BAC Calculator

Estimate your blood alcohol concentration with the Widmark formula — from your drinks, weight, and time — plus how long until you're back to zero.

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A US standard drink is 14 g of alcohol — a 355 mL (12 oz) 5% beer, 150 mL (5 oz) of 12% wine, or 44 mL (1.5 oz) of 40% spirits. This Widmark estimate is educational only: real BAC varies with food, medication, and metabolism. Never drive after drinking — no calculator can tell you you're safe to drive.

Blood alcohol concentration (BAC) is the percentage of alcohol in your blood — 0.08% means 0.08 grams of alcohol per 100 mL. This calculator estimates your current BAC from what you’ve had, your body weight and sex, and how long you’ve been drinking, then shows roughly how long until you’re back under common legal thresholds and at zero.

The Widmark formula

Nearly every BAC estimate, including police and forensic approximations, starts from the equation Swedish chemist Erik Widmark published in the 1930s:

BAC% = (alcohol in grams ÷ (body weight in grams × r)) × 100 − (0.015 × hours)

The constant r is the Widmark factor — the share of your body that alcohol distributes into — averaging 0.68 for males and 0.55 for females. The subtraction is elimination: the liver clears about 0.015% BAC per hour, roughly one standard drink per hour, at a fixed rate you cannot speed up.

What the thresholds mean

  • 0.08% — the per-se legal driving limit in most US states (0.05% in Utah, and lower limits in much of Europe).
  • 0.05% — noticeable impairment of judgment and coordination; a criminal offense in many countries.
  • 0.02–0.04% — the range where measurable impairment already begins, and the limit for US commercial drivers (0.04%).

Impairment starts below every legal limit. Reaction time, tracking, and judgment degrade from the first drink — which is why the only safe amount for driving is zero.

Why this is only an estimate

The Widmark model assumes alcohol is fully absorbed and averages out the things that make individuals different: food in the stomach slows absorption; medications, liver conditions, and fatigue change the response; elimination rates genuinely vary from about 0.010 to 0.025% per hour. Breath or blood testing is the only way to measure actual BAC — treat this tool as a way to understand the math, never as clearance to drive.

Frequently asked questions

How is BAC calculated?

The calculator uses the Widmark formula: alcohol consumed in grams, divided by your body weight times a body-water constant (0.68 for males, 0.55 for females), minus what your body has already eliminated — about 0.015% per hour since drinking started.

What is one standard drink?

In the US, one standard drink contains 14 grams of pure alcohol: a 12 oz (355 mL) beer at 5%, a 5 oz (150 mL) glass of wine at 12%, or a 1.5 oz (44 mL) shot of 40% spirits. Stronger or larger servings count as more than one drink.

How long until my BAC is zero?

The body eliminates roughly 0.015% BAC per hour — about one standard drink per hour — and nothing speeds that up. Coffee, cold showers, or food after drinking don't lower BAC; only time does. The calculator shows the estimated hours until 0.08%, 0.05%, and zero.

Can I use this to decide whether to drive?

No. This is an educational estimate — real BAC varies with food, medications, health conditions, and individual metabolism, and impairment starts well below legal limits. If you've been drinking, don't drive. No calculator can tell you that you're safe.

References