Pack years = (number of cigarette packs smoked per day) × (number of years smoked)
Pack years = (cigarettes smoked per day ÷ 20) × years smoked
Example: 1 pack per day for 10 years = 10 pack years
Example: 2 packs per day for 15 years = 30 pack years
Example: 10 cigarettes per day for 20 years = 10 pack years
If smoking history varies over time, calculate each period separately and add the results
Use 20 cigarettes as 1 pack when converting cigarettes to packs