Health📅 18 March 2025⏱ 5 min read
How to Calculate Blood Alcohol Content More Accurately
The one-unit-per-hour rule is a myth for most people. BAC depends on your weight, sex, drink strength, and food intake. Here's the Widmark formula and what the numbers actually mean.
JW
James WhitfieldPersonal Finance & Maths WriterJames has written about personal finance, health metrics, and everyday mathematics for over six years. He holds a BSc in Mathematics from the University of Leeds.
Understanding how alcohol metabolism actually works — not the pub myth — is genuinely useful for making informed decisions. Here's the science-based calculation and what it means in practice.
The Widmark Formula
BAC (g/100ml) = [Alcohol consumed (g) / (Body weight (kg) x r)] - (Metabolism rate x Hours)
r = Widmark factor (body water distribution)
Male: r = 0.68
Female: r = 0.55
(Women have proportionally less body water, so the same amount
of alcohol produces a higher BAC)
Metabolism rate: 0.015 g/100ml per hour (average)
Range: 0.01-0.02 depending on individual
Alcohol in grams:
Units x 8g per unit
Or: Volume (ml) x (ABV% / 100) x 0.789 (density of alcohol)
Worked Example
75kg male drinks 4 pints of 4.5% beer over 3 hours.
Step 1: Calculate alcohol consumed
1 pint = 568ml
4 pints = 2,272ml
Alcohol = 2,272 x (4.5/100) x 0.789 = 80.7g
Step 2: Apply Widmark formula
BAC = [80.7 / (75 x 0.68)] - (0.015 x 3)
= [80.7 / 51] - 0.045
= 1.582 - 0.045
= 1.54 g/litre = 0.154 g/100ml
UK drink drive limit: 0.08 g/100ml (80mg/100ml)
This person is nearly double the drink drive limit after 3 hours.
BAC Effects by Level
BAC (g/100ml)Typical EffectsDriving Status
0.01-0.03Mild relaxation, slight warmthLegal everywhere
0.03-0.06Lowered inhibition, mild impairmentLegal (but impaired)
0.05-0.08Reaction time affected, coordination reducedIllegal in Scotland (0.05 limit)
0.08-0.15Clear impairment, slurred speechIllegal UK (England/Wales limit: 0.08)
0.15-0.25Significant impairment, nausea likelySignificantly illegal
Above 0.30Serious risk of unconsciousnessMedical emergency risk
Why the "One Unit Per Hour" Rule Is Wrong
The average metabolism rate is 0.015 g/100ml per hour.
One unit = 8g alcohol
BAC from one unit (70kg male):
= 8 / (70 x 0.68) = 0.168 g/100ml
Time to metabolise to 0: 0.168 / 0.015 = 11.2 hours
NOT 1 hour.
"One unit per hour" is approximately true for BAC rise rate —
not the time to return to zero. Starting at BAC 0.15 after
an evening out, it takes 10+ hours to reach zero, not a few hours.
Factors That Affect BAC
- Food: Eating before and during drinking slows alcohol absorption, reducing peak BAC by 20-40%
- Drink speed: Rapid drinking overwhelms absorption, producing higher peaks
- Carbonation: Sparkling drinks (champagne, beer) absorb faster than still drinks
- Medications: Many common medications significantly amplify alcohol effects
- Tolerance: Regular drinkers show fewer behavioural effects at the same BAC, but the same physiological impairment remains