Pack-Years Calculator

How it is calculated

The tool computes pack-years = (cigarettes per day / 20) × years smoked. The divisor 20 is the number of cigarettes in one pack, so the first part converts your daily count into packs per day. Multiplying by the number of years gives the cumulative exposure, shown with two decimals. Neither input may be negative, and decimals such as 1.5 years or 7.5 cigarettes are accepted.

The calculation assumes a single steady habit over the whole period. If the amount changed, work out each stretch separately (for example 10 a day for 5 years, then 20 a day for 8 years) and add the pack-years together. The tool does not subtract years spent not smoking, and it does not cover pipes, cigars or vaping, whose pack equivalents are not defined here.

Common mistakes

• Counting the years since starting instead of the years actually smoked; quit periods must be left out.
• Entering packs per day in the cigarettes field, such as 1 for a pack; the field takes single cigarettes, so a pack a day is 20.
• Using one average for a habit that changed a lot over time, which can hide a heavy period; split it into stretches and add the results.
• Assuming a pack always holds 20: in places where packs hold a different number, convert to cigarettes first, because the tool divides by 20.
• Treating pack-years as a measure of risk by itself; the same number can come from heavy short-term or light long-term smoking, which are not equal in effect.