Future population depends on fertility, survival, and age structure. This Lab varies expected fertility relative to a reference path and shows how a logistic curve changes through 2100.

UN reference, Lab model

The UN’s WPP 2024 summary gives roughly 8.2 billion people in 2024 and 10.2 billion in 2100 for its medium reference, following a peak in the 2080s. Low and high variants shift fertility by approximately half a birth per woman relative to the medium trajectory. These are assumption-based scenarios, not confidence intervals.

The slider represents that fertility shift, not a constant global fertility rate. The plotted curves are our own teaching model: a simple logistic cannot reproduce the peak and subsequent decline of the official projection.

World population · 2024–2100Illustrative logistic model
Medium reference · Δ = 0
0510152020242040206020802100Billions of people
Population in 2100 (bn)
10.2
Model plateau K (bn)
10.39
Versus medium in 2100 (bn)
0

Solid: selected model. Dashed curves: model presets −0.5, 0 and +0.5. Horizontal dashed line: model plateau. This logistic approximation is not a UN forecast: it cannot reproduce a peak followed by decline. K is a model parameter, not Earth’s carrying capacity.

Annual model values
YearPopulation (billions)
20248.2
20258.26
20268.32
20278.38
20288.43
20298.49
20308.54
20318.59
20328.65
20338.7
20348.74
20358.79
20368.84
20378.88
20388.93
20398.97
20409.01
20419.06
20429.1
20439.14
20449.17
20459.21
20469.25
20479.28
20489.32
20499.35
20509.38
20519.41
20529.44
20539.47
20549.5
20559.53
20569.56
20579.58
20589.61
20599.63
20609.66
20619.68
20629.7
20639.73
20649.75
20659.77
20669.79
20679.81
20689.83
20699.85
20709.86
20719.88
20729.9
20739.91
20749.93
20759.94
20769.96
20779.97
20789.99
207910
208010.01
208110.03
208210.04
208310.05
208410.06
208510.07
208610.08
208710.09
208810.1
208910.11
209010.12
209110.13
209210.14
209310.15
209410.16
209510.16
209610.17
209710.18
209810.19
209910.19
210010.2

ONU · WPP 2024 · Variant methodology

What the slider recalculates

The model trajectory is:

P(t)=K1+(K/P0−1)e−r(t−2024).P(t)=\frac{K}{1+(K/P_0-1)e^{-r(t-2024)}}.

We fix P0=8.2P_0=8.2 and r=0.035r=0.035. The central plateau K0K_0 is calibrated to reach P(2100)=10.2P(2100)=10.2. The control Δf\Delta f changes that plateau using an illustrative sensitivity chosen for this experiment:

K(Δf)=K0 e0.75Δf,−0.5≤Δf≤0.5.K(\Delta f)=K_0\,e^{0.75\Delta f},\qquad -0.5\leq\Delta f\leq0.5.

The coefficient 0.75 is not a UN estimate. The plateau is the model’s mathematical asymptote, not an ecological limit or validated forecast. In the low scenario it can fall below the initial value and cause immediate decline: this model omits age-structure momentum. Explore that mechanism in the fertility and replacement Lab.

Sources: UN, WPP 2024 summary and projection methodology. The simulation runs locally and downloads no data when controls change.