Estimate how many years it takes for money to double at a given rate of return, or the return you need to double it in a set number of years, using the Rule of 72.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut for compound growth. Divide 72 by the annual rate of return to estimate how many years it takes to double your money.
Years to double ≈ 72 ÷ annual rate (%)
Rate needed ≈ 72 ÷ years
Worked example
At 8% a year, money doubles in about 72 ÷ 8 = 9 years. The exact answer is 9.006 years, so the shortcut is almost perfect. At 8%, $10,000 becomes about $20,000 in 9 years, $40,000 in 18 and $80,000 in 27.
How accurate is it?
The rule is most accurate for rates between about 6% and 10%. At very low or very high rates it drifts a little. The exact formula is:
Years = ln(2) ÷ ln(1 + rate)
The calculator shows both, so you can see the difference.
Using the rule on costs, too
- Inflation: at 3% inflation, prices double in about 24 years, so money sitting in cash loses half its buying power in that time.
- Debt: a credit card balance at 24% doubles in about 3 years if you do not pay it down.
- Fees: a 2% yearly fee compounds too. Over about 36 years it can leave you with roughly half the balance you would have had without it.
Frequently asked questions
Why 72 and not 70?
72 divides evenly by many common rates (2, 3, 4, 6, 8, 9, 12), which makes mental math easy. The Rule of 70 is slightly more accurate for low rates such as inflation.
Does it work with monthly contributions?
No. It applies to a single lump sum. For regular deposits, use the compound interest calculator.
Related tools
Results are estimates for planning and education only. They are not financial, tax, legal or medical advice. Check important numbers with a qualified professional.