Rule of 72 Calculator
Find out how long it takes money to double at any rate. Compare the Rule of 72 shortcut with the exact answer.
The Rule of 72 is a mental shortcut. Divide 72 by the rate and you get the years needed to double your money.
At 7% that is about 10 years. At 12% it is 6 years.
How it works
The rule works because ln(2) is about 0.693, and 72 is close to 69.3 while dividing neatly. It is most accurate between 6% and 10%.
The calculator also gives the exact doubling time using logarithms. The table shows both at common rates.
The formula
Exact = ln(2) / ln(1 + r).
Tips
- Use the rule in reverse too. To double in 8 years you need about 9% a year.
- It also shows how inflation halves your buying power. At 3% inflation prices double in 24 years.
Frequently asked questions
Why 72 and not 70?
Both work. 72 divides evenly by more numbers, which makes mental math easier.
How accurate is it?
Within a few months for rates between about 4% and 12%. It drifts at very high or very low rates.
This is not financial advice. Nothing on this site is investment, tax, legal or financial advice of any kind.
The calculators give simplified estimates from the numbers you enter. They ignore taxes, fees, rate changes and real market returns, and they can be wrong.
Do not make financial decisions based on these results. Speak to a licensed financial adviser, accountant or lender before acting, and read the full disclaimer.