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How it works

How the estimate is calculated

The model behind the number, in the open: the sources, the math and where it runs out.

Written by Lee H. Last reviewed 4 August 2026.

Where the baseline comes from

The starting point is the US Social Security Administration's 2023 period life table, the same table used in the 2026 Trustees Report. Given an age and a sex, it returns the average number of years someone at that age is expected to live. Add that to the current age and the result is a baseline life expectancy: the average for someone of that age and sex in the general population, nothing more specific than that.

That table includes smokers and non-smokers together, because it describes the whole population. The app calls this figure "average life expectancy for someone your age and sex," which is exactly what it is, and never "what you would have had if you had never smoked." Starting from the general-population figure slightly understates the true never-smoker baseline. That is a conservative error, and it is the direction we would rather be wrong in.

Every other country

Outside the United States, the baseline comes from the World Health Organization's Global Health Estimates 2019 life tables, indicator LIFE_0000000035, covering 185 member states by age and sex. The 2019 tables are used because 2020 and 2021 carry COVID-19 excess mortality, which would lower every baseline for reasons that have nothing to do with smoking. The United States keeps the newer, single-year SSA table above, the one every worked example on this page is pinned to.

WHO publishes remaining life expectancy only at the start of each age group (0, 1, 5, 10, and on in five-year steps to 85), where the calculator needs a value at every whole age. Two rules turn one into the other:

  • Up to 85, a straight line between the two nearest published ages. Remaining life expectancy falls in almost a straight line across five years, so this reproduces every value WHO actually published exactly, and comes within a few hundredths of a year of any smoother curve in between.
  • Past 85, the SSA table's own shape, scaled to that country's value at 85. WHO's oldest group is open ended, so there is nothing published to interpolate against. The US table has single years all the way to 119, so its curve is carried forward, scaled so the two tables agree exactly at 85.

The same honesty rule applies to every country, word for word: each WHO table is a general-population table that includes smokers, so it is "average life expectancy for someone your age and sex in that country." The minutes-per-cigarette rate below and the recovery ceiling both come from the UK and US cohorts described on this page, and the model applies them in every country, because they are the best published figures available.

How the cost of smoking is applied

The per-cigarette cost comes from Jackson, Jarvis and West, "The price of a cigarette: 20 minutes of life?", Addiction, 2025, a University College London analysis commissioned by the UK Department of Health and Social Care: an average of 17 minutes of life per cigarette for men and 22 minutes for women (19.5, the unweighted average of the two, for anyone who prefers not to say). Multiply cigarettes per day by 365.25 by years smoked to get a lifetime total, multiply that by the minutes-per-cigarette figure, and convert to years.

Extended in a straight line, that math overshoots at the extremes: the 20-minute figure was itself derived from lifetime smokers losing roughly 10 years on average, but a straight line through 20 cigarettes a day for 50 years comes to about 11.8 years, past the study's own headline number. So the model stays linear through the range that covers almost every real user, up to 8 years of straight-line loss, and only above that softens the curve toward a ceiling of 12 years, so the two never disagree at the extreme. Below the 8-year mark, which covers the great majority of people, the figure is the plain multiplication above with nothing smoothed.

How the benefit of quitting is applied

The benefit of quitting comes from Jha et al., "21st-Century Hazards of Smoking and Benefits of Cessation in the United States", New England Journal of Medicine, 2013, which tracked how much of a smoker's excess mortality risk is avoided depending on the age they quit: essentially all of it below 35, roughly 90% quitting between 35 and 44, 60% between 45 and 54, and 40% between 55 and 64. The app moves smoothly between those bands, so nothing jumps at a birthday.

That recoverable share does not arrive all at once. It grows on a curve that is fast at first and slows over time, roughly half of it within about 3 years and nearly all of it by 15, matching milestones from the wider cessation literature: coronary risk roughly halved within a year of quitting, stroke risk approaching a never-smoker's within about 5, and coronary risk converging with a never-smoker's by about 15. The specific curve used to model that shape is a choice made for a clear, explainable estimate. Its overall shape is anchored to that evidence; the exact mathematical curve is not itself a number published in any single study.

A worked example

Take a 40-year-old man who has smoked 15 cigarettes a day for 20 years, and quits today.

  • Baseline life expectancy: the table gives a 40-year-old man 38.59 remaining years, so 40 + 38.59 = 78.59.
  • Lifetime cigarettes: 15 × 365.25 × 20 = 109,575.
  • Years lost to smoking so far: 109,575 cigarettes at 17 minutes each, converted to years, is 3.54. That is below the 8-year point where the curve would start to soften, so nothing is capped.
  • Share of that recoverable by quitting at 40: 0.885, just under the band figure of 0.90. Age 40 sits just past the midpoint of the 35 to 44 band, so interpolation toward the next band (45 to 54, 0.60) has already begun.
  • Years recoverable if he stays quit indefinitely: 3.54 × 0.885 = 3.13.
  • Forecast on the day he quits: 78.59 minus 3.54, with nothing recovered yet, = 75.05.
  • Forecast after 5 years smoke-free: recovery is front-loaded, so most of the 3.13 recoverable years have already accrued: 77.19.
  • Forecast if he stays quit forever: approaches 78.18, the full 3.13 years recovered, and still just under the 78.59 baseline. The forecast can approach the population average. It can never pass it.

What this cannot tell you

Three limits, stated plainly:

  • The baseline is not a never-smoker figure. The SSA table and every WHO table alike describe the whole general population, smokers included. Each is the closest well-documented public figure available for its country, but none of them is the life expectancy of someone who never smoked, which would be somewhat higher. If anything, the app's baseline is a slight underestimate of what quitting is really worth.
  • Anything past age 64 is extrapolation. Jha et al.'s published recovery bands stop at age 64. The app continues the same slope beyond that and floors it at a fifth of the maximum recoverable share, so quitting at 70 and quitting at 90 are treated the same. That is a reasonable way to extend an unfinished curve, but it is not something the study measured, and this page will not present it as if it were.
  • The exact recovery curve is this site's modeling choice. The milestones it is anchored to (risk roughly halved by year one, converging with a never-smoker's by year fifteen) come from the literature. The specific mathematical shape connecting those milestones was chosen by this site for a clear estimate.

The figure this model produces is a motivational estimate built from population averages. It is not a prediction about any one person, it does not diagnose, treat or prevent any condition, and it is not medical advice. No guarantee is made or offered about the accuracy of any figure or calculation on this site. Talk to a doctor or a quitline about stopping smoking. In the US, that is 1-800-QUIT-NOW.

Sources

Run it yourself

The same model, in your browser

Everything above is what the calculators do. They give the same numbers the app gives.

Quit smoking calculator

Years lost so far, and the years quitting returns, for your own history.

Life expectancy calculator

The same forecast at one, five and ten years out, and where it stops rising.

Where the ten years figure comes from

The CDC statement, the Surgeon General's report, and the paper underneath both.

The guides

The questions people ask about these numbers, answered one at a time.

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