The Rule of 72 is the most useful mental maths shortcut in personal finance. It tells you approximately how many years it takes to double your money at any given annual interest rate — without a calculator. The formula is simple: divide 72 by the annual interest rate. At 5%, your money doubles in approximately 14.4 years. At 7%, it doubles in approximately 10.3 years.
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The Rule of 72 — how it worksFormula: Years to double = 72 ÷ annual interest rate (%)At 2% AER: 36 years to doubleAt 3% AER: 24 years to doubleAt 4% AER: 18 years to doubleAt 5% AER: 14.4 years to doubleAt 6% AER: 12 years to doubleAt 7% AER: 10.3 years to doubleAt 10% AER: 7.2 years to doubleAt 12% AER: 6 years to double
The Rule of 72 — full reference table
| Annual rate (%) | Years to double (Rule of 72) | Exact years (compound) | Difference |
|---|---|---|---|
| 1% | 72 years | 69.7 years | 2.3 years |
| 2% | 36 years | 35.0 years | 1.0 years |
| 3% | 24 years | 23.4 years | 0.6 years |
| 4% | 18 years | 17.7 years | 0.3 years |
| 5% | 14.4 years | 14.2 years | 0.2 years |
| 6% | 12 years | 11.9 years | 0.1 years |
| 7% | 10.3 years | 10.2 years | 0.1 years |
| 8% | 9 years | 9.0 years | 0.0 years |
| 10% | 7.2 years | 7.3 years | 0.1 years |
| 12% | 6 years | 6.1 years | 0.1 years |
| 15% | 4.8 years | 4.9 years | 0.1 years |
The Rule of 72 is most accurate between 2% and 10%. Below 2% or above 15% the approximation becomes less precise, but it remains a useful planning shortcut at any rate.
Why the Rule of 72 works
The mathematical basis of the Rule of 72 comes from the natural logarithm of 2 (approximately 0.693), which describes the exact doubling time under continuous compounding. For practical compounding frequencies (monthly, annual), 72 is a convenient approximation of 69.3 that is also highly divisible — it divides evenly by 1, 2, 3, 4, 6, 8, 9, 12 and 18, making mental arithmetic easy.
The formula only works for compound interest — not simple interest. With simple interest (which is rare for savings accounts but common for some loans), doubling time is simply 100 divided by the rate.
Rule of 72 applied to UK savings rates in 2026
Here is how the Rule of 72 applies to the rates available to UK savers in April 2026:
| Account type | Typical rate (April 2026) | Years to double £10,000 |
|---|---|---|
| Current account (no interest) | 0% | Never |
| Instant access savings (low rate) | 1.5% AER | 48 years |
| Easy-access savings account | 4.5% AER | 16 years |
| Best easy-access savings | 5.0% AER | 14.4 years |
| 2-year fixed rate bond | 5.3% AER | 13.6 years |
| Stocks and shares ISA (hist. avg) | 7–8% avg | 9–10.3 years |
The difference between leaving money in a low-rate account at 1.5% versus moving to the best easy-access account at 5% AER is the difference between doubling in 48 years versus 14.4 years — a 33-year gap from one simple action.
Using the Rule of 72 in reverse — working out required rate
The Rule of 72 also works backwards. If you want to double your money in a specific number of years, divide 72 by the years to find the required rate:
- Want to double in 10 years? You need approximately 7.2% AER
- Want to double in 15 years? You need approximately 4.8% AER
- Want to double in 20 years? You need approximately 3.6% AER
- Want to double in 30 years? You need approximately 2.4% AER
This is particularly useful for retirement planning. If you have 20 years until retirement and want to double your pension pot, you need to achieve at least 3.6% real annual returns — achievable with a diversified equity portfolio but not with cash savings at current inflation levels.
Find a regulated IFA to advise on the right savings structure in the Kaeltripton Financial Index.
Rule of 72 and inflation — the doubling of prices
The Rule of 72 applies equally to inflation — it tells you how quickly the purchasing power of money halves. At 3% inflation, prices double in 24 years. At 5% inflation, prices double in 14.4 years.
This is why cash savings earning less than the inflation rate represent a real-terms loss. If inflation is 3% and your savings account pays 1.5% AER, your purchasing power is declining at approximately 1.5% per year — and halving in roughly 48 years even while your nominal balance grows.
Rule of 72 applied to debt
The Rule of 72 works equally well for debt. At a credit card rate of 24% APR, unpaid debt doubles in 3 years. At a personal loan rate of 9% APR, it doubles in 8 years. At a mortgage rate of 4.5%, it doubles in 16 years — but only if you make no repayments, which is why mortgages are structured as repayment rather than interest-only by default.
This article is for informational purposes only. All figures are illustrative. Past returns on equity investments are not a reliable indicator of future results.
Frequently asked questions
What is the Rule of 72?
The Rule of 72 is a mental maths shortcut for estimating how long it takes to double your money at a given annual interest rate. Divide 72 by the annual interest rate (as a percentage) to get the approximate number of years. At 6%, for example: 72 ÷ 6 = 12 years to double.
How accurate is the Rule of 72?
Very accurate for rates between 2% and 10%. The error is typically less than 0.3 years in this range. At 8%, the Rule of 72 gives exactly 9 years — matching the precise compound interest calculation. The rule becomes less accurate at very high or very low rates.
Does the Rule of 72 work for investments?
Yes, provided the investment generates a consistent annual return. For cash savings with a fixed AER, the rule is highly accurate. For equity investments with variable annual returns, the rule gives a rough guide based on the assumed average annual return — actual doubling time will vary.
How long does it take to double £10,000 at 5%?
Using the Rule of 72: 72 ÷ 5 = 14.4 years. The precise calculation gives 14.2 years. At the end of that period, £10,000 has grown to approximately £20,000 through compound interest alone, with no additional contributions.
Sources and verification
- Exact doubling times calculated using: t = ln(2) / ln(1 + r/n) with monthly compounding
- Moneyfacts — UK savings rates, April 2026
- Bank of England — base rate 3.75%, March 2026
- ONS — UK CPI inflation data, March 2026
