.3 hr. This is a rough estimate; do not use it to decide whether you can drive.

Try this example →

Common mistakes

  • Entering 8 or 80 for a BAC of 0.08%; the field expects the value as a percent, 0.08.
  • Mixing up g/dL, permille (‰) and percent: a reading of 0.8 ‰ equals 0.08%.
  • Starting the clock from the last drink rather than from the time of the BAC reading, when you still were absorbing alcohol.
  • Expecting to enter negative hours to look at the past; the tool only projects forward.
  • Treating a result of 0.000% as proof it is safe to drive, when the true elimination rate can be slower than 0.015 per hour.

Sources

About this calculator

Blood alcohol concentration doesn't stay put after drinking stops — the liver metabolizes alcohol at a roughly steady rate, commonly approximated as 0.015% BAC per hour (the Widmark elimination rate), though the true rate varies with body composition, liver function, food intake and other factors. This calculator projects your BAC forward in time from a known reading, for a number of hours you enter, using that constant elimination rate, and estimates how many hours remain until you reach 0.00%. It does not work backward to estimate an earlier reading.

The Widmark model is the same underlying approach used informally in alcohol education, driving-safety awareness and health self-monitoring to illustrate how slowly the body actually clears alcohol compared to how quickly it's consumed. It's an estimate, not a measurement — actual elimination rates vary by roughly ±30% between individuals, so this tool is for general awareness only and should never be used to decide whether it's safe to drive or how to interpret a legal limit.

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Comments (1)

  • Editorial team

    Both the hours field and the time-to-zero line count from the moment of your reading, not from now. If some time has already passed since the reading, subtract it before interpreting the answer. The projection also assumes a fixed 0.015 per hour, so it is an illustration of how slowly alcohol clears and never a basis for deciding whether to drive. Which units do your BAC readings come in?

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