Free tool
Pack-year calculator
Work out your smoking history in pack-years, even if the amount you smoked changed over the years, and see whether you fall within the US (USPSTF, American Cancer Society) or NHS lung cancer screening criteria. Everything runs in your browser.
Your smoking history
20 pack-years
One pack-year is 20 cigarettes a day for one year.
Lung cancer screening check
- Within the USPSTF criteria for yearly low-dose CT screening.
- Within the American Cancer Society criteria for yearly low-dose CT screening.
- In the age range for an NHS lung health check, where it is offered in your area.
This is a general guide, not a medical decision. Screening also depends on your overall health. Talk to your doctor or GP about whether screening is right for you.
The pack-year formula
Pack-years = packs smoked per day × years smoked. A pack is 20 cigarettes, so if you count cigarettes, divide by 20 first. The USPSTF defines one pack-year as the equivalent of smoking an average of 20 cigarettes a day for a year.
If you smoked different amounts at different times, calculate each period and add them up. Leave out any years you didn't smoke.
Pack-year examples
| Smoking history | Pack-years |
|---|---|
| 1 pack a day for 20 years | 20 |
| 10 cigarettes a day for 40 years | 20 |
| 2 packs a day for 15 years | 30 |
| 15 a day for 10 years, then 30 a day for 10 years | 22.5 |
What your pack-years mean for lung cancer screening
- USPSTF (US, 2021): yearly low-dose CT for adults aged 50 to 80 with a 20 pack-year history who smoke now or quit within the past 15 years. Screening stops once you have not smoked for 15 years.
- American Cancer Society (US): yearly low-dose CT for people aged 50 to 80 who smoke or used to smoke and have at least a 20 pack-year history, with no cut-off for years since quitting.
- NHS (England): lung health checks for people aged 55 to 74 who have ever smoked, in areas where the programme runs.
Meeting the criteria is a reason to talk to your doctor, not a diagnosis. Your doctor will also consider your overall health.
Frequently asked questions
What is a pack-year?
A pack-year is a way to measure how much you have smoked over time. One pack-year equals smoking an average of 20 cigarettes (one pack) a day for one year.
How do you calculate pack-years?
Multiply the number of packs you smoked per day by the number of years you smoked. If you smoked cigarettes rather than counting packs, divide cigarettes per day by 20 first. If your smoking changed over time, work out each period separately and add them together.
What does a 20 pack-year smoking history mean?
It means the equivalent of a pack a day for 20 years, for example 10 cigarettes a day for 40 years, or 2 packs a day for 10 years. In the US, 20 pack-years is the threshold used in lung cancer screening guidelines.
Who should get lung cancer screening?
In the US, the USPSTF recommends yearly low-dose CT screening for adults aged 50 to 80 with at least 20 pack-years who smoke now or quit within the past 15 years. The American Cancer Society uses the same age and pack-year limits but does not stop screening based on years since quitting. In England, the NHS offers lung health checks to people aged 55 to 74 who have ever smoked, where the programme runs.
Do years I didn't smoke count?
No. Only count the years you actually smoked. If you quit for a few years and started again, leave the smoke-free years out.
Does quitting lower my lung cancer risk if I have many pack-years?
Yes. Risk falls the longer you stay smoke-free: after about 10 years, lung cancer risk is around half that of someone who still smokes. See quit smoking timeline for the full recovery timeline.
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Sources
- US Preventive Services Task Force: Lung cancer screening (2021)
- American Cancer Society: Can lung cancer be found early?
- NHS: Lung cancer screening
This calculator is educational and does not replace medical advice.