Pack-Years Calculator

How it is calculated

The tool multiplies two numbers: pack-years = packs per day × years smoked. Packs per day can be a fraction, for example 0.5 for half a pack, and years can be fractional too. The result is rounded to one decimal and printed with the label pack-years. Both values must be zero or more. If your smoking habit changed over time, calculate each period separately and add the results; the tool takes one constant average. A pack is usually 20 cigarettes, so a person smoking 10 cigarettes a day has 0.5 packs. The figure measures cumulative exposure only. It is not a risk score or a diagnosis.

Common mistakes

• Entering cigarettes per day (such as 20) in the packs field. That would give a figure twenty times too high; 20 cigarettes is 1 pack, so divide first.
• Averaging a changing habit into one rough number. Heavy years followed by light years are better calculated as separate periods and summed.
• Counting the years since starting rather than the years actually smoked, so a long quit break is included.
• Entering a monthly or weekly pack count as the daily one, which inflates the result sevenfold or more.
• Reading a pack-year figure as a risk score. It records cumulative exposure; what it means for a particular person is for a doctor to judge.