What is blood alcohol concentration?
Blood alcohol concentration, or BAC, is the amount of alcohol present in blood. It is usually written as a percentage: a BAC of 0.08 means 0.08 grams of alcohol in every 100 millilitres of blood. Small numbers hide big effects. That 0.08 figure is the widely used legal driving limit, and levels just a few hundredths of a percent above it start shrinking reaction, coordination and judgement in measurable ways. Since alcohol is absorbed quickly and eliminated slowly, BAC climbs with every drink and falls at an almost constant rate, which makes it predictable with a reasonable estimate.
Most people are not screened at the spot they most often need data — after a gathering, before driving, or while planning the next day. A BAC estimate built from your drinks, your weight and the time that has passed gives a realistic snapshot of where your level sits, and more importantly, how long it will take to come down.
The engine behind the estimate is the classic Widmark equation, the same century-old method that modern breathalyzer mathematics draws from. Use the tool to estimate, never to decide whether to drive: when in doubt, the answer is always a zero-BAC alternative.
How the BAC estimate is calculated
The model treats your body as a tank of water of limited size, distributes every gram of alcohol you drank into that volume, then lets a constant elimination rate chip away at the result:
BAC% = (0.1 × alcohol_grams) / (weight_kg × r) − 0.015 × hours
The two multipliers carry the biology:
- r, the distribution ratio. Men carry alcohol in a larger share of body water on average (commonly taken as 0.68), so the same drink dilutes more. The typical value for women is 0.55, which is one reason the same number of drinks produces a higher peak for a woman of the same weight as for a man.
- 0.015 per hour. The liver processes about one standard drink's worth of alcohol every two hours; expressed as a decline, BAC falls roughly 0.015% each hour regardless of weight.
Because both constants carry "on average" signs, the result is an estimate, usually within a couple hundredths of a percent for a normal adult — plenty of signal for planning, and no substitute for a calibrated breath test.
The drink presets and how they are built
To compute alcohol intake the tool needs grams of pure alcohol, which depend on volume, alcohol by volume (ABV) and the density of ethanol (0.789 g/ml):
grams = volume_ml × ABV ÷ 100 × 0.789
A 330 ml regular beer at 5% yields 330 × 0.05 × 0.789 ≈ 13 g. The preset menu in the tool carries the common portion and ABV for beer, wine, shots and spirit mixers so you do not have to do that multiplication yourself. Because real bars vary (a generous pour, a draft pint instead of a stubby), the presets are approximations and your glass may hold more than the label suggests. When in doubt, count the drink a size or an ABV above the preset, being generous about memory.
Two drink rows are enough to model the typical mixed evening — a couple of beers followed by a different drink, or wine with dinner plus a digestif. If your night mixes more than two kinds, fold the extra into the closest preset and raise the serving counts proportionally.
Reading the gauge and the limit lines
The gauge colours mirror the impairment ladder:
| BAC range | What the research shows |
|---|---|
| 0.00–0.02 | Negligible. Most people behave normally; staying in this band over hours of a party still demands a sober math realistic estimate. |
| 0.02–0.05 | Mild warmth and release. Concentration and judgement of speed start to slip for a meaningful share of people. |
| 0.05–0.08 | Recoverable slipper increases: glare recovery, tracking and decision quality degrade. Multiple jurisdictions set limits inside this band, not at 0.08. |
| 0.08–0.30 | Over the legal limit. Coordination, reaction time and reasoning are measurably impaired; walking and speech show it. |
| 0.30+ | Severe depression of the nervous system. Blackouts, risk of unconsciousness and a real medical emergency. |
Treat the 0.08 line as a floor for protection, not a target to steer under. Reaction time is already slower across 0.05 in many studies, and every hour with alcohol behind the wheel converts a statistical delay into distance: at 80 km/h, a delay of 0.2 seconds equals about four and a half metres of travel you no longer control.
What determines your peak and your recovery
The estimate balances you from the moment the drinks stop arriving. The arithmetic of the formula only mentions weight, r and time, but reality adds several players:
- Food. A man with a full meal absorbs alcohol more slowly, pushing the achieved peak lower and later. Drinking on an empty stomach concentrates in blood quickly.
- Body composition. The distribution ratio is a body-water statistic; people with more lean mass dilute alcohol more than the average number suggests.
- Tolerance. Do not. Tolerance changes how the brain copes with a level, not the level itself. A frequent drinker at 0.10 looks fine in conversation and is still impaired behind the wheels according to every laboratory measure.
- Medication and fatigue. Both interact brutally with alcohol. The formula has no row for these — plan a buffer of at least one extra zero for safety.
The outputs "hours until elimination reaches zero" is your planning number: it divides the current estimate by the 0.015/hour burn. That assumes you have stopped drinking — if the last drink sits inside the counting hours, the estimate under-states.
Standard drinks and moderation maths
Guidelines speak in "standard drinks" because they even out beer cans, wine glasses and shots. A standard drink commonly equals 14 grams of pure alcohol — roughly a 330 ml beer at 5%, a 150 ml glass of wine at 12%, or 40 ml of spirit. The tool reports how many standard drinks your two rows total, so you can talk to your own doctor or national guidance in the same unit they use.
Counting drinks is also the fastest way to spot a heavy evening: four standard drinks inside two hours for an 80 kg man calculate to a BAC estimate in the low 0.06–0.08 band and climb steeply from there. The same volume paced over an evening meal keeps a lower peak, but does not make you sober: metabolism is a fixed rate, not a repayment plan with interest.
Because alcohol is a dense source of energy — roughly 7 kcal per gram, burned alongside whatever the drink is made of — a party easily sponsors hundreds of calories nobody planned. The shape the serving still fits into a daily energy budget, the calorie calculator shows what those drinks cost in terms of food, and the macro calculator places the drink's carbs within your day if you are tracking meals.
Alcohol, weight and long-term health
Alcohol interacts with weight management twice: directly through calories and indirectly through perception. Decision-making under alcohol produces bar food and sleep debt; both nudge the daily balance toward gain. Conversely, very low body weight (and low body water) raises BAC faster at the same grams because r dilutes less — a point worth knowing both for the estimate and in general.
Body metrics matter here more than on the scale. The distribution ratio is tied to body water rather than total mass, which is why the same body-weight models its leaner friends differently. Tools like the body fat calculator and the BMI calculator can help you know the composition behind the number; the BAC estimator is the one place where a heavy, lean person and a light, soft one of the same weight are treated nearly as equals — a reminder that BAC reading always lives in the body, not on the label.
Limitations and the responsible way to use it
Estimates break down precisely when they matter most: after a long night, on medication, or for someone outside the average adult. The formula assumes steady state of the rates, a patient liver, and no recent alcohol from earlier sessions — out of drinking window skews the start. It also cannot see how fast you drank: pace matters because the peak depends on the balance between absorption and elimination before elimination begins.
The rules of the road are brutally simple and neither this tool nor any other drinks it away: if you have been drinking, do not drive. Zero is the only safe number behind the bullet. Use the estimate to plan ahead — confirming by your own measure that last night's two beers leave no trace before your morning commute — and always honour the local legal limit.
Conclusion
Blood alcohol concentration is a small percentage with a large story. Widmark's formula, drink presets and a body-weight input turn it into an estimate you can hold in your hand: current BAC, standard drinks, and the hour the number finally lands at zero. Feed it an honest report of your evening, read the gauge and the table, and let the strongest number of the tool — the hours until zero — draw the boundary of your plans.
Disclaimer
Results are provided as estimates for informational purposes only and may be inaccurate. Always verify outcomes with a qualified professional before making financial or personal decisions based on these calculations.