- Home
- Finance & Loans
- Rule of 72 Calculator
Rule of 72 Calculator
The Rule of 72 is a mental shortcut: divide 72 by the annual rate of return to estimate how many years it takes money to double. Enter a rate to see the estimate next to the exact answer, plus the time to triple.
- Exact years to double
- 11.9 years
- Doubled amount
- $20,000.00
- Years to triple (Rule of 114)
- 19 years
Saved setups
Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.
Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Enter the yearly rate of return or interest rate.
- Read the Rule of 72 estimate and compare it with the exact figure.
- Flip it around in your head: 72 ÷ years gives the rate needed to double in that time.
Worked examples
Money growing at 6% a year
- Years to double (Rule of 72)
- 12 years
- Exact years to double
- 11.9 years
- Doubled amount
- $20,000.00
- Years to triple (Rule of 114)
- 19 years
Money growing at 9% a year
- Years to double (Rule of 72)
- 8 years
- Exact years to double
- 8.04 years
- Doubled amount
- $10,000.00
- Years to triple (Rule of 114)
- 12.67 years
How accurate is it?
Very, for everyday rates. It is nearly exact around 8% (9 years versus 9.01) and close on either side: at 4% it says 18 years against an exact 17.67, and at 12% it says 6 against 6.12. At very low rates 70 is a slightly better numerator — at 2% the exact answer is 35.0 years — and for continuous compounding 69.3 is exact.
It works for anything that compounds
Inflation at 3% halves the buying power of cash in about 24 years. A credit card balance at 24% doubles in about 3 years if nothing is paid. An economy growing 2% a year doubles in about 36 years.
Questions people ask
How long does it take to double your money at 6%?
About 12 years by the Rule of 72 (72 ÷ 6). The exact answer is 11.90 years.
What return do I need to double my money in 10 years?
About 7.2% a year, since 72 ÷ 10 = 7.2. The exact figure is 7.18%.
Why 72 and not another number?
The mathematically exact numerator for small rates is 69.3 (100 × ln 2). 72 is used because it is close, compensates for annual compounding at typical rates, and divides evenly by 2, 3, 4, 6, 8, 9 and 12.