Pack Year Calculator
Calculate your smoking history in pack-years, the standard measure doctors use to estimate lifetime tobacco exposure. If your smoking changed over time, add up to three periods. Add your age to see whether lung cancer screening guidelines may apply to you.
What is a pack-year?
A pack-year is a way of measuring how much a person has smoked over their lifetime. One pack-year equals smoking one pack of 20 cigarettes a day for one year. The same total can be reached in different ways: 20 cigarettes a day for 10 years and 10 cigarettes a day for 20 years both equal 10 pack-years. Doctors use pack-years because the risk of smoking-related diseases rises with total lifetime exposure.
Pack-year formula
Pack-years = (cigarettes per day ÷ 20) × years smoked
Examples: Someone who has smoked 10 cigarettes a day for 15 years has (10 ÷ 20) × 15 = 7.5 pack-years. Someone who smoked 20 a day for 20 years and then cut down to 10 a day for 10 more years has 20 + 5 = 25 pack-years. If your smoking changed over time, add each period together, which this calculator does for you.
Why do doctors use pack-years?
Pack-years give doctors a quick, standard number for your tobacco exposure. They help estimate your risk of conditions such as lung cancer, chronic obstructive pulmonary disease (COPD), heart disease and stroke, and they are used to decide who should be offered lung cancer screening. The higher the number, the higher the risk, although even light smoking is harmful.
Lung cancer screening guidelines
The US Preventive Services Task Force recommends a yearly low-dose CT scan for adults who meet all of these:
| Criterion | Requirement |
|---|---|
| Age | 50 to 80 years |
| Smoking history | 20 pack-years or more |
| Smoking status | Currently smoke, or quit within the past 15 years |
Screening can find lung cancer early, when it is more treatable. Guidelines differ between countries, so ask your doctor what applies where you live.
Pack-years and health risk
There is no safe level of smoking, but risk rises with pack-years. People with 20 or more pack-years have a much higher risk of lung cancer and COPD than people who have never smoked. Other forms of tobacco, such as bidi, shisha (hookah) and chewing tobacco, also carry serious health risks, but they are not measured accurately by the pack-year formula, which is designed for cigarettes. Quitting at any age lowers your risk over time.
Benefits of quitting smoking
Within weeks of quitting, circulation and breathing improve. After about a year, the extra risk of heart disease is roughly halved compared with someone who still smokes. Over 10–15 years, the risk of lung cancer falls to about half that of a current smoker. Support from a doctor, stop-smoking programmes and medicines such as nicotine replacement therapy greatly improve your chances of quitting for good. Staying active also helps; see our Calories Burned Calculator and BMI Calculator to track healthy changes.
Frequently Asked Questions
How do I calculate pack-years?
Divide the number of cigarettes you smoke per day by 20, then multiply by the number of years you have smoked. For example, 15 cigarettes a day for 20 years is (15 ÷ 20) × 20 = 15 pack-years.
What is considered a heavy smoking history?
A history of 20 pack-years or more is generally considered heavy and is the threshold used for lung cancer screening in the United States.
Do pack-years go down after I quit?
No. Pack-years measure your total lifetime exposure, so the number stays the same after you quit. However, your actual health risk does fall the longer you stay smoke-free.
What if my smoking changed over the years?
Calculate pack-years separately for each period and add them together. This calculator lets you enter up to three different periods.
Who should get lung cancer screening?
In the US, yearly screening with a low-dose CT scan is recommended for people aged 50–80 with at least 20 pack-years who currently smoke or quit within the past 15 years. Ask your doctor about the guidelines in your country.