Rule of 72 Calculator
Estimate how many years it takes to double your money, or the rate needed to double it, with the Rule of 72 next to the exact answer.
The Rule of 72 is a mental shortcut: divide 72 by a yearly interest rate to estimate how many years it takes money to double. It also works backwards — divide 72 by the years you have to find the rate you need. This calculator shows the shortcut next to the exact compound-interest answer so you can see how close it is.
How to use this tool
- Choose whether to solve for years to double or the rate needed.
- Enter the yearly rate of return, or the number of years.
- Compare the Rule of 72 estimate with the exact answer.
- Use the table to see where the rule is most accurate.
Formula
Years ≈ 72 ÷ rate · Exact years = ln 2 ÷ ln(1 + rate ÷ 100)
The rate is a yearly percentage (8 for 8%) compounded once a year. In reverse, rate ≈ 72 ÷ years, and the exact percentage rate is 100 × (2^(1 ÷ years) − 1). With continuous compounding the exact constant is 100 × ln 2 ≈ 69.3147, often rounded to 69.3.
Example
At 8% a year, 72 ÷ 8 = 9 years to double. The exact figure is ln 2 ÷ ln 1.08 ≈ 9.006 years, so the shortcut is off by about two days. Going the other way, doubling your money in 10 years needs about 72 ÷ 10 = 7.2% a year; the exact rate is 7.177%.
Frequently asked questions
Why 72 and not 70?
72 divides evenly by 2, 3, 4, 6, 8, 9 and 12, so it is easy to use in your head, and it is most accurate near 8%. The Rule of 70, or 69.3, is closer for low rates and for continuous compounding, which is why economists often use it for GDP or population growth.
How accurate is the Rule of 72?
Within a few percent for rates between about 4% and 12%. It gets worse at high rates: at 25% the rule says 2.9 years, but the true doubling time is 3.1 years.
Can I use it for inflation or debt?
Yes. At 3% inflation prices double in about 24 years, so cash loses half its buying power in that time. A card balance at 24% APR doubles in roughly 3 years if nothing is paid.
Is there a rule for tripling?
Use about 114 instead of 72. At 8%, money triples in about 114 ÷ 8 = 14.25 years, against an exact ln 3 ÷ ln 1.08 ≈ 14.27 years.
This tool gives an estimate for general information only. It is not financial, tax, legal or medical advice — check with a qualified professional before you rely on the result.
Last updated: 2026-10-30