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Blood Alcohol (BAC) & Drink-Driving Limit Calculator — Sri Lanka

Estimate your blood alcohol concentration from what you drank, your weight and the time elapsed, using the Widmark formula — then see how it stacks up against Sri Lanka's 80 mg/100 ml legal driving limit. Private, in-browser, no signup.

By Induwara AshinsanaUpdated Jul 6, 2026
Estimate your BAC
Widmark formula · SL 80 mg limit

Estimate only — never use it to decide to drive. Real BAC varies with food, metabolism, medication and health. If you have had any alcohol, do not drive.

kg

30–200 kg

h

0–24 h

What you drank

330 ml @ 4.8% ABV · 12.5 g pure alcohol each

Estimated blood alcohol

0mg/100 ml

= 0% BAC

Under the limit by 80 mg

Sri Lanka's legal drink-driving limit is 80 mg of alcohol per 100 ml of blood (Motor Traffic Act). Being under it is not a green light to drive.

Pure alcohol
12.5 g
1.2 standard drinks
Peak BAC
26.3 mg
Before any elimination
Until under 80 mg
0 min
Already under the limit
Until fully sober
0 min
BAC back to ~0

Cross-checked against the imperial Widmark formula (agrees within 0.1%).

Runs entirely in your browser — nothing you enter is uploaded. Sources: Motor Traffic Act (80 mg/100 ml limit), Widmark 1932, WHO standard drink (10 g), ethanol density 0.789 g/ml.

How it works

The calculator uses the Widmark equation — the model forensic toxicologists have relied on since 1932 and the basis for the US NHTSA's BAC tables. It runs in four steps, all of them shown on this page so the maths is auditable rather than a black box.

  1. Grams of pure alcohol per drink. Each drink is volume(ml) × ABV% × 0.789. The 0.789 g/ml is the density of ethanol. A 330 ml Lion Lager at 4.8% is 330 × 0.048 × 0.789 = 12.5 g. The tool sums this across every drink and quantity.
  2. Peak BAC. BAC = (alcohol grams ÷ (r × weight grams)) × 100, giving g/100 ml. The Widmark distribution ratio r is 0.68 for men and 0.55 for women, reflecting average body-water fraction.
  3. Subtract elimination over time. The liver clears alcohol at a near-linear rate, so the tool subtracts β × hours, with β defaulting to 0.015 g/100 ml per hour (adjustable under Advanced). The result is floored at zero.
  4. Convert to statutory units and compare. Multiply g/100 ml by 1000 to get mg/100 ml, the unit written into the Motor Traffic Act, and compare against the 80 mg limit. The gauge also estimates the hours until you fall below 80 mg and until BAC returns to zero.

As an independent check, the same peak BAC is recomputed through the imperial form of the Widmark equation (fluid ounces of ethanol, body weight in pounds, factor 5.14). The two methods agree to within about 0.1%, and the result card flags when that cross-check passes. Standard drinks are reported by dividing total alcohol grams by the WHO standard-drink size of 10 g.

Limits of the model: Widmark assumes a fasted, steady state. Food in the stomach, drinking pace, medication, illness and individual metabolism all move real BAC away from the estimate. This tool never reports a "safe to drive" verdict — it reports a number, and the number is an estimate.

Worked examples

Male · 70 kg · 3 Lion Lagers · 2 hours

Under the limit (48.8 mg)

  1. Per beer: 330 ml × 4.8% × 0.789 = 12.50 g
  2. Total alcohol: 12.50 × 3 = 37.49 g (3.7 standard drinks)
  3. Peak BAC: (37.49 ÷ (0.68 × 70,000)) × 100 = 0.0788 g/100 ml
  4. After 2 h: 0.0788 − (0.015 × 2) = 0.0488 g/100 ml
  5. = 48.8 mg/100 ml → UNDER 80 mg (31 mg to spare)
  6. Time to sober: 0.0488 ÷ 0.015 ≈ 3 h 15 min

Female · 55 kg · 4 arrack tots · 1.5 hours

Over the limit (82.4 mg)

  1. Per tot: 30 ml × 33.5% × 0.789 = 7.93 g
  2. Total alcohol: 7.93 × 4 = 31.72 g (3.2 standard drinks)
  3. Peak BAC: (31.72 ÷ (0.55 × 55,000)) × 100 = 0.1049 g/100 ml
  4. After 1.5 h: 0.1049 − (0.015 × 1.5) = 0.0824 g/100 ml
  5. = 82.4 mg/100 ml → OVER 80 mg (2.4 mg over)
  6. Time under limit: (0.0824 − 0.08) ÷ 0.015 ≈ 10 min; sober ≈ 5 h 30 min

Male · 90 kg · 1 glass of wine · 1 hour

Well under the limit (12.7 mg)

  1. Wine glass: 150 ml × 12% × 0.789 = 14.20 g (1.4 standard drinks)
  2. Peak BAC: (14.20 ÷ (0.68 × 90,000)) × 100 = 0.0232 g/100 ml
  3. After 1 h: 0.0232 − (0.015 × 1) = 0.0082 g/100 ml
  4. = 8.2 mg/100 ml → far UNDER 80 mg
  5. Even so, any alcohol impairs judgement — do not drive.

Frequently asked questions

Sources & references

The 80 mg/100 ml threshold, the Widmark constants (r = 0.68/0.55, β = 0.015 g/100 ml/h), the 10 g standard drink and the 0.789 g/ml ethanol density were last cross-checked against these sources on 2026-07-06.

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