Doubling Time in Real Life: 5 Examples

Savings account

A savings account earning 4 percent compounded annually doubles in just under 18 years using the exact formula. The Rule of 70 says 17.5 years and the Rule of 72 says 18. Use this to decide whether a higher rate is worth the friction of switching banks. For more financial scenarios, see Doubling Time in Finance.

Startup growth

A startup that grows its user base 12 percent per month doubles in about 6 months. That is the speed founders chase, but it also explains why growth at that rate is unsustainable for long: in two years the user base would be over 16 times larger than it started.

Population

A city growing at 1.5 percent per year doubles in about 47 years. Long-range city planning lives or dies on those numbers, which is why public sector forecasts care so much about a single percentage point. See Population Doubling Time for more.

Bacterial culture

Some bacteria double every 20 minutes. That is roughly a continuous growth rate of 2.08 per hour, which is why a single cell can become a visible colony in less than a day.

Bandwidth use

If a service sees usage climb 25 percent per quarter, its bandwidth doubles in roughly 3.1 quarters, or about 9 months. Capacity planning that ignores this number tends to discover it on a Friday night.

For the math behind these results, see The Doubling Time Formula.