acalculator

How many pack-years did I smoke?

Type how much you smoked a day and for how many years. Add a row for each time the amount changed. The calculator adds up your smoking history in pack-years.

Your numbers

I count
Smoking periods
Row 1
Add a row when the amount changed, for example 1 pack a day for 10 years, then half a pack for 5 years.
Pack-years
20

Your smoking history is 20 pack-years.

20 pack-years or more

  1. Under 20 pack-years
  2. 20 pack-years or more
Cigarettes smoked
146,000
Years smoked
20
Screening line
At or over 20 pack-years: the history part of the USPSTF screening advice is met (it also needs age 50 to 80, and smoking now or quitting within 15 years).

Pack-years: 20. Your smoking history is 20 pack-years.

This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use

How to calculate

Works out a smoking history in pack-years, packs a day times years smoked, over one or more periods, and says whether it reaches the 20 pack-years in the USPSTF lung cancer screening advice.

Example with the default inputs (I count Packs a day, Smoking periods [Amount a day 1, Years 20]): Your smoking history is 20 pack-years.

Method: pack-years = Σ (packs a day × years) over each period, where packs a day = cigarettes a day ÷ 20.

  • A pack holds 20 cigarettes.
  • Each period has one steady amount a day; split your history into rows when it changed.
  • Arithmetic is exact on the typed decimals; pack-years show to one decimal, rounded half up.
  • Pack-years measure exposure only. They are one part of screening advice and not a diagnosis.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. I count Packs a day, Smoking periods 1 20 gives Pack-years 20, Cigarettes smoked 146,000.Source: National Cancer Institute, NCI Dictionary of Cancer Terms, pack year (packs smoked per day × years smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year, retrieved 2026-10-02; US Preventive Services Task Force, Lung Cancer: Screening (2021): yearly screening for adults 50 to 80 with a 20 pack-year history who smoke now or quit within the past 15 years. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/lung-cancer-screening, retrieved 2026-10-02
  2. I count Packs a day, Smoking periods 2 0.5 gives Pack-years 1.Source: National Cancer Institute, NCI Dictionary of Cancer Terms, pack year (packs smoked per day × years smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year, retrieved 2026-10-02: 2 packs a day for half a year is 1 pack-year
  3. I count Cigarettes a day, Smoking periods 10 30 gives Pack-years 15, Years smoked 30.Source: National Cancer Institute, NCI Dictionary of Cancer Terms, pack year (packs smoked per day × years smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year, retrieved 2026-10-02
  4. I count Packs a day, Smoking periods 1.5 10; 0.5 12 gives Pack-years 21, Years smoked 22.Source: US Preventive Services Task Force, Lung Cancer: Screening (2021): yearly screening for adults 50 to 80 with a 20 pack-year history who smoke now or quit within the past 15 years. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/lung-cancer-screening, retrieved 2026-10-02

How it works

For each period of smoking, with a steady amount a day:

pack-years = packs a day × years

  • If you count cigarettes, packs a day = cigarettes a day ÷ 20 (20 cigarettes to a pack).
  • The total is the sum of every period's pack-years.
  • Cigarettes smoked = pack-years × 20 × 365 (a 365-day year), shown as a whole number.
  • Years smoked is the sum of the years of every period.
  • Screening line. At 20 pack-years or more (19.95 and up, which shows as 20 at one decimal) the page says the pack-year part of the USPSTF 2021 lung cancer screening advice is met; the advice also needs an age of 50 to 80, and smoking now or having quit within the past 15 years. Under 20 it says the history is below the line.

Rules

  • Each row's amount is from 0 to 200 a day and its years from 0 to 100; there are 1 to 10 rows.
  • Arithmetic is exact on the typed decimals. Pack-years and years show to one decimal place, rounded half up.
  • Pack-years measure exposure. They are not a diagnosis or a risk score.

Worked examples by hand

1 pack a day for 20 years. 1 × 20 = 20 pack-years: at the screening line. Cigarettes: 20 × 20 × 365 = 146,000.

2 packs a day for half a year. 2 × 0.5 = 1 pack-year, NCI's example.

10 cigarettes a day for 30 years. 10 ÷ 20 = 0.5 packs a day; 0.5 × 30 = 15 pack-years.

1.5 packs a day for 10 years, then 0.5 packs a day for 12 years. 1.5 × 10 + 0.5 × 12 = 15 + 6 = 21 pack-years over 22 years.

Other questions people ask

What is a pack-year?

A measure of how much a person has smoked over time. One pack-year is smoking 1 pack (20 cigarettes) a day for 1 year. It is worked out as packs smoked a day × years smoked.

How do I calculate pack-years?

Multiply the packs smoked a day by the years smoked. 1 pack a day for 20 years is 20 pack-years; 2 packs a day for half a year is 1 pack-year.

How do I calculate pack-years from cigarettes a day?

Divide the cigarettes a day by 20 to get packs a day, then multiply by the years. 10 cigarettes a day for 30 years is 0.5 × 30 = 15 pack-years.

What if I smoked different amounts at different times?

Work out each period on its own and add them up. 1.5 packs a day for 10 years and then half a pack a day for 12 years is 15 + 6 = 21 pack-years. Add one row for each period.

Why do 20 pack-years matter?

The US Preventive Services Task Force (2021) advises a yearly low-dose CT scan for lung cancer for adults aged 50 to 80 with a history of 20 pack-years or more who smoke now or quit within the past 15 years. Talk with your clinician about screening.

Do years since quitting lower my pack-years?

No. Pack-years count what you smoked, so they do not go down after you quit. The time since quitting matters separately: USPSTF screening stops once a person has not smoked for 15 years.