Pack Year Calculator

Estimate cumulative smoking exposure in pack-years from cigarettes smoked per day and years smoked, using the standard (cigarettes per day / 20) x years formula. Supports two smoking periods if your daily amount changed over time.

Quick Facts

Formula
Pack-years = (cigarettes/day / 20) x years
One pack equals 20 cigarettes. Separate smoking periods with different daily amounts are summed.
Screening threshold
20 pack-years
The 2021 USPSTF lung cancer screening guidance uses a 20 pack-year mark for adults 50-80 who currently smoke or quit within the past 15 years.

Your Results

Calculated
Total pack-years
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(cigarettes/day / 20) x years, summed
Total years smoked
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Sum of both periods
Average cigarettes/day
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Weighted by years in each period
20 pack-year screening mark
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Pack-year criterion only, not full eligibility

Ready

Enter your smoking history, then calculate your pack-years.

About the pack-year calculation

A pack-year is the standard way clinicians quantify cumulative cigarette exposure. One pack-year means smoking an average of one pack (20 cigarettes) per day for one year. The formula is simple: divide cigarettes smoked per day by 20 to get packs per day, then multiply by the number of years smoked. When someone's daily amount changed over time, each distinct period is calculated separately and the pack-years are added together.

How to get an accurate total

  • Estimate cigarettes per day as an average over each period, not just a peak or a light day - rough recall is normal, but try to anchor to a typical day.
  • Split your history into periods wherever your average daily amount changed meaningfully, and enter each period separately; this calculator supports two periods.
  • Years smoked should reflect actual smoking time - exclude any period you fully quit, even briefly, if you want a precise sub-period breakdown.

Putting the result in context

Pack-years are a cumulative exposure measure, not a diagnosis or a risk score by themselves. Clinical guidance most often uses pack-years as one entry criterion (alongside age and how recently you quit) for discussing lung cancer screening, and as a general marker of cumulative respiratory and cardiovascular risk. A higher pack-year total does not predict an individual outcome; it summarizes exposure.

When to consult a professional

This calculator performs the standard pack-year arithmetic only. It does not assess your age, family history, or overall eligibility for lung cancer screening, and it gives no dosing or treatment advice. Share your pack-year total with a healthcare provider to discuss screening eligibility and cessation support.

Frequently Asked Questions

What is a pack-year and how is it calculated?
A pack-year equals smoking an average of one pack (20 cigarettes) per day for one year. The formula is (cigarettes smoked per day / 20) x years smoked at that rate. Someone who smoked a pack a day for 10 years has 10 pack-years; someone who smoked two packs a day for 10 years has 20 pack-years. If your daily amount changed, calculate each period separately and add the results.
Why does lung cancer screening use a 20 pack-year threshold?
The U.S. Preventive Services Task Force updated its guidance in 2021 to recommend annual low-dose CT screening for adults aged 50 to 80 with at least a 20 pack-year smoking history who currently smoke or who quit within the past 15 years. Pack-years alone do not determine eligibility - age and quit timing matter too - so use this total to start a conversation with a clinician, not as a self-diagnosis.
How do I calculate pack-years if my smoking amount changed over time?
Break your smoking history into periods where your average daily cigarette count was roughly stable, calculate pack-years for each period separately with (cigarettes per day / 20) x years, then add the periods together. This calculator supports two periods; for more than two, add each additional period's pack-years to the total by hand.

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