BAC calculator
A 70 kg man who had 2 standard drinks is estimated at 0.0288 % blood alcohol after 2 hours under the Widmark model. See how weight, drinks and time interact: an educational estimate, never permission to drive. Free in your browser.
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- Your data stays on your device
Run — free
Worked example
- Sex
- Male
- Weight (kg)
- 70
- Number of drinks
- 2
- Hours
- 2
- Alcohol (g)
- 28
- Widmark factor
- 0.68
- Blood alcohol concentration (%)
- 0.0288
Estimates blood alcohol content (BAC) as a percentage with the Widmark equation from sex, body weight, number of standard drinks and hours elapsed. Educational estimate only, not legal or medical advice.
How the Widmark estimate is built
The calculation follows three plain steps. First, your drinks become grams of pure alcohol: each standard drink counts as 14 grams unless you set another size, so 2 drinks are 28 grams. Second, those grams are divided by your body weight multiplied by a distribution ratio, 0.68 for men and 0.55 for women. Third, the body is assumed to clear 0.015 percentage points for every hour since the first drink. For a 70 kg man after 2 drinks and 2 hours, that gives 0.0288 %. Once enough time has passed, the figure stops at zero instead of turning negative.
What the number cannot tell you
Widmark describes an average person, not you. It ignores food, differences in drink strength, medication, tolerance and individual metabolism, so a real blood alcohol level can be far from the estimate. A common mistake is treating every glass as one standard drink, which is why the size of a standard drink can be changed to match another definition. Another is counting the hours from the last drink instead of the first. Use the result to learn how weight, drinks and time interact. It is not legal or medical advice and it never tells you that you can drive.
Determinism, limits and a safety caveat
Crucially, Widmark is a population average: it ignores food, drink strength variation, medication, tolerance, sex-specific body composition beyond the r constant, and individual metabolism, so real BAC varies widely. Treat the number as an educational estimate only. It is not legal or medical advice and does not authorize driving.
What you can do with it
Educational BAC illustration
Show how drink count, body weight and elapsed time interact under the Widmark model, purely for learning.
FAQ
What is a standard drink here?
14 grams of alcohol by default (one US standard drink).
Can I change the elimination rate?
Yes. The beta field defaults to 0.015 BAC percent per hour and can be adjusted; the result is clamped at zero.
Is this legal or medical advice?
No. It is an educational estimate only, not legal or medical advice, and does not authorize driving. Real BAC varies with many factors Widmark ignores.
Why so many decimals?
BAC values are small, so decimals defaults to 4 (range 0 to 4).
Why does the calculator ask for sex?
Because the Widmark model uses a different distribution ratio for each: 0.68 for men and 0.55 for women. With the same weight and the same drinks, the lower ratio gives a higher estimated blood alcohol content.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/dev/bac-calculator \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"sex":"male","weight_kg":70,"drinks":2,"hours":2}'const res = await fetch("https://api.kit.forhosting.com/dev/bac-calculator", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"sex": "male",
"weight_kg": 70,
"drinks": 2,
"hours": 2
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/dev/bac-calculator",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"sex": "male",
"weight_kg": 70,
"drinks": 2,
"hours": 2
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/dev/bac-calculator", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"sex":"male","weight_kg":70,"drinks":2,"hours":2}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"sex":"male","weight_kg":70,"drinks":2,"hours":2}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/dev/bac-calculator", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"sex": "male",
"weight_kg": 70,
"drinks": 2,
"hours": 2
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "dev.bac_calculator",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
The Widmark equation it evaluates
This calculator applies the classic Widmark model. The result is clamped at zero, because once enough time passes the estimate cannot go negative. It first converts drinks to grams of alcohol as alcohol_g = drinks * grams_per_drink, where grams_per_drink defaults to 14 (one US standard drink). It then computes raw BAC as alcohol_g / (r * weight_kg * 10) minus beta * hours, where r is the Widmark distribution ratio, 0.68 for male and 0.55 for female, and beta is the metabolic elimination rate, defaulting to 0.015 per hour. The worked example, a 70 kg male with 2 drinks after 2 hours, gives alcohol_g 28 and bac_percent 0.0288. The response echoes sex, weight_kg, drinks, hours, alcohol_g and r, and tags method "widmark" so a stored result is self-describing.
Fields, ranges and tunable coefficients
Four fields are required: sex (enum "male" or "female", which selects r), weight_kg (a positive number up to 300), drinks (a non-negative number, decimals allowed for a half drink), and hours (a non-negative number of hours since the first drink). Three optional fields tune the physiology: grams_per_drink (default 14, positive) so you can model a different standard-drink definition; beta (default 0.015, non-negative) to adjust the elimination rate; and decimals, an integer from 0 to 4 that defaults to 4, since BAC figures are small and need precision. Every input is checked for type, finiteness and range before arithmetic, and violations return a typed invalid_input error naming the field, for example a weight above 300 kg or a negative drink count.
Determinism, limits and a safety caveat
A non-finite intermediate value is rejected as invalid_input rather than propagated.
Integration use cases
Tune the standard-drink definition
Override grams_per_drink to reflect a UK unit or a strong cocktail instead of the 14 g US default, and see the effect.
Back a research or app calculation
Call the API with the same JSON contract to compute estimates in bulk, keeping the deterministic Widmark math server-side.
Technical questions
Which formula does it use?
The Widmark equation: alcohol_g / (r * weight_kg * 10) minus beta * hours, with r 0.68 for male and 0.55 for female.