How to use the Rule of 72 Calculator
- 1 Enter the annual rate or expected return, for example 8.
- 2 Optionally enter an amount to see it doubled.
- 3 Read the years to double from the Rule of 72.
- 4 Compare it with the exact compound answer shown below.
Worked example
8% a year. The Rule of 72 gives 72 ÷ 8 = 9 years to double. The exact answer with annual compounding is ln 2 ÷ ln 1.08 = 9.01 years, almost the same. So $10,000 becomes about $20,000 in 9 years and roughly $40,000 in 18 years.
Features
Shortcut and exact answer
See 72 ÷ rate next to the precise logarithm result.
Quadrupling time
Twice the doubling time shows when money grows four times.
Works for debt too
At 24% APR, card debt doubles in about 3 years if unpaid.
When to use it
Investing
At 8% a year, money doubles in about 9 years and quadruples in about 18.
Savings accounts
At 4%, savings take about 18 years to double.
Inflation
At 3% inflation, prices double in about 24 years, halving what money buys.
Credit card debt
Unpaid debt at high APRs doubles alarmingly fast.
Frequently asked questions
What is the Rule of 72?
A quick way to estimate the years it takes an amount to double: divide 72 by the annual percentage rate. At 6%, money doubles in about 12 years.
How accurate is it?
Very close for rates between about 6% and 10%. At very low or very high rates the exact answer differs more.
Why 72?
72 is close to the exact constant of about 69.3 for continuous compounding, adjusted for annual compounding, and it divides evenly by many common rates.
Can I use it for inflation?
Yes. Divide 72 by the inflation rate to see how many years until prices double.
Is there a Rule of 69 or 70?
Yes. 69 or 70 is more accurate for continuous compounding or low rates, but 72 is easiest to use in your head.
How do I find the rate needed to double my money?
Divide 72 by the number of years. To double money in 6 years you need about 72 ÷ 6 = 12% a year.
Does it work for fees?
Yes. Even a 1% yearly fee slows growth noticeably over decades, and higher fees eat into long-term returns faster, which is why low-cost funds matter.
Can I use it for population or prices?
Any steady percentage growth works. If a city grows 2% a year, its population doubles in about 36 years.
Is doubling guaranteed at that time?
Only if the rate stays the same every year. Savings rates change and investment returns vary, so treat the result as an estimate, not a promise.
Results are estimates based on the official rates linked above. Confirm important figures with the issuing authority or a qualified adviser before you act on them.
Reviewed by ToolsUnlocked Team · Last updated