Doubling time
How many years it would take a population to double in size, if it kept growing at its current rate.
In short
Doubling time is CAGR turned into something easier to picture. A growth rate on its own is abstract; "this country will double in 23 years" is not.
How it's calculated
The exact answer is ln(2) divided by the growth rate. In practice, most people use a shortcut called the rule of 70: divide 70 by the annual growth rate in percent. The rule is an approximation — it is accurate to within a few percent for growth rates up to about 10%, which covers all realistic population growth rates.
Typical values
Why it matters here
A ridgeline that looks similar in height can be growing at very different speeds. Doubling time makes that difference concrete: two countries with the same 2023 population can be 20 years or 100 years away from twice that size. It also explains why the world curve looks flat for so long and then turns sharply — doubling time has been shrinking for most of modern history.
At the peak of the 20th-century population boom, around 1963 to 1968, the world was growing at about 2.1% per year — a doubling time of just 33 years. Today global growth is around 0.9%, implying a doubling time of about 78 years. For many high-income countries the growth rate is already negative, so doubling time no longer applies at all. The UN World Population Prospects publishes annual growth rates by country from 1950 onward.
Why small rates compound quickly
Doubling time reveals how deceptive small growth rates can be. A steady 1% a year sounds modest, but over 700 years it results in a population more than a thousand times its starting size. This compounding effect is why populations that maintained even modest growth over centuries eventually produced the rapid rise visible on the canvas after 1800 — and why even a small permanent reduction in fertility has such a large long-run effect on population size.
Frequently asked questions
What is population doubling time?
Population doubling time is the number of years it would take a population to double in size if it continued growing at its current annual rate. It is calculated by dividing 70 (or more precisely ln(2) × 100) by the percentage growth rate.
What is the rule of 70?
The rule of 70 is a quick approximation: divide 70 by the annual percentage growth rate to get the approximate doubling time in years. For example, a 2% growth rate gives roughly 35 years. It works because ln(2) ≈ 0.693, and 70 is a round number close to 69.3.
How fast did world population double in the 20th century?
World population doubled roughly every 35 to 45 years during peak growth in the mid-20th century, when annual growth rates reached about 2%. The world population grew from around 2.5 billion in 1950 to 8 billion by 2022 — more than tripling in one lifetime. Our World in Data's population growth page charts this full arc from prehistory to today.
What happens to doubling time when a population is shrinking?
When a population is shrinking, doubling time becomes meaningless. Demographers talk instead about halving time — how long before the population is half its current size. Countries like Japan and several Eastern European nations are already in this territory. Our World in Data maps current population growth rates, making it easy to see which countries are already in negative territory.