Rule of 72 calculator
See how many years it takes your money to double at a given yearly return.
Years to double at 8%
The rule of 72 says 9.0 years; the exact answer is 9.0. To double in 10 years you need about 7.2% a year.
How this calculator works
The rule of 72 is a mental shortcut: 72 divided by the yearly return gives the years to double. At 8%, money doubles in about 9 years.
It also works backward. To double in 10 years, you need about 7.2% a year.
Years to double ≈ 72 ÷ yearly return (%). Exact: ln 2 ÷ ln(1 + r)Common questions
How accurate is it?
Very close for returns between about 5% and 10%. At higher rates, 69 or 70 gives a closer estimate.
Does it work for inflation?
Yes. At 3% inflation, prices double in about 24 years, so cash loses half its buying power.
Does it work for debt?
Yes. A card balance at 24% interest doubles in about 3 years if unpaid.
FatDCA calculators are educational estimates, not investment, tax or legal advice. Results depend on the assumptions you enter, and past returns do not predict future returns.
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