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The short answer
Divide 72 by the yearly compound rate to estimate how many years money takes to double: at 12% a year, about six years. The same shortcut on inflation shows how fast cash loses value; at September 2026's 14.63% inflation, prices would roughly double in under five years, so compare any return with inflation first.
The formula
The rule of 72 is a quick way to estimate how long money takes to double at a steady compound rate: divide 72 by the yearly percentage rate. At 12% a year, 72 ÷ 12 = 6, so money doubles in about six years. It works because of how compound growth behaves, and it is close enough for planning at the rates most savers see. It assumes returns are reinvested and the rate stays the same, which rarely happens exactly.
- Years to double
- 72 ÷ yearly rate (%)
- Rate needed to double in a set time
- 72 ÷ number of years
- Years for inflation to halve buying power
- 72 ÷ inflation rate (%)
Labelled examples in naira
Each row starts with an example ₦1,000,000. The exact column uses the compound formula, years = log 2 ÷ log(1 + rate), so you can see how close the shortcut is.
| Yearly rate (example) | Rule of 72 | Exact | ₦1,000,000 becomes |
|---|---|---|---|
| 5% | 14.4 years | 14.2 years | ₦2,000,000 |
| 10% | 7.2 years | 7.3 years | ₦2,000,000 |
| 15.89% | 4.5 years | 4.7 years | ₦2,000,000 |
| 20% | 3.6 years | 3.8 years | ₦2,000,000 |
| 24% | 3.0 years | 3.2 years | ₦2,000,000 |
The 15.89% row uses the 364-day Treasury bill stop rate at the CBN auction of 23 September 2026, as reported by Nairametrics; it is an example of a published rate at one date, not a promise of what you will earn. The rule gets slightly less accurate as rates rise: at 24% it is off by about two months.
Use it on inflation, not just interest
The same rule tells you how fast prices erode savings. The NBS put headline inflation at 14.63% year on year in September 2026 (reported 16 October 2026). 72 ÷ 14.63 is about 4.9 years, so at that pace prices would roughly double, and the buying power of cash that earns nothing would roughly halve, in under five years.
Real returns double much more slowly
What matters for your future spending is the return after inflation. Using the figures above as an example: a 15.89% return with 14.63% inflation is a real return of about 1.1% a year, because 1.1589 ÷ 1.1463 is about 1.011. At 1.1%, 72 ÷ 1.1 is about 65 years to double in real terms. That is the honest picture of many "high" naira returns when inflation is also high. Read more in is your savings beating inflation?.
Working backwards: what rate do you need?
Say you have ₦2,000,000 and want ₦4,000,000 in five years for a business. 72 ÷ 5 = 14.4, so you need about 14.4% a year, compounded, with nothing withdrawn. If you can only find 10%, either add money along the way, extend the timeline to about seven years, or reduce the target. For a step-by-step method with monthly additions, see how much to save each month for a goal and compound interest explained.
The rule of 72 and saving every month
Most people do not invest one lump sum and wait; they add money every month. The rule of 72 does not handle that directly, but it still helps you think. Each monthly deposit has its own doubling clock: money added in year one has the longest to grow, money added in the final year barely grows at all. That is why starting earlier matters more than any single high-rate year. Two examples at the same 10% rate: ₦500,000 saved at age 30 doubles roughly every 7.2 years, so by about 44 it is near ₦2,000,000 and by about 51 near ₦4,000,000. The same ₦500,000 saved at 44 reaches only about ₦1,000,000 by 51. All figures are illustrations at a constant assumed rate.
Where the rule breaks down
- Simple interest: if interest is not reinvested, money takes longer to double. At 10% simple interest it takes exactly ten years.
- Changing rates: Treasury bill and savings rates change often, so a doubling time based on today's rate is only an estimate.
- Fees and tax: management fees and withholding tax reduce the effective rate. Use the rate after costs.
- Losses: the rule assumes steady growth. Shares and property do not grow in a straight line.
- Flat-rate loans: a lender quoting a flat monthly rate is charging more than the same number as a compound annual rate, so do not use the rule on flat rates without converting.
What to check first
- Is the rate yearly, monthly or daily? Convert to yearly before dividing.
- Is it compound, with returns reinvested?
- Is it after fees and tax?
- How does it compare with the latest inflation figure?
Where Pryme fits
Pryme's Savings Boxes show the current rate for each Box in the app before you commit. Apply the rule to that rate after any lock period, and compare it with inflation, to see what a Fixed Box could do for a dated goal.
Questions people ask
How long does it take to double money in Nigeria?
Divide 72 by the yearly compound rate. At 15.89%, the 364-day T-bill stop rate on 23 September 2026, that is about 4.5 years (exactly about 4.7), if the rate held and returns were reinvested.
What is the rule of 72 formula?
Years to double equals 72 divided by the yearly percentage rate. For example, 72 ÷ 8 = 9 years at 8% a year compounded.
Does the rule of 72 work with inflation?
Yes. 72 divided by inflation estimates how long prices take to double. At 14.63% inflation that is about 4.9 years.
Does the rule of 72 work for monthly interest rates?
Convert to a yearly compound rate first. Using a monthly figure directly gives a doubling time in months, and only if interest compounds monthly.
Sources
- Nairametrics: NTB stop rates fall across tenors after CBN rate cut, 24 Sep 2026 · accessed Oct 2026
- Voice of Nigeria: Nigeria's headline inflation drops to 14.63 per cent, 16 Oct 2026 · accessed Oct 2026
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