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Blood Alcohol Content Calculator

Q: Why does the estimate differ from a breathalyser?

Widmark uses population-average water fractions and elimination rates and ignores absorption time and food. Individual results commonly deviate by ±0.02 % BAC or more, which is why it must not be used for driving decisions.

Q: How do I convert between %, g/L and mg/100 mL?

0.05 % = 0.5 g/L (0.5 ‰) = 50 mg/100 mL. Multiply percent by 10 for g/L and by 1000 for mg/100 mL.

Q: Which grams-per-drink value should I use?

14 g for a US standard drink (12 oz 5 % beer, 5 oz 12 % wine, 1.5 oz 40 % spirits), 10 g for Australian, WHO and most European standard drinks, 8 g for a UK unit. Use standard-drinks to convert a specific beverage.

Estimates blood alcohol concentration from body weight, sex, number of standard drinks and hours since drinking started with the Widmark formula (Widmark 1932), reported in %, g/L and mg/100 mL together with the peak value and the hours until the estimate returns to zero.

When to use

You need a rough BAC estimate for a known number of drinks and elapsed time, the peak BAC before elimination, or how long until the estimate reaches zero.

Do not use when: You need to decide whether someone can drive or is legally intoxicated: the result is informational only, not medical or legal advice, and real BAC varies widely between people; to count drinks or grams of alcohol first, use standard-drinks.

Formula

peak_bac_percent = standard_drinks × grams_alcohol_per_drink / (r × weight_kg × 1000) × 100 with r = 0.68 (male) or 0.55 (female); bac_percent = max(0, peak_bac_percent − elimination_rate_percent_per_hour × hours_since_first_drink); bac_g_per_l = bac_percent × 10; bac_mg_per_100ml = bac_percent × 1000; hours_to_zero = bac_percent / elimination_rate_percent_per_hour

Widmark's formula assumes instantaneous absorption and constant (zero-order) elimination; food, drinking pace, body composition, medication and genetics change real BAC by ±0.02 % or more. Informational only, not medical or legal advice; never use it to decide whether to drive.

Inputs

Parameter	Type	Unit	Required	Description
`weight_kg`	number	kg	yes	Body weight in kilograms. Range: ≥ 20, ≤ 400
`sex`	enum: male \| female		yes	Selects the Widmark distribution ratio r: 0.68 for men, 0.55 for women (average body-water fractions).
`standard_drinks`	number		yes	Number of standard drinks consumed since the first drink. Range: ≥ 0, ≤ 60
`grams_alcohol_per_drink`	number	g	default 14	Grams of pure ethanol in one standard drink: 14 g in the US (NIAAA), 10 g in Australia and much of Europe, 8 g for a UK unit. Range: ≥ 1, ≤ 50
`hours_since_first_drink`	number	h	yes	Time elapsed since drinking started. Range: ≥ 0, ≤ 48
`elimination_rate_percent_per_hour`	number	%/h	default 0.015	BAC eliminated per hour (Widmark β). Typical adults eliminate 0.010–0.020 %/h; 0.015 is the usual average. Range: ≥ 0.008, ≤ 0.03

Outputs

Output	Type	Unit	Description
`bac_percent`	number	%	Blood alcohol concentration now, in grams per 100 mL of blood (% w/v), floored at 0.
`bac_g_per_l`	number	g/L	Same value in grams per litre (‰), the unit used in most of continental Europe.
`bac_mg_per_100ml`	number	mg/100 mL	Same value in milligrams per 100 mL, the unit used in the UK and Ireland.
`peak_bac_percent`	number	%	Widmark BAC if all alcohol were absorbed at once, before any elimination.
`hours_to_zero`	number	h	Time from now until the estimate reaches 0: bac_percent / elimination rate.
`interpretation`	string		Typical-effects band for the estimated BAC (< 0.02, 0.02–0.05, 0.05–0.08, ≥ 0.08 %); legal driving limits vary by country.
`warning`	string		Fixed safety warning returned with every result.

Example

80 kg male, 4 US drinks, 2 h: {"weight_kg":80,"sex":"male","standard_drinks":4,"grams_alcohol_per_drink":14,"hours_since_first_drink":2} → {"peak_bac_percent":0.103,"bac_percent":0.073,"bac_g_per_l":0.729,"bac_mg_per_100ml":72.9,"hours_to_zero":4.86}

60 kg female, 2 Australian drinks (10 g), 1 h: {"weight_kg":60,"sex":"female","standard_drinks":2,"grams_alcohol_per_drink":10,"hours_since_first_drink":1} → {"peak_bac_percent":0.061,"bac_percent":0.046,"hours_to_zero":3.04}

GET https://tttkmbb.com/api/v1/calculate/bac-widmark?weight_kg=80&sex=male&standard_drinks=4&grams_alcohol_per_drink=14&hours_since_first_drink=2

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/bac-widmark (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/bac-widmark · Markdown: https://tttkmbb.com/health/bac-widmark.md · JSON definition: https://tttkmbb.com/health/bac-widmark.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="bac-widmark"
OpenAPI operationId: estimate_blood_alcohol_widmark
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Blood alcohol content (Widmark formula) (reference)
NIAAA – What Is a Standard Drink? (government)

FAQ

Why does the estimate differ from a breathalyser?

Widmark uses population-average water fractions and elimination rates and ignores absorption time and food. Individual results commonly deviate by ±0.02 % BAC or more, which is why it must not be used for driving decisions.

How do I convert between %, g/L and mg/100 mL?

0.05 % = 0.5 g/L (0.5 ‰) = 50 mg/100 mL. Multiply percent by 10 for g/L and by 1000 for mg/100 mL.

Which grams-per-drink value should I use?

14 g for a US standard drink (12 oz 5 % beer, 5 oz 12 % wine, 1.5 oz 40 % spirits), 10 g for Australian, WHO and most European standard drinks, 8 g for a UK unit. Use standard-drinks to convert a specific beverage.

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Standard Drinks Calculator — Convert a beverage's volume and ABV into standard drinks and grams of alcohol to enter here.