Pack-year calculator

What is a pack-year?

A measure of how much a person has smoked: packs per day multiplied by years of smoking (National Cancer Institute). One pack-year is one pack a day for a year, or two packs a day for half a year.

How many cigarettes are in a pack?

20 — so 10 cigarettes a day is half a pack. The USPSTF describes one pack-year as an average of 20 cigarettes a day for a year.

Example: how many pack-years?

1.5 packs a day for 20 years is 30 pack-years.

Who should get lung cancer screening?

The USPSTF (2021) recommends a yearly low-dose CT scan for adults aged 50 to 80 with a 20 pack-year history who smoke now or quit within the past 15 years.

When does screening stop?

Once a person has not smoked for 15 years, or if a health problem limits life expectancy or the ability to have lung surgery.

Worked example: smoking that changed over the years

Pack-years add up across periods, each with its own rate. Suppose someone smoked 10 cigarettes a day for 6 years and then a full pack a day for 15 years. Ten cigarettes is half a pack, so the first period is 0.5 × 6 = 3 pack-years, the second is 1 × 15 = 15, and the total is 18 pack-years. At 62 and still smoking, that does not meet the 20 pack-year criterion.

Does the screening rule use the pack-years alone?

No. The 2021 USPSTF recommendation (grade B) has three conditions: age 50 to 80, a smoking history of at least 20 pack-years, and either current smoking or quitting within the past 15 years. A 62-year-old with 30 pack-years who quit 10 years ago meets the criteria; the same person who quit 18 years ago does not meet them. The test is a low-dose CT scan done once a year. Screening is meant for people without symptoms; anyone with symptoms should see a clinician.

Common mistakes with pack-years

Counting cigarettes as packs: 10 cigarettes a day is 0.5 pack, not 10. Counting years since the first cigarette instead of years actually smoked, so that long breaks are included. Rounding a pack a day for a few months up to a whole year. Using a pack size other than 20 without adjusting; the USPSTF definition is based on 20 cigarettes. And treating the number as a risk score: pack-years describe exposure only, and the decision to be screened is made with a clinician.

Different habits, the same number

A pack-year is a product, so different patterns can give the same total. Each of these is 20 pack-years:

• 0.25 packs a day (5 cigarettes) for 80 years = 20 pack-years
• 0.5 packs a day (10 cigarettes) for 40 years = 20 pack-years
• 1 pack a day (20 cigarettes) for 20 years = 20 pack-years
• 2 packs a day (40 cigarettes) for 10 years = 20 pack-years

The number treats light, long smoking and heavy, short smoking as equal. It is a convenient summary for screening rules, not a measure of risk in an individual, and it does not capture how long ago someone quit or whether they smoked other tobacco products.

How the screening criteria have changed

The USPSTF's 2013 recommendation covered adults aged 55 to 80 with at least 30 pack-years who smoke or quit within the past 15 years. The 2021 update lowered the starting age to 50 and the smoking history to 20 pack-years, keeping the 15-year window and the upper age limit of 80. Under the 2021 rule, a 52-year-old current smoker with 22 pack-years meets the criteria; under the 2013 rule that person would have been too young and below the pack-year threshold. In 2023 the American Cancer Society went a step further: its guideline uses the same age range and 20 pack-years but no longer requires that a former smoker quit within 15 years. Organizations differ here, and an insurer may follow one set or the other, so ask your clinician which applies to you.

Sources and related calculators

Sources: U.S. Preventive Services Task Force, “Lung Cancer: Screening” (2021 recommendation, including its definition of a pack-year); USPSTF 2013 statement (Annals of Internal Medicine, Moyer, 2014); American Cancer Society, lung cancer screening guideline update (2023); National Cancer Institute definition of pack-year. Related: standard drink calculator, eGFR calculator and blood pressure calculator.

For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.