What this calculator does
Pack-years express total smoking exposure as a single number: one pack a day for one year is one pack-year. It combines intensity and duration so that different smoking histories can be compared on one scale.
The figure matters because screening criteria are written in it. Twenty pack-years is a common threshold for lung cancer screening eligibility, which twenty a day for ten years reaches exactly.
The formula
Divide cigarettes per day by 20, the number in a standard pack, and multiply by the number of years smoked.
| Term | Meaning |
|---|---|
| Pack-year | One pack of 20 cigarettes a day for one year. |
| Screening threshold | Twenty pack-years is widely used as an eligibility criterion for lung cancer imaging. |
| Cumulative exposure | Intensity multiplied by duration, which is what risk models use. |
The inputs explained
| Field | What to enter |
|---|---|
| Cigarettes per day | Average cigarettes smoked per day over the period. |
| Years smoked | Number of years smoked at that rate. |
When to use it
Checking screening eligibility
Screening guidelines specify a pack-year threshold alongside age and quit date.
Recording a smoking history
Pack-years is the standard way a history is documented in medical notes.
Understanding cumulative risk
Seeing intensity and duration combine makes the arithmetic of exposure concrete.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What do different histories add up to?
Four smoking histories with the same arithmetic.
| Cigarettes per day, over 30 years | Pack-years | Note |
|---|---|---|
| 10 a day | 15.0 | 20 pack-years or more is commonly used as a screening threshold for lung cancer imaging: discuss with a clinician. |
| 20 a day | 30.0 | 20 pack-years or more is commonly used as a screening threshold for lung cancer imaging: discuss with a clinician. |
| 30 a day | 45.0 | 20 pack-years or more is commonly used as a screening threshold for lung cancer imaging: discuss with a clinician. |
| 40 a day | 60.0 | 20 pack-years or more is commonly used as a screening threshold for lung cancer imaging: discuss with a clinician. |
Questions
What if my smoking varied over the years?
Calculate each period separately at the rate for that period and add the results. A decade at ten a day plus a decade at thirty is five plus fifteen, giving twenty pack-years in total.
Does stopping reduce the figure?
No. Pack-years measure cumulative past exposure, so the number does not fall. Risk does fall after quitting, substantially and progressively, but that is captured separately in screening criteria through the years-since-quitting condition.
What counts as a pack?
Twenty cigarettes, which is the international convention. Packs sold in some countries contain a different number, so count actual cigarettes rather than packets bought.
Am I eligible for screening?
That depends on your age, pack-years and how long ago you quit, and the criteria differ between countries and have changed over time. Your doctor can confirm what applies to you.
For lung-related measures generally, see the BMI calculator as a broader health baseline. For cardiovascular risk from bloods, see the cholesterol ratio calculator.