How many years until your money doubles? Enter an interest rate and compounding frequency to get the exact doubling time, alongside the popular Rule of 72 and Rule of 70 shortcuts so you can see how close they are.
Time to Double Money Calculator
Find exactly how long money takes to double, and compare with the Rule of 72.
Results are estimates for educational purposes and are not financial advice. Actual returns depend on your provider's terms, fees and taxes.
Doubling time formulas
Exact: t = ln(2) ÷ [ n × ln(1 + r/n) ]
Rule of 72: t ≈ 72 ÷ R | Rule of 70: t ≈ 70 ÷ R
- r
- Annual rate as a decimal (for the exact formula).
- R
- Annual rate as a percentage (for the rules of thumb).
- n
- Compounding periods per year.
- ln(2)
- Natural log of 2 ≈ 0.693147.
How to use
- Enter the annual interest rate or expected return.
- Choose the compounding frequency.
- Optionally enter a starting amount to see a growth chart up to the doubling point.
- Press Calculate.
Worked example
At 6% compounded annually:
- Exact: t = 0.693147 ÷ ln(1.06) = 0.693147 ÷ 0.058269 = 11.90 years.
- Rule of 72: 72 ÷ 6 = 12 years.
- Rule of 70: 70 ÷ 6 = 11.67 years.
- Because interest is credited once a year, the balance first passes double at the end of year 12.
With monthly compounding at 6% the exact doubling time drops to 11.58 years.
Accuracy of the Rule of 72 (annual compounding)
| Rate | Exact years | Rule of 72 |
|---|---|---|
| 2% | 35.00 | 36.00 |
| 4% | 17.67 | 18.00 |
| 6% | 11.90 | 12.00 |
| 8% | 9.01 | 9.00 |
| 10% | 7.27 | 7.20 |
| 12% | 6.12 | 6.00 |
The Rule of 72 is most accurate around 8%. It slightly overestimates at low rates and underestimates at high rates.
Practical uses
- Quick mental comparisons between savings rates or investment returns.
- Understanding how inflation halves purchasing power — at 3% inflation, prices double in about 23.4 years.
- Seeing how high-interest debt can double if left unpaid — at 24% APR compounded monthly, in about 2.9 years.
Assumptions and limitations
- Constant rate, no deposits or withdrawals, no taxes or fees.
- The rate must be greater than 0%.
Frequently asked questions
Why 72?
72 is close to 100 × ln(2) ≈ 69.3 adjusted upwards for annual compounding at typical rates, and it divides evenly by many numbers (2, 3, 4, 6, 8, 9, 12), which makes mental arithmetic easy.
When should I use the Rule of 70 or 69.3?
70 is often used for economic growth and inflation rates. 69.3 is exact for continuous compounding.
How long to triple my money?
Replace ln(2) with ln(3) ≈ 1.0986. At 6% annually that is about 18.85 years.