Blood Alcohol (BAC) Estimator

About the Blood Alcohol (BAC) Estimator

Add what you drank — size, strength (% ABV) and how many — when you started and finished, and your sex, weight and height (and age, for men). The estimator works out how much pure alcohol that is, your estimated blood alcohol concentration (BAC) now, the highest it is likely to have reached, and roughly when it falls back to zero, as a range for slow, average and fast elimination. A chart shows the curve against the drink-drive limits of India (30 mg per 100 ml), Scotland (50), England, Wales and Northern Ireland (80) and the US (0.08%).

It uses the method forensic scientists use for such estimates — the Widmark equation with Watson’s body-water formula — and still the result can be well off for you: food, how fast you drank, your liver and your body build all change it. No estimate can tell you whether it is safe or legal to drive. If you have been drinking, do not drive.

Why you cannot use this to decide whether to drive

• The spread between people is large. Ethanol leaves the blood at about 10–15 mg/100 ml per hour after drinking on an empty stomach, 15–20 after food, and 25–35 in alcohol-dependent people during detoxification. The body water — and so how high BAC rises — is estimated from your height and weight, not measured.
• The uncertainty of any calculated BAC is wide. One forensic analysis put it at a coefficient of variation of about ±21%; a later review showed it has to be worked out case by case and is far larger at low levels. That is why the result is shown as a range.
• BAC can keep rising after you stop. Alcohol in the stomach and intestine is still being absorbed, so your level can go up after the last drink. This model assumes each drink is absorbed as you drink it.
• Legal limits are not safety limits. WHO notes that the risk of a crash starts at low blood alcohol levels and rises significantly from 0.04 g/dl — half the 0.08% limit used in the US and England. Some drivers, such as US commercial drivers, have lower limits.

If you have been drinking, do not drive: use a taxi, public transport or a driver who has not been drinking.

How the estimate works

• Alcohol in grams = volume (ml) × ABV ÷ 100 × 0.789 (the density of ethanol). Four 568 ml pints of 5% beer contain 89.6 g.
• Total body water (Watson): men TBW = 2.447 − 0.09516 × age + 0.1074 × height (cm) + 0.3362 × weight (kg); women TBW = −2.097 + 0.1069 × height (cm) + 0.2466 × weight (kg). For a man of 40, 180 cm and 80 kg that is 44.9 litres.
• Peak with nothing removed: C0 (mg/100 ml) = 100 × grams × water fraction of blood ÷ TBW, using 0.825 for men and 0.838 for women. Here 100 × 89.6 × 0.825 ÷ 44.9 = 164.8 mg/100 ml. The equivalent Widmark factor is r = TBW ÷ (water fraction × weight) = 0.68.
• Elimination: BAC = C0 − β × hours since the first drink, with β = 10 (slow), 15 (average) and 20 (fast) mg/100 ml per hour. After 3 hours of drinking that gives 135, 120 and 105 mg/100 ml — 0.135%, 0.120% and 0.105% — and zero after 16.5, 11.0 and 8.2 hours.

Units: 0.08% = 0.08 g per 100 ml = 80 mg per 100 ml = 0.8 g per litre (0.8‰). Breath tests measure a different number (micrograms per 100 ml of breath) and convert it.

Drink-drive limits

• India: more than 30 mg of alcohol per 100 ml of blood (0.03%), detected by a breath analyser or any other test including a laboratory test, is an offence under section 185 of the Motor Vehicles Act 1988. The penalty is up to six months in prison or a ₹10,000 fine (or both) for a first offence, and up to two years or ₹15,000 for a repeat offence.
• England, Wales and Northern Ireland: 80 mg per 100 ml of blood (35 µg per 100 ml of breath, 107 mg per 100 ml of urine).
• Scotland: 50 mg per 100 ml of blood (22 µg breath, 67 mg urine).
• United States: a BAC of 0.08% or more is a crime in every state except Utah, where the limit is 0.05%; drivers of commercial vehicles must not drive with 0.04% or more.

GOV.UK puts it plainly: it is impossible to say exactly how many drinks a limit equals — it is different for each person.

Signs of alcohol overdose — get help

Call your emergency number (112 in India, 999 in the UK, 911 in the US) straight away if someone who has been drinking has mental confusion, difficulty staying conscious or cannot be woken, vomiting, seizures, slow or irregular breathing, a slow heart rate, clammy skin, dulled responses such as no gag reflex, a very low body temperature, or bluish or pale skin. Do not wait for all the signs, and do not leave them alone or to “sleep it off”: BAC can keep rising while a person is unconscious, and they can choke on vomit. If they are unconscious or lying down, roll them onto one side with an ear towards the ground. Cold showers, coffee and walking do not reverse an overdose (NIAAA).

Sources

• Maskell PD et al. Forensic alcohol calculations in transgender individuals undergoing gender-affirming hormonal treatment. J Forensic Sci — the total-body-water form of the Widmark equation and the water fractions of blood
• Searle J. Alcohol calculations and their uncertainty. Med Sci Law
• Watson PE, Watson ID, Batt RD. Total body water volumes for adult males and females estimated from simple anthropometric measurements. Am J Clin Nutr
• Maskell PD et al. Total body water is the preferred method to use in forensic blood-alcohol calculations. Forensic Sci Int
• Jones AW. Evidence-based survey of the elimination rates of ethanol from blood with applications in forensic casework. Forensic Sci Int
• The Motor Vehicles Act, 1988, section 185, as amended by the Motor Vehicles (Amendment) Act, 2019
• GOV.UK, The drink drive limit
• GHSA, Alcohol-impaired driving; 49 CFR 382.201
• NIAAA, Understanding the dangers of alcohol overdose
• WHO, Road traffic injuries fact sheet

Limitations

• It is an estimate for adults from average equations. Food, drinking speed, medicines, liver disease, pregnancy and body build all change real levels.
• It assumes the drinks were spread evenly between your first and last drink and absorbed as you drank them; real absorption takes longer, especially after a meal.
• Low results (below about 20 mg/100 ml) are the least precise, because the body removes alcohol more slowly at very low levels.
• It does not show when you will be fit or legal to drive, work, operate machinery or swim — none of these can be judged from an estimate.
• Watson’s equations were checked for BMI about 17–80 in women and 17–67 in men; outside that the estimate is less certain.

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