Toolstia / Rule of 72 calculator
Rule of 72 calculator
The rule of 72 is a mental shortcut. Divide 72 by the yearly return to guess how many years it takes to double your money. At 8% a year, it is about 9 years. This tool applies the rule and also shows the exact answer.
Enter a number above zero.
The rule of 72 is a quick mental shortcut: divide 72 by the yearly rate. It is close for rates between about 4% and 15%.
How it works
- Choose what you want to find.
- Enter the annual return, or the years.
- Compare the rule of 72 answer with the exact one.
How it works
Doubling time in years is roughly 72 divided by the percentage rate. To find the rate needed to double in a set time, divide 72 by the years.
Exact answer
The exact doubling time with yearly compounding is ln 2 divided by ln of 1 plus the rate. The tool shows both so you can see how close the shortcut is.
How accurate
The rule is best for rates from about 6% to 10%. At very low or very high rates it drifts. Some people use 69 or 70 for continuous compounding.
Inflation too
Use it for prices as well. At 3% inflation, prices double in about 24 years, which halves what money can buy.
Questions
Why 72?
It divides evenly by many numbers, which makes mental maths easy, and it fits common rates well.
Does it include contributions?
No. It is for a single lump sum. Use the compound interest calculator for regular saving.
Is anything uploaded?
No. Your numbers stays in your browser and is never sent anywhere.