Rule of 72 calculator
Estimate how long money takes to double at a fixed annual rate and compare the shortcut with exact compounding.
Your estimate
- Exact doubling time (years)
- 11.9
Assumes a fixed positive annual growth rate and no deposits, withdrawals, taxes, or fees.
How the doubling shortcut works
Worked example
At 6% annual growth, 72 divided by 6 gives 12 years. The compound growth formula gives about 11.90 years.
Divide 72 by the annual growth rate in percent. For the exact result, divide the natural logarithm of 2 by the natural logarithm of one plus the rate as a decimal. The shortcut is less accurate at very low and very high rates.
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When the doubling shortcut is useful
Compare the shortcut with the exact result
At 2% annual growth, the shortcut gives 36 years and the exact formula gives about 35. At 18%, the shortcut gives four years and the exact formula gives about 4.19. The shortcut can run high or low depending on the rate. If timing matters for a goal, use the exact result and remember that it assumes the same rate every year.
Check what would actually make the balance double
The estimate assumes a lump sum grows from compounding alone. New deposits can make the balance reach twice its starting amount earlier, but some of that growth would be your own contributions. If you plan to add money along the way, use a savings projection so you can see deposits and interest separately.