Rate of Natural Increase and Doubling Time

Rate of natural increase, or RNI, measures how fast a population grows from births and deaths alone, leaving migration out entirely. You get it by subtracting the crude death rate from the crude birth rate, both measured per 1,000 people, then converting that difference to a percentage. A birth rate of 32 per 1,000 and a death rate of 9 per 1,000 gives a difference of 23 per 1,000, which is 2.3 percent. That number looks small, but population growth compounds every year, the same way interest compounds in a bank account. A steady 2.3 percent a year does not add 2.3 percent of the original population each year forever; it adds 2.3 percent of a base that keeps getting bigger, so growth accelerates in absolute terms even while the rate stays fixed.

The rule of 70 turns that percentage into something concrete: years until the population doubles. Divide 70 by the annual growth rate. At 2.3 percent, 70 divided by 2.3 gives roughly 30 years, well within one lifetime. That is why even growth rates under 3 percent, which sound modest, can double a country's headcount inside a generation.

RNI only accounts for births and deaths. Add net migration, immigrants minus emigrants, and you get total population growth, which can run higher or lower than RNI alone, or even flip its direction entirely in countries with heavy in- or out-migration. This connects RNI to the demographic transition model: as a country moves through its stages, birth rates fall faster than death rates eventually do, so RNI itself declines over time even in a growing population. That decline in rate is not a decline in size. A country can have a shrinking RNI and still be adding people every year, as long as RNI stays positive, because a positive rate on a larger base still produces more people than the same rate on a smaller one did before.