In more detail
The Widmark formula is BAC (%) = alcohol (g) / (body weight (g) × r) × 100 − 0.015 × hours. The factor r reflects how much of the body is water that alcohol spreads into, about 0.68 for men and 0.55 for women, and 0.015% per hour is a typical elimination rate. In the US BAC is written as grams per 100 mL (0.08%); much of Europe uses grams per litre or per mille, so 0.5 g/L is the same as 0.05%. BAC usually peaks 30 to 90 minutes after the last drink, later on a full stomach.
Why it matters
Driving limits are set in BAC: 0.08% in most of the US and 0.05% in many European countries, often lower for new or professional drivers. A "standard drink" also differs by country: 14 g of alcohol in the US, 10 g in many others, and a UK unit is 8 g. Food, medication, liver function and body composition all shift real BAC, so an estimate can be well off; the only safe rule is not to drive after drinking.
Example
Three US standard drinks (42 g of alcohol), two hours after starting:
Man, 80 kg: 42 / (80,000 × 0.68) × 100 − 0.015 × 2 = 0.047 %
Woman, 60 kg: 42 / (60,000 × 0.55) × 100 − 0.015 × 2 = 0.097 %
The same drinks give the lighter woman roughly twice the BAC, above the 0.08% US limit.