Blood Alcohol: How the Body Burns It Off

Set a body and an evening. The page estimates how much water the body holds, spreads the alcohol through it, burns it off at the rates healthy people span, and shows when the level falls below the legal limit you choose, as a range of times rather than one.

Worked example — Blood Alcohol: How the Body Burns It Off

Total body water (Watson, 1980)	44.65 L
Ethanol drunk	89.6 g
Peak at the average rate, when the drinking ends	1.26 g/L · 0.126 %BAC
The same, at the fast edge: 0.20 g/L an hour	02:05 · 6 h 05 min after the first drink
The same, at the slow edge: 0.10 g/L an hour	08:05 · 12 h 05 min after the first drink
Alcohol burnt per hour, at the average rate	7.88 g an hour · 0.99 UK units

These are the values the calculator shows when the page opens. Change any input above to recompute them.

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Lesson

The theory — Blood Alcohol: How the Body Burns It Off

Blood alcohol concentration is the mass of ethanol in a volume of blood. This page gives it in grams per litre, g/L, the unit most European limits are written in, often as per mille: 0.5 g/L is 0.5 ‰. The American %BAC is grams per 100 mL, so the same level is ten times smaller as a number: 0.5 g/L is 0.05 %BAC, and an American 0.08 %BAC is 0.8 g/L. The page's model is Widmark's: the alcohol spreads through the body's water, so its concentration is the grams drunk divided by that water, and the liver removes it at a steady rate.

What each symbol means

A: grams of ethanol drunk: volume × strength × 0.789 g/mL
TBW: total body water in litres, from sex, age, height and weight by Watson's equations
V: the blood the alcohol would fill at the same concentration, TBW divided by the 0.85 L of water in a litre of blood
c₀: A/V, the level if every gram arrived at once and none were burnt
β: the elimination rate, in g/L an hour: 0.10 to 0.20 in healthy people, 0.15 on average

Assumes: Body water follows Watson's equations, which were fitted to published dilution measurements on 458 men and 265 women. A litre of whole blood holds about 0.85 L of water. The alcohol is counted in the blood as it is drunk, spread evenly over the drinking time: absorption from the stomach and gut is not modelled, because no set of absorption parameters was read from a source for this page. Real absorption takes time, so the real peak is later and lower than the one drawn; in this arithmetic, once absorption is complete, the time a limit is crossed after the peak does not depend on how absorption went. The fall is a straight line at one rate for the whole night.
Breaks when: Watson's equations predict a person's body water only to about a tenth either way. Below about 0.2 g/L the enzyme that burns alcohol is no longer saturated and the fall slows, which is the Michaelis-Menten curve taking over from the straight line. People who drink heavily and often burn alcohol faster, up to 0.25 to 0.35 g/L an hour in alcoholics during detoxification, in Jones's review, outside the band drawn here. And a level is not an impairment: the page says nothing about how anyone at a given level can drive, work or judge.

The morning after is a span of hours, not a time 🖖

Press Pub evening. At the average rate of 0.15 g/L an hour the level falls below 0.5 g/L at 04:05. Alan Wayne Jones, reviewing the published drinking studies for forensic casework in 2010, put the normal range for healthy people at 0.10 to 0.15 g/L an hour after drinking on an empty stomach and 0.15 to 0.20 after food. At the fast edge the crossing comes at 02:05, at the slow edge at 08:05: six hours apart, for the same man and the same four pints. Whenever both edges cross the line, the slow one takes twice as long as the fast one, counted from the first drink, because the same fall is covered at half the speed. Nothing on the page can tell you which edge you are on, so the chart shades the whole band and the verdict gives both ends.

A unit an hour is true of a large body 🖖

The NHS describes a UK unit, 8 g of alcohol, as around what the average adult can process in an hour. Press Pub evening: at the average rate the 80 kg man burns 7.88 g an hour, 0.99 units, and the rule fits him. Press Same round, 60 kg: the woman burns 5.35 g, two thirds of a unit. The rate on this page is a fall in concentration, the same 0.15 g/L an hour for both of them, and the grams it removes are that fall times the volume the alcohol is spread through. Her body holds 30.34 L of water against his 44.65. The same smaller volume made her peak higher, 2.06 g/L against his 1.26, so she reaches 0.5 g/L at 09:25, five hours and twenty minutes after him. Jones notes that women's levels fall slightly faster; to catch him up she would need 0.25 g/L an hour, beyond the healthy range.

The limit you drive under moves the morning by hours 🖖

Press 0.2 g/L limit, Pub evening and 0.8 g/L limit in turn. It is one evening each time, and at the average rate it falls below Sweden's 0.2 g/L at 06:05, below most of the EU's 0.5 g/L at 04:05 and below England's 0.8 g/L at 02:05. At any one rate, two limits are crossed a fixed time apart: the gap between them divided by the rate. From 0.8 to 0.2 g/L is 0.6 g/L, which takes four hours at the average rate, three at the fast edge and six at the slow one. The drinks and the body do not enter that gap, provided the peak cleared both lines. The strictest line is also where the page is least sure of itself: Jones puts the steady fall on firm ground only above about 0.2 g/L, and below that the enzyme is less and less saturated and the fall slows, so the real crossing of a 0.2 g/L limit is, if anything, later than the one drawn.

Problem solved in full

Working backwards from a blood sample 4 steps
A driver with the body of the Pub evening preset is stopped at 02:00 and gives a blood sample at 08:00. It reads 0.30 g/L. What was the level at 02:00, and how much alcohol was in the body then? Press Pub evening for the body's numbers.

Open this problem in the tool
1. The page runs forwards, from drinks to a level. A court asks the question backwards. Six hours passed between the stop and the sample, and in that time the level fell at the steady rate, so the level at 02:00 is the sample plus six hours of elimination.
  
  $C (02 : 00) = C (08 : 00) + β \times 6 = 0.30 + 6 β$
2. There is no single rate to use. Across Jones's healthy range, 0.10 to 0.20 g/L an hour, six hours add between 0.60 and 1.20 g/L.
  
  $β = 0.10 : 0.90 β = 0.15 : 1.20 β = 0.20 : 1.50 g/L$
3. So the level at 02:00 lies between 0.90 and 1.50 g/L, with 1.20 at the average rate. Every value in that range is over both the 0.5 and the 0.8 g/L limits.
  
  $A = C V : 0.90 \times 52.53 = 47.28 g 1.50 \times 52.53 = 78.80 g$
4. The body's volume turns a level into grams. For this body V is 52.53 L, and one pint of 5% beer is 22.41 g.
  
  $\frac{47 . 28}{22 . 41} = 2.11 \frac{78 . 80}{22 . 41} = 3.52$
Answer

Between 0.90 and 1.50 g/L at 02:00: 47.28 to 78.80 g of ethanol still in the body, 2.11 to 3.52 pints of 5% beer. The top of that range is 1.67 times the bottom, from the rate alone, before any doubt about the body water. The calculation also assumes all the alcohol had been absorbed by 02:00. If the driver had finished a drink just before being stopped, part of it was still in the stomach, the level at 02:00 was lower than this arithmetic gives at whichever rate applied, and a straight line drawn back from the morning cannot see it. That assumption is why back-calculation, in Jones's words, "has many proponents and critics".

$\frac{1 . 50}{0 . 90} = 1.67$

References (5)

The elimination band the page draws, 0.10 to 0.20 g/L an hour, its 0.15 average, the faster rates of alcoholics in detoxification, and the Michaelis constant below which the straight-line fall stops holding: A. W. Jones, "Evidence-based survey of the elimination rates of ethanol from blood with applications in forensic casework." Forensic Science International 200(1-3), 2010, pp. 1-20. DOI 10.1016/j.forsciint.2010.02.021
The total-body-water equations the page uses for men and for women, from height, weight and (for men) age: P. E. Watson, I. D. Watson and R. D. Batt, "Total body water volumes for adult males and females estimated from simple anthropometric measurements." The American Journal of Clinical Nutrition 33(1), 1980, pp. 27-39. DOI 10.1093/ajcn/33.1.27
The 0.85 L of water in a litre of whole blood, which turns body water into the blood-equivalent volume and so fixes the page's g/L: C. Ialongo, "Blood alcohol concentration in the clinical laboratory: a narrative review of the preanalytical phase in diagnostic and forensic testing." Biochemia Medica 34(1), 2024, pp. 24-41. DOI 10.11613/BM.2024.010501
The line insight block 2 tests, quoted from the NHS: one UK unit, 8 g, is about what the average adult can process in an hour: NHS, "Calculating alcohol units". National Health Service, England.
The drink-driving limits the page offers, 0.2, 0.5 and 0.8 g/L, and which countries use each: European Road Safety Observatory, "Safety Performance Indicator (SPI): Alcohol and Drugs". European Commission, 2023, Table 2 (after ETSC, December 2021).