Rule of 72 Calculator
The Rule of 72 is a quick mental maths shortcut: divide 72 by an annual interest rate to estimate how many years it takes to double your money. It also works in reverse — divide 72 by your target doubling time to find the required rate.
Rule of 72
Reference table: rates 1%–20%
| Annual rate | Years to double (Rule of 72) | Exact years (ln(2)/ln(1+r)) |
|---|
Related: Compound Growth · CAGR
How the Rule of 72 Calculator works
The Rule of 72 is a classic mental maths shortcut used by investors, financial advisers, and anyone wanting a quick sense of how compound interest works. Divide 72 by an annual interest or growth rate and you get a close approximation of the number of years required to double a sum of money. It works in reverse too: divide 72 by the number of years to find the rate needed to double your money in that time.
For UK savers and investors, the rule provides a quick sanity check. At a 4% savings rate it takes about 18 years to double your money. At a 7% investment return it takes roughly 10 years. The rule also highlights the damaging effect of high-interest debt: a credit card at 20% APR will double the amount owed in just over 3.5 years if left unpaid. This calculator also shows the exact logarithmic answer alongside the Rule of 72 estimate so you can see how accurate the approximation is for any given rate.
Frequently asked questions
What is the Rule of 72?
The Rule of 72 is a quick mental maths shortcut for estimating how long it takes to double your money at a given compound interest rate. Simply divide 72 by the annual interest rate percentage. For example, at 6% per year, your money doubles in approximately 72 / 6 = 12 years. It also works in reverse: divide 72 by the number of years to find the approximate rate needed to double your money.
How accurate is the Rule of 72?
The Rule of 72 is most accurate for interest rates in the range of 6–10% per year. At these rates, the estimate is typically within a fraction of a year of the mathematically exact answer (which uses the logarithm formula: ln(2) / ln(1 + r)). At very low rates (below 3%) or very high rates (above 20%), the approximation becomes less precise, but it remains a useful rule of thumb.
Can the Rule of 72 be applied to debt?
Yes. The Rule of 72 applies equally to debt as to investments. If you have a credit card charging 18% APR and make no payments, the amount you owe will roughly double in 72 / 18 = 4 years. This makes the rule a powerful illustration of how quickly high-interest debt can compound, which is why paying off expensive debt is often the highest-return financial decision a UK consumer can make.
What interest rate doubles money in 10 years?
Using the Rule of 72, you need an annual rate of approximately 72 / 10 = 7.2% to double your money in 10 years. The exact answer from the logarithm formula is approximately 7.18% per year. This is broadly consistent with long-run average returns from UK equity markets, though actual market returns vary significantly year to year.