The Pack Years Calculator provides a simple yet effective way to quantify long-term tobacco exposure. Smoking history is a critical factor in evaluating risk for numerous chronic diseases, including lung cancer, chronic obstructive pulmonary disease (COPD), cardiovascular disease, and stroke. Because both the amount and duration of smoking contribute to health risks, pack years combine these elements into one standardized measure.

One pack year is defined as smoking 20 cigarettes (one pack) per day for one year. For example, an individual who smokes one pack daily for 20 years has a 20 pack year history, as does someone who smokes two packs per day for 10 years. This calculation provides a consistent method for comparing smoking histories across patients and populations. 

Clinically, pack year history is used to guide preventive care decisions, such as eligibility for low-dose CT lung cancer screening, and to stratify risk in respiratory and cardiovascular conditions. It is also a valuable tool in research, where smoking exposure is a key variable in population health studies.