Calculating Pack Years of Smoking and Health Risks

The term "pack years" is a measure of how much someone has smoked. Since lung cancer is directly related to the number of cigarettes smoked, using pack years can help physicians identify which people have the greatest risk of developing lung cancer. It's not just lung cancer, however, and the number of pack years a person has smoked is helpful in evaluating the risk of heart disease, other lung diseases, and risks related to the other diseases and cancers caused by smoking.

One pack year of smoking would mean that someone had smoked one package of cigarettes (20 cigarettes) daily for one year.

Using pack years helps physicians not only calculate the probable risk of lung cancer but the risk of many other conditions associated with smoking. The number of pack years is also very helpful as an objective measure of the number of cigarettes smoked when looking at studies of smoking and disease.

The number of pack years smoked and risk of disease may have some limitations. For example, some studies suggest that women are more susceptible to carcinogens in cigarettes because women appear to develop lung cancer after fewer pack years of smoking than men.

Lung cancer risk
Illustration by Joshua Seong. © Verywell, 2018. 

Calculating Pack Years

Let's take a look at some examples so that you can calculate the number of pack years you smoked.

 If N stands for the number of packages of cigarettes smoked daily, and T stands for the number of years of smoking, then PY equals the pack years smoked.

The equation looks like this:

N x T = PY

Now let's do a few calculations:

  • Jill smoked 1 pack of cigarettes daily for 20 years. She has a 20 pack year history of smoking. Multiplying N (1 pack) times 20 (years smoked) equals 20 pack years.
  • Frank smoked 2 packs of cigarettes daily for 30 years. Multiplying N (2 packs) by N (30 years,) Frank has a 60 pack year smoking history.
  • Eleanor smoked 10 cigarettes (1/2 pack) per day for 30 years. Multiplying N (0.5 packs per day) by T (30 years,) Eleanor has a 15 pack year history of smoking.

Lung Cancer Risk

In general, the more pack years you have smoked, the greater the chance of getting cancer. When the number of pack years is put on a graph, there is almost a linear relationship between pack-years and cancer. The number of pack-years says more about your risk than the length of time you smoked.

That said, the relationship between pack years of smoking and lung cancer is statistical, and individual people don't always "follow the rules." Lung cancer occurs in never-smokers and in fact, lung cancer in never-smokers is in the top ten leading causes of cancer deaths in the United States. On the other hand, most of us know of someone who was a lifelong heavy smoker and never got lung cancer.

Pack Years, Former Smokers, and Lung Cancer Risk

Using the calculation of pack years is important for those who once smoked but have now quit. Unlike heart disease, the risk of lung cancer persists for a long period of time after a person quits smoking and never returns to normal.

Those who smoke continue to be at risk of lung cancer even decades after quitting. In other words, if you have a 40 pack-year history of smoking, but quit 12 years ago, you are still at risk. You may be eligible for lung cancer screening and should make sure you are aware of the early symptoms of lung cancer.

This persistent risk is easier to understand if you look at the number of people diagnosed today who currently smoke. Around 20 percent of people who died of lung cancer in 2018 never smoked. Since only around 10 to 20 percent of lung cancers occur in people who never smoked, the largest number of people diagnosed with lung cancer today are former smokers.

Quitting smoking does reduce your risk of lung cancer, but the reduction in risk drops off much more slowly than the reduction in heart disease.

Heart Disease Risk

The number of pack years someone has smoked is correlated not only with lung cancer but with heart disease as well. In fact, heart disease accounts for a large percentage of deaths in people who smoke, and secondhand smoke is more likely to lead to heart disease than lung cancer.

Lung Cancer Screening

Recently, doctors have studied the number of pack years of smoking to determine who should be screened for lung cancer. Studies suggest that people who have a 30 pack-year history of smoking, are between the ages of 55 and 80, and continue to smoke or have quit in the past 15 years, are candidates for CT lung cancer screening. Studies using these criteria have found that the mortality rate from lung cancer could be cut by 20 percent if people meeting these criteria undergo screening.


While the number of pack years a person has smoked is a useful tool in determining risk, it is not foolproof. There is some controversy that the duration of smoking may be an important factor to consider, especially in determining lung cancer risk. The age of onset of smoking may play an important role as well, for example, two people with the same calculated risk based on pack years, the one who began smoking at an earlier age may be at greater risk.

A Word From Verywell

Since you are looking for the definition of pack years, you may feel concerned about your smoking history (or that of a family member or friend.) We are here to let you know that even if you smoked in the past there are still things you can do to improve your wellness. If you smoked in the past, make sure to review the criteria for lung cancer screening. If you smoke, it's never too late to quit. And there are always things you can do to lower your lung cancer risk. For ideas on reducing your risk, you can eat superfoods that may reduce lung cancer risk based on solid scientific research.

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