Pack-Year Calculator — Smoking History & Screening Eligibility

A pack-year is the unit clinicians use to record how much someone has smoked over a lifetime. One pack-year equals smoking one pack — 20 cigarettes — every day for one year. It bundles two things into a single number: how heavily you smoked and for how long.

The formula comes straight from the US National Cancer Institute's definition:

pack-years = (cigarettes per day ÷ 20) × years smoked

Dividing by 20 converts cigarettes into packs; multiplying by years accumulates the exposure. When a smoking history has more than one phase — say, heavier in your twenties and lighter later — each phase is a separate term and you add them:

total = Σ (cig/day ÷ 20) × years, over every period

This calculator keeps full precision for the eligibility test and shows the total rounded to two decimals. It also cross-checks each period a second way — counting every cigarette (cig/day × years × 365.25), converting to packs, then dividing back by 365.25 — which gives the identical answer, confirming the arithmetic.

For screening, the tool applies the US Preventive Services Task Force (USPSTF) 2021 recommendation. It flags a possible eligibility for annual low-dose CT lung screening only when all three conditions hold at once: a total of at least 20 pack-years, an age between 50 and 80, and a status of currently smoking or having quit within the past 15 years. If any condition fails, the tool names the exact one so you understand why. The older National Lung Screening Trial (NLST) used a stricter bar — 30 pack-years, ages 55–74 — which is shown as context because some programmes still reference it. Pack-years are one input among several; a real screening decision belongs with your doctor.

• US National Cancer Institute — Dictionary of Cancer Terms, “pack year”
• USPSTF — Screening for Lung Cancer, final recommendation (JAMA 2021)
• National Lung Screening Trial (NLST) — reduced lung-cancer mortality with low-dose CT (NEJM 2011)

The formula and screening thresholds on this page were last cross-checked against the NCI and USPSTF sources on 2026-07-17. This tool is educational, not a diagnosis, and does not replace advice from a qualified clinician.