Step 1. The actuarial baseline
For each country and sex, the model builds a mortality curve using the Gompertz–Makeham law, the same family of model that actuaries have used for over a century. The force of mortality at age x is modelled as μ(x) = A + B·e^(b·x), where A is a small background risk (accidents, violence, higher for men), b sets how fast risk rises with age (human mortality roughly doubles every 8.5 years), and B is the overall level.
The model solves for B separately for every country and sex so that the model reproduces that population's published life expectancy. The baselines are calibrated to the WHO Global Health Observatory life tables (2000–2021 series, the current release) and to the United Nations World Population Prospects 2024 revision, which remains current until 2027.
Step 2, Remaining life expectancy (the key fix)
This is where most novelty calculators go wrong. They quote your country's life expectancy at birth and treat it as your age at death. That badly underestimates older people, because it ignores survivorship, the fact that simply by reaching your current age, you have already outlived every risk that claimed people younger than you.
Instead, the model integrates the survival curve from your current age onward to compute your conditional remaining life expectancy, then add it to your age. A 70-year-old is not expected to die at the national birth average; they are expected to live well beyond it. Reaching old age is itself strong evidence you will reach older age.
Step 3, Lifestyle modifiers
To that baseline the model adds or subtracts years for the factors you entered. Each modifier is grounded in published cohort or meta-analytic data, not invented:
Smoking uses a saturating curve anchored to Doll & Peto, a lifelong pack-a-day smoker loses about 10 years, scaled by pack-years rather than a naive linear rule. BMI follows the 2016 Lancet Global BMI Mortality Collaboration, with the lowest risk near BMI 22–24 and rising losses through the obese ranges. Physical activity, diet, alcohol, outlook, and social factors use effect sizes from PREDIMED, the WHO activity guidelines, the 2018 Lancet alcohol study, and related work cited in the sources section.
Step 4. The What-If explorer
Quitting a harmful habit does not recover the same number of years at every age. The what-if gains are age-adjusted: quitting smoking at 40 recovers most of the lost years, while quitting at 65 recovers far fewer, matching Doll & Peto's cessation findings. The explorer is meant to show direction and rough magnitude, not a guaranteed payout.
Honest limitations
Please read this part. (1) The model treats you as an average member of your category; it cannot see your genetics, medical history, or luck. (2) Modifiers are applied additively and independently, while in reality risk factors interact, so stacked changes are approximate. (3) For countries where life expectancy is heavily shaped by infant and child mortality, the adult survival curve is an approximation and may read slightly pessimistic for healthy adults. (4) A single "country" figure hides large within-country gaps by income and region. (5) The underlying mortality data is itself incomplete. In World Health Statistics 2026, WHO reported that only about one third of countries meet its standards for high-quality mortality data, and that of roughly 61 million deaths worldwide in 2023 only about a third carried cause-of-death information. Baselines for countries with weak civil registration are modelled rather than counted. (6) This is not a substitute for a clinical risk assessment from a physician. Treat the output as a conversation starter, never a verdict.