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CALCPARK

BAC Calculator

Estimate blood alcohol content from your drinks, weight, sex and hours since the first drink with the Widmark formula, plus the time back to 0.00 and under common limits.

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How to use it

  1. Choose units, your sex and body weight. Optionally switch to the Watson method and add height and age for a personalised distribution factor.
  2. Add each drink with the preset buttons, then adjust volume, alcohol by volume (ABV) and count – the label on the bottle gives the ABV.
  3. Enter how many hours have passed since your first drink. Leave 0 to see the peak value for the whole amount.
  4. Read the estimated BAC in %, ‰ and mg/100 mL, the time back to 0.00 and the time until the estimate is under 0.02, 0.05 and 0.08 %.
  5. Copy the summary or the link. The link stores the numbers only – no names or notes.

Example

A 70 kg man drinks two 355 ml beers at 5 % over two hours. Each beer is 355 × 0.05 × 0.789 = 14.0 g of ethanol, 28.0 g in total. Classic Widmark: 28 ÷ (0.68 × 70,000) × 100 = 0.059 % at peak. After two hours of elimination at 0.015 %/h the estimate is 0.059 − 0.030 = 0.029 % (0.29 ‰), and it reaches 0.00 about 1 h 55 min later. With the Watson method (175 cm, 30 years → 41.9 L of body water, r = 0.749) the peak is 0.053 % and the two-hour value 0.023 %.

Formula, factors and limits

A (g)        = volume (ml) × ABV ÷ 100 × 0.789 g/ml
BAC (%)      = A ÷ (r × W (g)) × 100 − β × t
Classic r    = 0.68 (men) / 0.55 (women)          – Widmark 1932
Watson r     = TBW ÷ (0.8 × W (kg))
  TBW men    = 2.447 − 0.09516 × age + 0.1074 × height (cm) + 0.3362 × W (kg)
  TBW women  = −2.097 + 0.1069 × height (cm) + 0.2466 × W (kg)
β            = 0.015 %/h by default (0.010–0.025 %/h)
1 % = 10 ‰ = 1,000 mg/100 mL

The Widmark model treats the whole dose as absorbed and evenly distributed at time zero, then removed at a constant rate. Real absorption takes 30–90 minutes and is slower on a full stomach, so shortly after drinking the true value is usually lower than the peak shown, and later it can be higher than the time-adjusted estimate. Studies of the Widmark factor give population means close to Widmark’s 0.68 / 0.55 but individual values from about 0.5 to 0.9; the Watson body-water variant (Watson, Watson & Batt 1980, applied to Widmark by Seidl and others) narrows the error for people far from average build but is still an estimate. Elimination averages about 0.015 %/h in social drinkers and is faster in heavy drinkers. The tool is not valid for under-18s, during pregnancy, or with liver disease, and it does not model medication interactions.

Sources: Posey & Mozayani 2007, Forensic Sci Med Pathol – The estimation of blood alcohol concentration: Widmark revisited, Watson, Watson & Batt 1980, Am J Clin Nutr – Total body water volumes for adult males and females, Seidl, Jensen & Alt 2000, Int J Legal Med – The calculation of blood ethanol concentrations in males and females, NIAAA – What is a standard drink?, NHTSA – The ABCs of BAC.

This is a statistical estimate, not a measurement, and not medical or legal advice. It can be wrong by 0.02 % or more for an individual. Never use it to decide whether you are fit to drive, work or care for others; only a breath or blood test gives an actual value.

What the numbers mean

BACTypical effects (NIAAA / NHTSA)
0.02–0.05 %Relaxation, slight loss of judgement; divided attention and visual tracking already decline
0.05–0.08 %Reduced coordination, slower reactions and alertness; legal limit for driving in most of Europe is 0.05 %
0.08–0.15 %Clear impairment of balance, speech, reaction time; 0.08 % is the US and UK (England/Wales) driving limit
0.15–0.30 %Vomiting, major loss of control, risk of blackouts
≥ 0.30 %Risk of loss of consciousness, respiratory depression and death – a medical emergency

Questions

How long until I am back to zero?
Divide the current estimate by the elimination rate: 0.06 % ÷ 0.015 %/h = 4 hours. The tool shows this as a time plus the moments the estimate drops under 0.08, 0.05 and 0.02 %. Sleep does not speed it up, and a reading of 0.00 does not mean you are rested or alert.
Why does the result differ from a breathalyser?
The formula uses averages for how alcohol spreads in the body and how fast it is removed, and assumes everything is already absorbed. Food, body composition, drinking speed, medication and genetics all shift the real value, so differences of ±0.02 % or more are normal.
Should I use the classic or the Watson method?
The classic 0.68 / 0.55 factors are Widmark’s population averages. The Watson method estimates total body water from height, weight and age, which generally fits people who are much heavier, lighter, older or taller than average better. Neither is a measurement.