Health⏱ 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.
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