Pack Years Calculator
Use this pack years calculator to find your smoking pack-year total and check it against lung cancer screening guidelines.
Updated October 2, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Pack Years Is Calculated
A pack-year is the standard way doctors measure how much someone has smoked over time, combining how many cigarettes were smoked per day with how many years that lasted. It's the dose metric used to decide who qualifies for lung cancer screening.
What Pack Years Measures
Pack-years is a cumulative measure of smoking exposure, not a measure of current lung health or cancer risk by itself. It gives a single number that reflects both intensity (cigarettes per day) and duration (years smoked) of smoking history.
The Formula
Pack-Years = (Cigarettes Smoked Per Day / 20) x Years Smoked
If smoking intensity changed over time, pack-years from each period are calculated separately and added together for a total.
How This Calculator Computes Your Result
Enter cigarettes per day and years smoked at that rate (add more than one period if your smoking history varied). We sum pack-years across all periods entered, then check the total, your age, and how long ago you quit (if applicable) against current screening guidelines.
What Your Result Means
The US Preventive Services Task Force recommends annual lung cancer screening with low-dose CT for adults who meet all three of these criteria¹:
Age 50 to 80
20 or more pack-years
Currently smoke, or quit within the past 15 years
Why Screening Eligibility Depends On More Than Pack-Years Alone
Pack-years captures cumulative dose, but lung cancer risk from smoking also depends on how recently someone quit, risk declines over time after quitting but remains elevated for years. That's why the screening guideline adds the 15-years-since-quitting window on top of the 20 pack-year threshold, someone who quit decades ago with a high pack-year count is treated differently from someone who quit last year¹. This calculator shows pack-years and the screening criteria as separate pieces for this reason, rather than collapsing them into one pass/fail number.
Where the Screening Benefit Was Actually Demonstrated
The 20 pack-year threshold isn't an arbitrary round number, it traces back to the National Lung Screening Trial, a large randomized trial that enrolled current and former heavy smokers and found that annual low-dose CT screening reduced lung cancer mortality by 20 percent compared to chest X-ray over the study period². That trial's enrollment criteria, heavy smoking history and a defined age range, are the direct basis for who current guidelines recommend screening today³.
A Note on Accuracy
Pack-years is a crude dose metric, it does not account for cigarette type, how deeply smoke was inhaled, or other carcinogen exposure. The 20 pack-year, age 50-80, quit-within-15-years criteria reflect current (2021) US screening guidance and may differ in other countries. This is not a personalized cancer risk score or a substitute for medical evaluation.
Frequently Asked Questions
How do you calculate pack-years?
Divide cigarettes smoked per day by 20, then multiply by the number of years smoked. Add the results together if your smoking rate changed over time.
How many pack-years qualify for lung cancer screening?
In the US, 20 or more pack-years is the threshold, combined with being age 50 to 80 and currently smoking or having quit within the past 15 years¹.
Does quitting smoking lower my pack-year count?
No. Pack-years reflects cumulative past exposure and doesn't decrease after quitting, but screening eligibility does take into account how long ago you quit, separately from the pack-year total itself.
Is a high pack-year count a diagnosis?
No. It's a dose metric used mainly to determine screening eligibility, not a diagnosis or an individualized risk score. Talk to a doctor about your personal risk and whether screening is right for you.
Sources
Screening for Lung Cancer: US Preventive Services Task Force Recommendation Statement, US Preventive Services Task Force, JAMA, 2021
Reduced Lung-Cancer Mortality with Low-Dose Computed Tomographic Screening, National Lung Screening Trial Research Team, New England Journal of Medicine, 2011
Lung Cancer Screening (PDQ), Patient Version, National Cancer Institute