Rule of 72 calculator
The mental shortcut everyone knows, next to the exact answer. And how much the shortcut is off by at your rate.
Your figures
Result
- In years, exactly
- 10.24
- What the rule of 72 says
- 10.29
- How far off the rule is (years)
- 0.04
- Your money, doubled
- $20,000
- Time to triple (years)
- 16.2
- Time to grow tenfold (years)
- 34.0
Milestones
| Milestone | Years | Amount |
|---|---|---|
| Double it (x2) | 10.2 | $20,000 |
| Triple it (x3) | 16.2 | $30,000 |
| Ten times over | 34.0 | $100,000 |
What we assume
- The return is effective annual and assumed constant, which no real investment is.
- No tax, fees or inflation are deducted.
- The rule of 72 is an approximation. The exact figure uses logarithms and is the one to trust.
How it is calculated
The rule of 72 says that dividing 72 by the annual percentage return gives roughly the number of years it takes to double:
years ≈ 72 / rate
The exact answer comes from logarithms:
years = ln(2) / ln(1 + rate)
Why 72 and not 69
Mathematically the cleanest constant is about 69.3, but 72 divides neatly by 2, 3, 4, 6, 8, 9 and 12, which is what makes it usable in your head. The trade-off is accuracy: it is very good between roughly 5% and 12%, and drifts outside that.
What it is actually for
Not precision. It is for noticing, without a calculator, that the difference between 5% and 7% is not small: 14 years to double against 10. Over a working life that gap is enormous.
An example
At 7% a year, the rule says 72 / 7 = 10.3 years. The exact figure is 10.24 years, so the shortcut is off by under a month. Tripling takes 16.2 years and growing tenfold takes 34.
Frequently asked questions
How accurate is the rule of 72?
Very good between about 5% and 12%, where it is off by a fraction of a year. Outside that range the error grows, which is why we show the exact figure next to it.
Does it work for inflation?
Yes, and it is sobering: at 3% inflation, prices double in 24 years. The same maths applies to anything growing at a constant rate.
What about a negative return?
Then the money never doubles, it halves. The rule works for halving too: 72 divided by the annual loss gives roughly how long until you have half left.
Keep calculating
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- How much to save each month Say what you want and when. We work out the monthly amount, and how much of it the return contributes instead of you.
- Rental yield calculator Gross yield is the number in the advert. Net yield, after costs and empty months, is the one you actually get.
- Simple interest calculator Simple interest always pays on the original capital and never on what you have already earned. Here it sits next to compound, which is where the difference shows.
- CAGR calculator Two figures and a term give the constant rate that would have taken the first to the second. It is the honest way to compare investments of different lengths.
- ROI calculator ROI is a simple division that gets misused constantly: 50% in six months and 50% in ten years give the same figure. So here it comes annualised too.
Updated on 2026-08-26. Calculations run in your browser; nothing you type is sent to a server.