Pack-Year Calculator
Pack-years measure how much someone has smoked over time: one pack a day for one year is one pack-year. US guidelines use the total, with age and quit date, to decide who should have a yearly low-dose CT scan for lung cancer.
| Result | Interpretation |
|---|---|
| Below 20 | Under 20 pack-years |
| 20 and above | 20 or more pack-years |
Source: US Preventive Services Task Force; Krist AH, Davidson KW, Mangione CM, et al. Screening for Lung Cancer: US Preventive Services Task Force Recommendation Statement. JAMA 2021;325(10):962-970. (doi:10.1001/jama.2021.1117); Wolf AMD, Oeffinger KC, Shih TY, et al. Screening for lung cancer: 2023 guideline update from the American Cancer Society. CA Cancer J Clin 2024;74(1):50-81. (doi:10.3322/caac.21811)
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How to calculate pack-years
Pack-years = packs per day × years smoked
One pack is 20 cigarettes, so 10 cigarettes a day is half a pack. One pack-year is the same as smoking a pack a day for one year, or two packs a day for half a year.
Worked example
15 cigarettes a day for 30 years: (15 ÷ 20) × 30 = 22.5 pack-years.
Pack-year chart
| Years smoked | 10 a day | 20 a day (1 pack) | 30 a day | 40 a day (2 packs) |
|---|---|---|---|---|
| 10 | 5 | 10 | 15 | 20 |
| 15 | 7.5 | 15 | 22.5 | 30 |
| 20 | 10 | 20 | 30 | 40 |
| 25 | 12.5 | 25 | 37.5 | 50 |
| 30 | 15 | 30 | 45 | 60 |
| 40 | 20 | 40 | 60 | 80 |
Who should be screened for lung cancer
Both major US guidelines recommend a yearly low-dose CT scan of the chest for adults aged 50 to 80 with at least 20 pack-years. They differ on people who quit long ago:
| USPSTF (2021) | American Cancer Society (2023) | |
|---|---|---|
| Age | 50 to 80 | 50 to 80 |
| Smoking history | 20 pack-years or more | 20 pack-years or more |
| Quit date | Smoke now, or quit within the past 15 years | Years since quitting does not matter |
| Test | Low-dose CT every year | Low-dose CT every year |
Under the USPSTF, screening stops once someone has not smoked for 15 years. Both guidelines also advise against screening people whose other health problems substantially limit life expectancy.
FAQs
How many pack-years do you need for lung cancer screening?
20 or more, between ages 50 and 80. The USPSTF lowered this from 30 pack-years and age 55 in 2021.
I quit more than 15 years ago. Should I be screened?
The USPSTF says no, but the American Cancer Society’s 2023 guideline removed the quit-date rule and would still recommend screening. This is worth discussing with your physician.
How do I count years I smoked less?
Work out each period separately and add them. For example, 10 years at a pack a day (10 pack-years) plus 10 years at half a pack (5 pack-years) is 15 pack-years.