Pack-Year Calculator
Key takeaways
A pack-year is the number of packs of cigarettes smoked per day multiplied by the number of years smoked. One pack a day for one year is one pack-year. Doctors and screening programs use it to describe smoking history.
How the calculator works
The tool turns your smoking history into one number. Enter how many cigarettes you smoked per day and for how many years. If your habit changed, add a row for each period. The calculator divides cigarettes per day by 20 to convert them to packs and multiplies by the years.
For example, smoking 20 cigarettes a day for 10 years is 1 pack × 10 years = 10 pack-years. Smoking 10 cigarettes a day for 10 years is half a pack × 10 years = 5 pack-years. Smoking 40 cigarettes a day for 5 years is 2 packs × 5 years = 10 pack-years.
What a pack-year means
The US National Cancer Institute defines a pack-year as the number of packs of cigarettes smoked per day multiplied by the number of years the person has smoked. It is a way to describe lifetime exposure in a single figure so clinicians can compare smoking histories.
The measure has limits. It treats a heavy smoker over a short time the same as a light smoker over a long time, and it says nothing about when someone quit.
Why a doctor may ask for it
Doctors record smoking history because it relates to the risk of lung disease and some cancers. It is also used to decide who should be offered lung cancer screening.
In its 2016 recommendation, the Canadian Task Force on Preventive Health Care advised annual low-dose CT screening, up to three times, for adults aged 55 to 74 with at least 30 pack-years who currently smoke or quit less than 15 years ago. The Task Force described this as a weak recommendation based on low-quality evidence. The site of the Task Force states that its mandate ended on 31 March 2026, so check with your province: provincial lung screening programs set their own age ranges and smoking-history criteria, and some use years of smoking rather than pack-years.
Other things to remember
- A cigarette-only calculation does not include cigars, pipes, hookah or vaping.
- If you do not remember exactly, use your best estimate and tell your doctor it is an estimate.
- Quitting stops your total from growing. Learn what changes in the lungs on the smoker’s lungs page, or find free help to quit.
Pack-year calculator
Add one row for each period when your smoking habit was different.
- Your total
- Total years smoked
One pack-year means smoking one pack (20 cigarettes) a day for one year. Your doctor decides what the number means for you.
Health information is general and educational. It does not replace advice from a doctor, pharmacist or nurse. Read the full disclaimer.
Frequently asked questions
How do you calculate pack-years?
Multiply the number of packs smoked per day by the number of years smoked. A pack is 20 cigarettes, so 10 cigarettes a day for 10 years is 5 pack-years.
What if my smoking changed over time?
Calculate each period separately and add the results. The calculator above has a row for each period.
Does a pack-year number tell me if I am at risk?
It is one piece of information. Screening programs combine it with age, how long ago you quit and other factors, and each province sets its own rules. Ask your doctor or nurse.
Sources
- Pack year (NCI definition, SEER glossary copy) — US National Cancer Institute (SEER). Accessed October 27, 2026.
- Recommendations on screening for lung cancer — Canadian Task Force on Preventive Health Care (2016 guideline). Accessed October 27, 2026.