Financial Calculators
Rule of 72 Calculator
Divide 72 by the return and you have the doubling time. The oldest mental shortcut in finance.
Your money doubles in about
6 years
by the Rule of 72
- Exact doubling time
- 6.12 years
The Rule of 72 divides 72 by the annual return to estimate doubling time. It is a mental shortcut; the exact figure uses logarithms and is shown alongside.
Finance's favourite party trick
Divide 72 by an annual return and you get, almost magically, the years needed for money to double: 12% doubles in about 6 years, 8% in about 9, 6% in about 12. The rule works because of a neat property of logarithms, and it is accurate enough for mental planning across normal return ranges.
It cuts both ways: at 6% inflation, prices double every 12 years too, which is exactly why parked money must earn more than that.
The rule, and the exact version
Approximate: years to double = 72 / rate\nExact: years = ln(2) / ln(1 + rate/100)Worked example: at a 12% return, the Rule of 72 says money doubles in about 6.0 years; the exact figure is 6.12 years. Close enough to plan with, fast enough to do in your head.
Frequently asked questions
How accurate is the rule?
Within a few percent for rates between about 4% and 15%, which covers most real planning. Outside that band, use the exact formula shown alongside.
Can I use it for inflation?
Yes, and you should: 72 divided by the inflation rate tells you how quickly prices double and idle cash halves in purchasing power.
What about tripling?
A sister shortcut: the Rule of 114 estimates tripling time the same way.
Why 72 and not 70?
72 divides cleanly by many common rates (6, 8, 9, 12), making the mental arithmetic easy; 69.3 would be mathematically purest but useless at a dinner table.
Doubling is a schedule, not a dream.
Pick the rate, know the date. We will help you build the portfolio behind it.