Rule of 72
Rule of 72
How long until your money doubles
A one-line shortcut for compounding. Enter a rate of return to see the classic Rule of 72 estimate alongside the exact answer, plus what your balance looks like in 10 and 20 years.
Inputs
Your assumption
Results
Time to double
Rule of 72 estimate9 yrs
Exact compounding9.01 yrs
Value in 10 years$21,589
Value in 20 years$46,610
Reference
Doubling time by rate
2% per year≈ 36.0 years
4% per year≈ 18.0 years
6% per year≈ 12.0 years
8% per year≈ 9.0 years
10% per year≈ 7.2 years
12% per year≈ 6.0 years
15% per year≈ 4.8 years
20% per year≈ 3.6 years
How it works
The maths
The shortcut approximates the logarithm of 2 divided by the logarithm of one plus the rate — accurate enough for mental arithmetic in the normal range of returns.
approx years = 72 ÷ rate exact years = ln(2) ÷ ln(1 + rate)
FAQ
Rule of 72 — frequently asked questions
It is a mental shortcut: divide 72 by the annual percentage return and you get the approximate number of years for the money to double. At 8% a year, doubling takes roughly nine years.
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Educational tool. Not financial advice. More tools