Pack-Year Calculator — Smoking History & Lung Screening
Get your pack-years from cigarettes per day and years smoked, and see if you meet USPSTF and ACS lung cancer screening criteria. Several periods allowed.
Pack-year formula
Pack-years = (cigarettes per day ÷ 20) × years smoked. The US National Cancer Institute defines the pack-year as a way to measure the amount a person has smoked over a long period of time: 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year. If your consumption changed, calculate each period separately and add them up.
Example: 20 cigarettes a day for 15 years, then 10 a day for 10 years = (1 × 15) + (0.5 × 10) = 20 pack-years.
How to use the calculator
- Enter how many cigarettes you smoked per day and for how many years.
- If the amount changed (for example 10 a day in your twenties and 30 later), add a period for each stage.
- Say whether you still smoke or have quit, and how many years ago you quit.
- Add your age to see whether you meet the USPSTF and American Cancer Society lung screening criteria.
Pack-year examples
Worked examples with the standard formula (1 pack = 20 cigarettes).
| Cigarettes per day | Years | Pack-years |
|---|---|---|
| 10 | 20 | 10 |
| 20 | 20 | 20 |
| 15 | 30 | 22.5 |
| 30 | 10 | 15 |
| 40 | 15 | 30 |
Lung cancer screening criteria
USPSTF 2021: annual screening with low-dose CT for adults aged 50 to 80 with a 20 pack-year history who currently smoke or have quit within the past 15 years (grade B). Screening stops once a person has not smoked for 15 years or develops a health problem that substantially limits life expectancy or the ability or willingness to have curative lung surgery. It replaced the 2013 criteria (ages 55 to 80, 30 pack-years).
American Cancer Society 2023: annual low-dose CT for asymptomatic people aged 50 to 80 who currently smoke or formerly smoked and have at least 20 pack-years (strong recommendation). Years since quitting are not a criterion to start or stop. A shared decision-making discussion should come before the first scan, and people who smoke should receive cessation counselling.
Smoking and older age are the two most important risk factors for lung cancer: risk increases with the cumulative quantity and duration of smoking. Outside the US, criteria and programmes vary, so ask your doctor what applies where you live.
FAQ
How do you calculate pack-years?
Divide the cigarettes you smoke a day by 20 to get packs per day, then multiply by the years you have smoked. The US National Cancer Institute gives the example that 1 pack-year equals 1 pack a day for 1 year, or 2 packs a day for half a year. So 10 cigarettes a day for 20 years is 10 pack-years, and 30 a day for 10 years is 15.
What if I smoked different amounts over the years?
Split your history into periods with a roughly stable amount, calculate each one and add them up. For example, 10 a day for 10 years (5 pack-years) plus 20 a day for 15 years (15 pack-years) gives 20 pack-years. The calculator accepts up to three periods, which covers most smoking histories.
How many pack-years qualify for lung cancer screening?
Both the USPSTF (2021) and the American Cancer Society (2023) use 20 pack-years and ages 50 to 80. The difference is quitting: the USPSTF includes people who smoke now or quit within the past 15 years, while the ACS sets no limit on years since quitting. Meeting the criteria means discussing annual low-dose CT with your doctor, not that a scan is mandatory.
Do my pack-years go down if I quit?
No. Pack-years measure cumulative exposure, so they stay the same after you stop; they simply stop increasing. What changes is the time since quitting: lung cancer risk decreases as it grows, and the USPSTF uses it to stop screening after 15 years without smoking. The American Cancer Society does not use it.
Can I count cigars, pipes or vapes?
Not with this calculator. Pack-years are defined for cigarettes (1 pack = 20 cigarettes), and there is no validated equivalent for cigars, pipes, e-cigarettes or vapes. If you have used them, tell your doctor separately: the exposure still matters even though it cannot be converted into pack-years.
Sources
- National Cancer Institute. NCI Dictionary of Cancer Terms: pack year.
- Krist AH et al.; US Preventive Services Task Force. Screening for Lung Cancer: US Preventive Services Task Force Recommendation Statement. JAMA. 2021;325(10):962-970. PMID 33687470.
- US Preventive Services Task Force. Lung Cancer: Screening. Final recommendation statement (2021).
- Wolf AMD et al. Screening for lung cancer: 2023 guideline update from the American Cancer Society. CA Cancer J Clin. 2024;74(1):50-81. PMID 37909877.
Suggested article
Pack-Years: How to Calculate Your Smoking History and When It Qualifies You for Lung Cancer Screening
Pack-years sum up a lifetime of smoking in a single number. How to calculate them, what to do if your smoking changed over the years, and why 20 pack-years is the figure that matters most today.
Read the full article →CalcVita. (2026). Pack-Year Calculator — Smoking History & Lung Screening. CalcVita. Retrieved October 24, 2026, from https://calcvita.com/en/calculators/pack-years
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