How fast does money double at different interest rates?
At 1% a year, money takes about 70 years to double. At 6%, about 12 years. At 12%, about 6 years. Those are the exact figures behind a mental shortcut called the Rule of 72: divide 72 by the interest rate and you get a close estimate of the doubling time in years. The table below gives both the exact number and the Rule of 72 guess, so you can see how well the shortcut actually holds up.
The doubling table
| Rate | Exact years to double | Rule of 72 estimate |
|---|---|---|
| 1% | 69.7 | 72.0 |
| 2% | 35.0 | 36.0 |
| 3% | 23.4 | 24.0 |
| 4% | 17.7 | 18.0 |
| 6% | 11.9 | 12.0 |
| 8% | 9.0 | 9.0 |
| 9% | 8.0 | 8.0 |
| 12% | 6.1 | 6.0 |
Reading the table
The first thing the table shows is how sharply doubling time falls as the rate rises. Between 1% and 4%, doubling time drops from almost 70 years to under 18 — a difference of more than 50 years for a 3-point change in rate. Between 8% and 12%, the whole range covers under 3 years. Doubling time doesn’t fall in a straight line as rates rise; it falls fast at low rates and then flattens out, because compound interest is multiplicative, not additive.
The second thing to notice is how well the Rule of 72 holds up through the middle of the table. At 6%, 8% and 9%, the estimate lands within a tenth of a year of the true figure — close enough for any practical purpose. It’s really only at the edges that the shortcut drifts: at 1% it overstates the true doubling time by more than two years, and at 12% it understates it by about a month. For most everyday rates, though, dividing 72 by the rate gives you a doubling time you can trust at a glance.
Why doubling is worth internalising
A percentage on its own is easy to shrug off. A doubling time is not — it converts an abstract rate into a timescale you can feel. Saying “2%” sounds harmless; saying “at 2% inflation, prices double in about 35 years” makes the erosion concrete. Flip it around and the same 35 years describes something else too: at 2% inflation, the buying power of money sitting idle is quietly cut in half over roughly the same stretch. A rate that sounds trivial on paper turns out to reshape a saver’s money within a single working lifetime.
The same logic applies to compound interest working in a saver’s favour. A rate that looks unremarkable on a statement — say 6% — is doing something dramatic underneath: it’s turning every sum into double itself roughly every 12 years. Two doublings in 24 years means a sum becomes four times its size; three doublings means eight times its size. The Rule of 72 is what makes that visible without needing a calculator — a quick way to translate any rate into “years until this doubles,” and from there into a sense of how a saving or borrowing decision will actually feel over time.
Try your own numbers
For a rate not shown here, or to see the effect over your own time horizon, use the compound interest calculator to work out exact doubling times and running balances.