Pack-Year Calculator (Smoking History and Lung Screening Eligibility)

1. Pick single or multiple periods. Choose single if your smoking stayed about the same. Choose multiple periods if the amount changed over the years.
2. Enter cigarettes per day and years. For each period, type the average cigarettes smoked per day and how many years you smoked that amount.
3. Add your age and status (optional). Enter your current age and whether you currently or formerly smoked to run the screening-eligibility check. Former smokers can add years since quitting.

This is an educational estimate and an eligibility guideline check. It is not medical advice, a diagnosis, or a screening order.

A pack-year is a way to measure how much a person has smoked over a long period of time. It lets a clinician compare very different smoking patterns using one number.

The National Cancer Institute defines it by multiplying the number of packs of cigarettes smoked per day by the number of years the person has smoked. One pack-year equals smoking one pack per day for one year, or two packs per day for half a year.

Because a standard pack holds 20 cigarettes, packs per day is just cigarettes per day divided by 20. So someone smoking 10 cigarettes a day is smoking half a pack a day, and 10 years of that is 5 pack-years.

Pack-years matter because lung cancer screening guidelines use a pack-year threshold, together with age and smoking recency, to decide who may benefit from a yearly scan. The number is a smoking-history measure, not a diagnosis.

The calculator applies the National Cancer Institute pack-year definition exactly, then sums the result across every smoking period you enter.

This is the same as packs per day times years smoked, because a standard pack is 20 cigarettes. The two forms always give the same answer.

1. Convert to packs per day. Divide cigarettes per day by 20 for each period.
2. Multiply by the years. Multiply that packs-per-day figure by the years smoked in the period.
3. Sum the periods. Add the pack-years from every period to get the lifetime total.

If you use a single period, only the first pair is used. In multiple-periods mode the tool adds up to three periods, which suits a smoker whose daily amount rose or fell over the years.

Here are the two worked examples this tool is tested against.

Single Period

1. Someone smokes 15 cigarettes a day for 20 years.
2. Packs per day = 15 / 20 = 0.75.
3. Pack-years = 0.75 x 20 = 15 pack-years.

Multiple Periods

1. Period A: 20 cigarettes a day for 10 years = (20 / 20) x 10 = 10 pack-years.
2. Period B: 10 cigarettes a day for 10 years = (10 / 20) x 10 = 5 pack-years.
3. Total = 10 + 5 = 15 pack-years.

Both land on 15 pack-years, which is below the 20 pack-year screening threshold. A person who smoked one pack a day for 30 years would reach 30 pack-years, which is above it. All three cases are loaded as presets.

Two major bodies publish lung cancer screening criteria. They agree on age and pack-years and differ on one point, so this tool shows both rather than merging them.

The single difference is the quit window. USPSTF 2021 keeps a 15-year limit since quitting. The American Cancer Society 2023 update removed it, so a former smoker with 20 or more pack-years can qualify under ACS regardless of how long ago they quit.

The USPSTF Stop Rule

USPSTF says screening should be discontinued once a person has not smoked for 15 years, or develops a health problem that substantially limits life expectancy or the ability or willingness to have curative lung surgery.

Both sets of criteria are guideline information. Only a clinician, through shared decision-making, decides whether screening is right for you.

The arithmetic is exact, but the inputs depend on memory and on how you count your smoking history.

Estimating Cigarettes per Day

Daily amounts often changed over the years. Use a realistic average for each period rather than your heaviest or lightest day, and split the history into periods when the amount shifted a lot.

Counting the Years

Count only the years you actually smoked. Long breaks when you did not smoke should not be counted as smoking years.

Pack Size

The formula assumes a 20-cigarette pack. If you count in packs, one pack per day equals 20 cigarettes per day. Roll-ups and other products are not standardised here.

Status and Quit Date

Whether you currently or formerly smoke, and how long ago you quit, change only the screening-eligibility check, not the pack-year total.

Checking Screening Eligibility

The most common use is to see whether your smoking history reaches the 20 pack-year threshold that the USPSTF and ACS use for lung cancer screening, alongside age and recency.

Preparing for an Appointment

Having a pack-year figure ready helps a conversation with a clinician about whether a yearly low-dose CT scan makes sense for you.

Understanding Your Risk Context

Pack-years are one input into smoking-related risk. To see the money side, try our guide to the lifetime cost of smoking, and to think about quitting, our addiction calculator.

Tracking Other Health Numbers

Smoking history sits with other measures worth knowing, such as your BMI and kidney function in our eGFR calculator.

1. Confusing Packs and Cigarettes

Twenty cigarettes a day is one pack a day, not 20 packs. Enter cigarettes per day here, and the tool divides by 20 for you.

2. Using Your Current Rate for All Years

If your amount changed, a single average can mislead. Use the multiple-periods mode to reflect the real history.

3. Counting Non-Smoking Years

Only years you actually smoked count. Including quit years inflates the total.

4. Treating the Eligibility Check as a Diagnosis

Meeting a pack-year threshold is not a diagnosis and does not mean you have lung cancer. It only flags a screening discussion.

5. Assuming the Two Guidelines Agree on Quitting

They do not. A former smoker who quit over 15 years ago can qualify under ACS 2023 but not USPSTF 2021.

6. Acting Without a Clinician

Screening decisions belong to you and your clinician together. Do not start or skip screening based only on this tool.

The calculator reproduces the NCI pack-year formula and the published USPSTF and ACS thresholds exactly. Its limits are in the smoking history you supply and in what a threshold can tell one person.

This is not a diagnosis or a screening order. Discuss lung cancer screening with a clinician who knows your full history.

Divide the cigarettes you smoked per day by 20, then multiply by the years you smoked. That is the same as packs per day times years, because a standard pack is 20 cigarettes. Add the periods together for a lifetime total.

One pack a day is 20 cigarettes a day, so packs per day is 1. One times 20 years equals 20 pack-years, which is the threshold both USPSTF and ACS use for screening.

Both the USPSTF 2021 and the American Cancer Society 2023 guidelines use 20 or more pack-years, together with ages 50 to 80. The older guidelines used 30 pack-years and a narrower age range.

They agree on age 50 to 80 and 20 pack-years and both recommend annual low-dose CT. They differ on quitting: USPSTF 2021 needs you to currently smoke or have quit within 15 years, while ACS 2023 removed that quit-year limit.

Yes. Enter your periods as usual, choose formerly smoked, and add years since quitting. The tool then checks the USPSTF 15-year window; ACS qualifies former smokers with 20 or more pack-years regardless of quit date.

Yes. Choose multiple periods and enter up to three cigarettes-per-day and years pairs. The tool sums the pack-years, which suits a history where your daily amount changed.

No. Pack-years are a smoking-history measure, and the eligibility check is a guideline flag for a screening discussion. It is not a diagnosis and does not mean you have lung cancer.

Yes. Both the USPSTF and the American Cancer Society recommend annual low-dose computed tomography, known as LDCT, for people who meet the screening criteria.

No. This is an educational estimate and a guideline check, not medical advice or a screening order. Screening is a shared decision you make with a clinician who knows your history.

No. The math runs in your browser. The Save button keeps results only in your own browser storage, and nothing is sent to our servers.

• NCI Dictionary of Cancer Terms: pack-year definition (National Cancer Institute; accessed 2026-10-05).
• Lung Cancer: Screening (2021 recommendation, Grade B) (US Preventive Services Task Force; age 50-80, 20 pack-years, quit within 15 years, annual LDCT; accessed 2026-10-05).
• American Cancer Society Lung Cancer Screening Guideline (2023) (American Cancer Society; age 50-80, 20 pack-years, quit-year limit removed; accessed 2026-10-05).