How many pack-years have you smoked?

A pack-year is one pack (twenty cigarettes) per day for one year. Half a pack a day for two years is also one pack-year. The unit lets doctors standardize smoking exposure — the way a credit score standardizes borrowers — so they can put you in a risk tier without a long interview.

Your number falls into a tier: low, moderate, or high. Twenty pack-years is the threshold most guidelines use for elevated risk. Thirty or more may qualify you for lung cancer screening. The calculator above gives you the raw number and the tier — the rest is a conversation to have with a doctor.