Rule of 72 Calculator
Estimate how long it takes to double your investment.
What this calculator does
This calculator applies the rule of 72 — divide 72 by an annual growth rate to estimate how long money takes to double. It returns that approximate figure alongside the mathematically exact doubling time from the logarithmic formula, plus a growth summary showing what a starting amount becomes across successive doublings.
Showing both the shortcut and the exact answer is deliberate. The rule's entire value is that you can do it in your head, so seeing how closely it tracks the precise calculation tells you when to trust the mental version and where it begins to drift.
When to use it
Use it when you want an instant sense of scale rather than a projection. Someone quotes a 7 percent return and you want to know what that means — about ten years to double. An easy-access account pays 4.5 percent, so sixteen years. The point is speed, not precision.
It is particularly good in reverse. If you need to double your money in twelve years, 72 divided by 12 says you need 6 percent, which rules some options in and others out immediately. It is also the sharpest way to show why charges matter: 6 percent doubles in 12 years, 5.5 percent takes 13. For anything involving contributions, withdrawals, or a specific target, use the compound interest calculator instead.
Understanding the inputs
The interest rate is the annual compound growth rate as a percentage. Use an effective annual rate rather than a nominal one — for a savings account that means the AER. The rule assumes annual compounding, so a nominal rate credited monthly doubles marginally faster than it suggests.
Decide too whether you want a nominal or a real answer. Entering 6 percent tells you when the balance doubles; entering 3.4 percent, which is 6 percent net of 2.5 percent inflation, tells you when your purchasing power doubles. For investments held outside an ISA, deducting expected tax first gives a more honest figure. The starting amount is optional and simply scales the growth summary into pounds.
How is this calculated?
Years to Double ≈ 72 / Annual Rate. Precise: ln(2) / ln(1 + r).
A worked example
Enter 6 percent and the rule gives 72 divided by 6, or 12 years. The exact answer is 11.90 years, so the shortcut overstates by about six weeks. Start with £20,000 and the doublings land at roughly £40,000 after 12 years, £80,000 after 24, and £160,000 after 36.
That progression is the argument for starting early. A 25-year-old putting £20,000 into a Stocks and Shares ISA and leaving it untouched until 61 ends near £160,000 without adding another pound. Someone starting the same £20,000 at 37 gets two doublings instead of three and finishes around £80,000. Twelve years of delay costs £80,000, and none of it is explained by saving less.
Limitations and assumptions
The rule assumes one constant annual return, which no market delivers. A fund averaging 6 percent over thirty years will have had individual years of plus 25 and minus 30 percent, and the order of those years matters greatly once you are withdrawing. Past performance does not predict future returns.
It also ignores platform and fund charges, tax outside an ISA, inflation unless you deliberately enter a real rate, and any deposits or withdrawals — so it describes a lump sum left entirely alone. Accuracy degrades outside roughly 4 to 12 percent. Use it for intuition and quick comparisons rather than planning a specific outcome, and treat nothing here as regulated financial advice.
Common Questions
- How does the rule of 72 work?
- Divide 72 by the annual growth rate and you get the approximate years for money to double. At 6 percent that is 12 years, at 8 percent 9 years, at 12 percent 6 years. It is a mental shortcut for compound growth needing no calculator and no formula.
- How accurate is it?
- Very accurate near 8 percent, where it gives 9.00 years against a true 9.01. At 6 percent it gives 12 against 11.90. Accuracy falls at the extremes: at 2 percent it says 36 years when the answer is 35.0, and at 20 percent it says 3.6 against 3.8. Between roughly 4 and 12 percent, treat it as exact.
- Why 72 rather than another number?
- The mathematically pure constant is 69.3, from the natural logarithm of 2. But 72 divides cleanly by 2, 3, 4, 6, 8, 9, and 12, which makes mental arithmetic effortless, and the slight overstatement happens to compensate for annual rather than continuous compounding at typical rates.
- Does it work in reverse?
- Yes, and this is often the more useful direction. Divide 72 by the years you have and you get the return required to double. Need to double in 10 years? You need about 7.2 percent. That immediately tells you whether a Cash ISA can manage it or whether you need equity exposure.
- Can I use it for inflation?
- Yes, and it is the fastest way to see inflation's effect. At 3 percent, prices double in 24 years, so a level annuity buys half as much by then. Some practitioners prefer the rule of 70 for inflation, because typical inflation rates sit at the low end where 72 slightly overstates the period.
- Does it work for debt?
- Uncomfortably well. A credit card balance at 24 percent APR doubles in roughly 3 years if you pay nothing, with the true figure at 3.2 years. An arranged overdraft at 40 percent EAR doubles in under two. The same mathematics that builds wealth at 6 percent destroys it far faster at these rates.
- What about tripling or quadrupling?
- Quadrupling is two doublings, so use 144 divided by the rate. Tripling uses roughly 114. At 6 percent, money doubles in 12 years, quadruples in 24, and multiplies eightfold in 36 — which is the whole argument for starting a pension in your twenties rather than your forties.
- Should I use nominal or real returns?
- Use a real return if purchasing power is what you care about. A 6 percent nominal return with 2.5 percent inflation is about 3.4 percent real, doubling buying power in roughly 21 years rather than 12. The nominal answer tells you when the statement balance doubles; the real one tells you when your wealth genuinely does.
- Where does the rule mislead?
- Anywhere returns are not steady. It assumes one constant compounding rate, so applying it to an equity fund's long-run average conceals that real sequences include years down 30 percent. It also ignores platform and fund charges, tax outside an ISA, and any contributions or withdrawals along the way.