How the estimate is calculated
The model behind the number, in the open: the sources, the arithmetic 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.
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: 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 arithmetic 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 interpolates smoothly between those bands rather than jumping 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, not a clean 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, because a page that hides them is worth less than one that does not:
- The baseline is not a never-smoker figure. The Social Security life table describes the whole general population, smokers included. It is the closest well-documented public figure available, but it is not 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, not a finding. 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 a modelling choice, not a published one. 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 for a clear estimate, not lifted from a single study.
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. Talk to a doctor or a quitline about stopping smoking. In the US, that is 1-800-QUIT-NOW.
Sources
- US Social Security Administration, 2023 period life table, Social Security area population.
- Jackson, S. E., Jarvis, M. J. and West, R., "The price of a cigarette: 20 minutes of life?", Addiction, 2025.
- Jha, P. et al., "21st-Century Hazards of Smoking and Benefits of Cessation in the United States", New England Journal of Medicine, 2013.