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- Shows how many years it takes to double your money at a given yearly rate.
- Or finds the rate you need to double your money in a set number of years.
- Compares the rule of 72 with the rules of 70 and 69.3 and the exact answer.
- Shows when your money would grow 2×, 4×, 8× and 16×.
How to use the rule of 72 calculator
Choose what to find
Years to double, or the rate needed.
Enter the rate or the years
For example 8% a year, or 6 years.
Read the estimate and the exact answer
See how close the shortcut is.
What is the rule of 72?
The rule of 72 is a quick way to estimate how long an investment takes to double. Divide 72 by the yearly interest rate, and the answer is roughly the number of years. It works because 72 is close to 100 × ln 2 (about 69.3), adjusted upward for yearly compounding, and it divides evenly by many common rates.
Rule of 72 formulas
Years to double ≈ 72 ÷ yearly rate (%)
Rate needed ≈ 72 ÷ years
Exact years = ln 2 ÷ ln(1 + rate)
The rule of 70 and the rule of 69.3 are closer for low rates and continuous compounding.
Example 1: 8% a year
How long to double?
- Rule of 72 72 ÷ 8 = 9 years
- Exact answer 9.01 years
- $10,000 becomes $20,000 after about 9 years
- $10,000 becomes $40,000 after about 18 years
Example 2: 6% a year
Slightly less accurate further from 8%.
- Rule of 72 12 years
- Exact answer 11.90 years
Example 3: double in 6 years
What rate do you need?
- Rule of 72 72 ÷ 6 = 12%
- Exact rate 12.25%
Where the rule of 72 helps
Investing
Compare how fast different returns grow your money.
Retirement planning
See how many times savings could double before you retire.
Debt
See how fast a credit card balance doubles if unpaid.
Inflation
Estimate how long until prices double.
For exact growth with deposits, use the compound interest calculator. To find the growth rate between two values, try the CAGR calculator.
Rule of 72 calculator FAQ
How does the rule of 72 work?
Divide 72 by the yearly rate to estimate the years to double. At 8%, money doubles in about 9 years.
How accurate is the rule of 72?
It is very close for rates between about 6% and 10%. At 8% it gives 9 years versus an exact 9.01 years.
What is the rule of 70?
The same idea using 70 instead of 72. It is a little more accurate for low rates, such as inflation or GDP growth.
How long does it take to triple money?
Use the rule of 114: divide 114 by the rate. At 8%, that is about 14.25 years.
What rate doubles money in 10 years?
About 7.2% using the rule of 72.
Does the rule of 72 work for debt?
Yes. It shows how fast an unpaid balance doubles at its interest rate.