Free bac calculator
Estimate your blood alcohol content in two seconds. Enter your standard drinks, body weight, sex, and the hours since your first drink, and this BAC calculator returns your estimated blood alcohol concentration using the Widmark formula — with the 0.08% legal limit, an effects-by-level chart, and how long it takes to sober up. An estimate only, never a green light to drive — updated live, as you type.
On this page15 sections
Results are estimates. Consult a professional.
What is blood alcohol content (BAC)?
Blood alcohol content (BAC) — also called blood alcohol concentration — is the percentage of alcohol in your bloodstream. A BAC of 0.08% means that 0.08 grams of pure alcohol are present in every 100 millilitres of blood. It is the single number police, courts, and breathalysers use to judge whether someone is too impaired to drive, and the figure this BAC calculator estimates the moment you enter how much you have had to drink.
BAC rises as you drink because alcohol is absorbed into the blood faster than your body can break it down, and it falls again once you stop, as the liver slowly clears it. The number you see is a snapshot of that balance at one moment in time — how much you have drunk, how big you are, and how long the body has had to work it off.
How the BAC calculator works: the Widmark formula
The calculator uses the Widmark formula, the method Swedish chemist Erik Widmark published in the 1930s and the one still treated as the scientific standard for estimating BAC. It works out how much pure alcohol you have consumed, spreads it through the water in your body, and then subtracts the amount your liver has eliminated since your first drink.
Three inputs drive the result. Standard drinks set the grams of alcohol (14 g each). Body weight and sex set how widely that alcohol is diluted — a heavier, higher-water body reaches a lower BAC from the same drinks. Hours since your first drink set how much the liver has already cleared. Change any field and the estimate updates live.
What is one standard drink?
The calculator counts standard drinks, not glasses. In the US a standard drink is any pour that contains 14 grams (about 0.6 fluid ounces) of pure alcohol. Because stronger drinks are poured smaller, the following each count as exactly one standard drink:
| Drink | Typical serving | Alcohol by volume |
|---|---|---|
| Regular beer | 12 oz (355 ml) | ~5% ABV |
| Malt liquor | 8–9 oz (240–270 ml) | ~7% ABV |
| Table wine | 5 oz (148 ml) | ~12% ABV |
| Distilled spirits (shot) | 1.5 oz (44 ml) | ~40% ABV (80 proof) |
Source: US NIAAA / NHTSA — each row contains roughly 14 g of pure alcohol.
A worked example using the BAC calculator
A man who weighs 80 kg (about 176 lb) has 4 standard drinks over an evening, and it has been 2 hours since his first drink. Here is the arithmetic the calculator runs.
Step 1 — Convert drinks to grams of alcohol
Each standard drink is 14 g, so 4 drinks is 4 × 14 = 56 g of pure alcohol.
Step 2 — Spread the alcohol through the body water
Body weight in grams is 80 × 1,000 = 80,000 g. For a man, r = 0.68. So the peak BAC, before any is eliminated, is (56 ÷ (80,000 × 0.68)) × 100 = 0.103 — already over the limit.
Step 3 — Subtract what the liver has eliminated
The body clears about 0.015 BAC per hour, so over 2 hours it removes 0.015 × 2 = 0.03. That leaves 0.103 − 0.03 = 0.073.
BAC levels and their effects
Impairment climbs steadily with BAC. The table below describes the typical effects at each level — they describe what alcohol does to the body and are independent of the formula. Effects vary between people and with tolerance, but driving impairment begins long before the legal limit.
| BAC | Typical effects |
|---|---|
| 0.02% | Slight relaxation and mood lift. Some loss of judgement; reduced ability to divide attention. |
| 0.05% | Lowered alertness and inhibition, impaired coordination and tracking of moving objects. |
| 0.08% | Legal driving limit in the US. Poor muscle control, slower reaction time, impaired judgement and reasoning. |
| 0.10% | Slurred speech, poor coordination, slowed thinking. Clearly impaired driving. |
| 0.15% | Major loss of balance and muscle control, vomiting likely, substantial driving impairment. |
| 0.20% | Confusion, disorientation, needing help to stand or walk. Risk of blackout. |
| 0.30% | Stupor, little comprehension of surroundings, possible loss of consciousness. |
| 0.40% and above | Coma, suppressed breathing and heart rate — potentially fatal. A medical emergency. |
Source: NHTSA and US health agencies — typical effects by BAC level. Effects vary by individual and tolerance.
The 0.08% legal limit and other thresholds
In every US state except Utah, the legal BAC limit for driving is 0.08% — drive at or above it and you can be charged with DUI or DWI. Utah sets a stricter 0.05% limit. But the limit is a ceiling, not a safe zone: many states allow a lesser impaired-driving charge below 0.08% if an officer judges you unfit to drive.
- 0.08% — the standard adult driving limit across the US (0.05% in Utah).
- 0.04% — the limit for drivers of commercial vehicles (CDL holders).
- 0.00–0.02% — zero-tolerance limits for drivers under 21 in every US state.
Limits differ around the world — many countries set 0.05%, and some enforce 0.02% or zero. Wherever you are, the calculator's number is an estimate; the legal and physical reality on the road is always less forgiving than a formula.
How the body eliminates alcohol
Once alcohol is in your blood, only time lowers your BAC. The liver breaks alcohol down at a roughly steady rate — on average about 0.015 BAC per hour — which is the figure this calculator subtracts. As a rough guide, that is the alcohol from about one standard drink an hour.
Because elimination is steady, the maths is unforgiving. A BAC of 0.15% takes about 10 hours to reach zero, and a 0.20% peak takes more than 13 hours. Someone who drinks heavily late at night can still be over the limit the next morning — the classic cause of a morning-after DUI, where a driver feels fine but is still legally impaired when they wake up. The only way to be sure is to give it enough time, then more.
Note too that absorption and elimination overlap. While your liver clears about 0.015 BAC per hour, the alcohol from a recent drink is often still being absorbed into your blood — so your BAC can keep rising for 30–90 minutes after your last drink, even though you have stopped. That delay is why the peak in the worked example sits above the figure two hours later, and why "I stopped an hour ago" is no guarantee you are below the limit.
Factors that change your BAC
The same number of drinks produces very different BAC levels in different people — which is exactly why no calculator can tell you it is safe to drive. The biggest factors:
- Sex. Women tend to have a lower distribution ratio (less body water on average), so the same drinks reach a higher BAC. The formula uses r = 0.55 for women and 0.68 for men.
- Body weight. A heavier body dilutes alcohol through more tissue, lowering the BAC from the same amount.
- Time and pace. Spacing drinks out lets the liver keep clearing alcohol; drinking fast spikes BAC before elimination can catch up.
- Food. Eating before and while drinking slows absorption, lowering and delaying the peak — but it does not reduce the total alcohol you absorb.
- Other factors. Hydration, medications, fatigue, tolerance, and individual metabolism all shift real BAC in ways no formula captures.
Tolerance is the most dangerous of these to misread. A regular drinker may feel sober at a BAC that would clearly impair someone else — but tolerance changes how alcohol feels, not how much is in the blood or how much it slows reaction time. Your BAC, and your legal liability behind the wheel, are exactly the same whether or not you feel drunk. That gap between feeling fine and being over the limit is precisely where this estimate is most likely to mislead, and why it should never be the basis for a decision to drive.
Limitations: why this is an estimate, never a green light to drive
This calculator is an educational estimate, not a measurement. It uses population averages for body water and a single fixed elimination rate, so your true BAC can be meaningfully higher or lower than the number shown — and the consequences of guessing wrong are severe.
- It cannot account for your exact metabolism, body composition, medications, or health.
- It assumes accurate drink counting — real pours are often larger and stronger than one standard drink.
- It uses a flat 0.015/hour elimination rate; real rates vary between people and over a single night.
- It is not a breathalyser and has no legal standing. Only a tested measurement reflects your real BAC.
Data sources and methodology
The estimate uses the Widmark formula, the established method for estimating BAC, with a standard drink defined as 14 g of pure alcohol (US NIAAA/NHTSA), a distribution ratio of 0.68 for men and 0.55 for women, and an average elimination rate of 0.015 BAC per hour. The effects-by-level descriptions and the 0.08% legal limit follow US National Highway Traffic Safety Administration (NHTSA) guidance.
US NHTSA — The ABCs of BAC; US NIAAA — standard drink and BAC information.Frequently asked questions about the free bac calculator
About this BAC calculator
This BAC calculator runs entirely in your browser. The numbers you type never leave your device — nothing is sent to a server, logged, or shared. It applies the Widmark formula (14 g per standard drink, an alcohol distribution ratio of 0.68 for men and 0.55 for women, and a 0.015-per-hour elimination rate), and updates instantly on every change. It is an educational estimate, not a measurement, and never a basis for deciding to drive.
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