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Pack-years is a measure of cigarette smoking calculated by multiplying the number of packs smoked per day by the number of years smoked. Medical professionals use this metric to assess lung cancer risk and determine screening eligibility. Even if you've quit smoking, your pack-year history remains important for health assessments.
Cigarette smoke contains more than 7,000 chemicals, at least 69 of which are known to cause cancer. Exact amounts per cigarette vary widely by brand and by how a person smokes, so we do not estimate a personal “dose”. Examples of harmful and potentially harmful constituents listed by the U.S. FDA:
Sources: FDA list of harmful and potentially harmful constituents (111 substances since April 2026) · National Cancer Institute
Based on published averages of about 11 minutes (Shaw et al., BMJ 2000) to about 20 minutes (Jackson et al., Addiction 2025) of life expectancy lost per cigarette. Not a personal prediction; quitting at any age regains years.
Pack years = (cigarettes per day ÷ 20) × years smoked. One pack is 20 cigarettes. Example: 15 cigarettes a day for 40 years is 15 ÷ 20 × 40 = 30 pack-years. If your smoking changed over time, calculate each period separately and add them up; years without smoking are not counted.
The U.S. Preventive Services Task Force (2021) recommends annual low-dose CT screening for adults aged 50–80 with at least 20 pack-years who currently smoke or quit within the past 15 years.
| Cigarettes/day | 10 yrs | 15 yrs | 20 yrs | 30 yrs | 40 yrs |
|---|---|---|---|---|---|
| 5 | 2.5 | 3.75 | 5 | 7.5 | 10 |
| 10 | 5 | 7.5 | 10 | 15 | 20 |
| 15 | 7.5 | 11.25 | 15 | 22.5 | 30 |
| 20 | 10 | 15 | 20 | 30 | 40 |
| 30 | 15 | 22.5 | 30 | 45 | 60 |
| 40 | 20 | 30 | 40 | 60 | 80 |
Step-by-step guide with worked examples: How to calculate pack years · What your total means: What is a pack year?
Multiply the number of packs smoked per day by the number of years you have smoked. For example, if you smoke 10 cigarettes a day (0.5 packs) for 20 years, the calculation is 0.5 × 20 = 10 pack-years.
It means the cumulative exposure equals smoking one pack (20 cigarettes) daily for 30 years, or two packs daily for 15 years, or any combination where packs × years = 30.
One pack-year is the equivalent of smoking one standard pack (20 cigarettes) every day for one full year.
It is shorthand for 20 pack-years: the total exposure reached by smoking one pack daily for 20 years, two packs daily for 10 years, etc. In the U.S., 20 pack-years plus age 50-80 is a key criterion for lung-cancer screening.
What Is a Pack Year?
Definition, formula, and clinical significance.
How to Calculate Pack Years
Step-by-step guide with 3 worked examples.
Pack Years & Lung Cancer Risk
Risk chart by exposure level, COPD, smoking index.
What Does 20 Pack Years Mean?
The USPSTF screening threshold explained.
What Does 30 Pack Years Mean?
Risk and screening for heavy smokers.
What Does 10 Pack Years Mean?
Risk levels for lighter smoking histories.
What Does 40 Pack Years Mean?
Heavy smoking history: risk, COPD and screening.
What Does 50 Pack Years Mean?
Very heavy exposure: what it means and next steps.
Pack-Year History Calculator
Smoked different amounts over the years? Add each period.
Pack Years After Quitting
Screening rules by years since quit; how risk falls.
Cigars, Pipes & Roll-Your-Own
What counts toward pack-years and what does not.
Vaping & Pack Years
Why vape use can’t be converted to pack-years.