This simulation runs a cohort-component demographic model: 18 five-year age bands per sex are aged forward using a mortality-derived survival curve, births are added from the total fertility rate applied to women of reproductive age, and net immigration is spread across the working-age bands. Each step redraws the age-sex pyramid and the trend chart, letting you watch a population's shape evolve from expansive to stationary to constrictive.
Eighteen age bands (0–4 up to 85+) for males and females, projected forward one time step at a time. Annual survival is derived from a baseline mortality curve — high in infancy, low through school and working ages, then rising exponentially with age — scaled by an adjustment factor (72/life expectancy)^1.8 so raising life expectancy visibly flattens the older bands. Births equal (TFR ÷ 35 reproductive years) × women aged 15–49 × the time step, split 51.2% male / 48.8% female.
Pick a preset — Rapid growth (TFR 5.0), Ageing society (TFR 1.3, life expectancy 84), Replacement (TFR 2.1) or Baby-boom echo — or set Total fertility rate, Life expectancy, Net immigration and Time step yourself. Choose a starting pyramid shape, then click Project one step (or Auto-play) to advance the population and watch Total population, Median age, Dependency ratio, Growth rate and Shape class update in the stats panel.
The "rule of 70" used for the doubling/halving-time stat (70 ÷ growth rate %) is the same shortcut demographers and economists use for compound growth of any kind. A TFR of about 2.1 is the long-run replacement level in low-mortality countries — exactly the model's own "Replacement" preset — while many European and East Asian countries now sit well below 1.5, driving the ageing, constrictive pyramids the "Ageing society" preset reproduces.
It is the standard demographic technique for projecting a population forward: each age-sex cohort is aged forward by the time step, losing people to age-specific mortality (survival), while new births are added at the bottom from women of childbearing age and net migration is added or removed across the working-age bands. Repeating this step by step traces out how the whole age-sex structure — not just the total population — changes over decades.
Deaths come from an annual survival probability per band, built from a baseline mortality curve (about 4% in the first year of life, roughly 0.15–0.35% through childhood and working ages, then rising exponentially after 50) and adjusted by (72 ÷ life expectancy)^1.8, so a higher life-expectancy slider genuinely lowers mortality at every age. Births equal (total fertility rate ÷ 35) × women aged 15–49 × the time step in years, added to the youngest band split 51.2% male and 48.8% female, the natural sex ratio at birth.
Expansive means a wide base and progressively narrower older bands — a young, fast-growing population typical of high-fertility countries. Stationary means roughly straight sides, indicating births and deaths are close to balance. Constrictive means a narrower base than middle, showing below-replacement fertility and an ageing population. Boom echo starts with a bulge partway up the pyramid, mimicking a country that had a fertility spike a generation ago whose children are now entering the population themselves.
Median age is found by walking up the age bands until the cumulative population reaches half the total, then interpolating within that band. The dependency ratio is the population under 15 plus the population 65 and over, divided by the working-age population aged 15–64, expressed as a percentage — a standard measure economists use to gauge how many dependents each working-age person effectively supports.
A country's pyramid shape is a direct record of its fertility and mortality history: a sustained high total fertility rate produces a wide base and an expansive pyramid (much of sub-Saharan Africa today), a fertility rate near 2.1 that has held steady for decades produces a stationary pyramid, and fertility that has fallen well below 2.1 for a generation or more — as in Japan, Italy, and South Korea — produces a constrictive pyramid with a shrinking base and a growing elderly population. Migration and past baby booms or wars can also leave permanent bulges or dents in the shape.