Smoking history
Pack-years calculator
The one number that sums up how much someone has smoked, worked out in your browser.
Written by Lee H. Last reviewed 14 September 2026.
The calculator
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10 pack-years
73,050 cigarettes over 20 years.
Below the 20 pack-years in the US lung cancer screening guideline.
How pack-years work
Divide the cigarettes you smoke a day by 20, then multiply by the years you have smoked. A pack a day for a year is one pack-year. Ten a day for twenty years is ten.
A pack-year always counts a pack as 20 cigarettes. In places where packs of 25 are common, the arithmetic still uses 20, so the number means the same thing to any doctor anywhere.
If your smoking changed over the years, work out each stretch on its own and add them up. Twenty a day for ten years is 10 pack-years, and ten a day for the next ten years is 5 more, so 15 in total.
What the number is used for
Doctors use pack-years because it captures both how much and how long in one figure, and the research on smoking and disease is often reported in it.
The best known use is lung cancer screening. The US Preventive Services Task Force recommends a yearly low-dose CT scan for adults aged 50 to 80 who have at least 20 pack-years and who smoke now or quit within the past 15 years. Other countries set their own rules. If your number is near that line, it is a good question to bring to a doctor.
What pack-years leave out
Pack-years treat a long light habit and a short heavy one as equal when they multiply out the same, and the research says duration matters more than that suggests. They also say nothing about how long ago you stopped, which matters a great deal.
The quit smoking calculator on this site starts from the same two inputs and adds your age, which is what decides how much of the loss comes back when you stop. For ten a day over twenty years, the worked example shows both numbers side by side.
This calculator does arithmetic on the numbers you enter. It does not diagnose, treat or prevent any condition and gives no medical advice, and screening decisions belong with a doctor. No guarantee is made or offered about the accuracy of any figure or calculation on this site. Talk to a doctor or a quitline about stopping smoking. In the US, that is 1-800-QUIT-NOW.
Keep reading
Where to go next
10 a day for 20 years
Ten pack-years in years of life, in money, and in what comes back.
Smoking for 30 years, or 40, or 50
Why more years of smoking costs more, and how much of it stopping returns.
Quit smoking calculator
The years smoking has likely cost you, and the years quitting returns.