Rule of 72 Calculator

Estimate how many years it takes money to double at a given return, or the return needed to double in a set time. The rule of 72 calculator puts the shortcut next to the exact answer.

Updated
Runs in your browser. Your data is not uploaded.
Find

Years to double
–

Estimate only. Not financial advice.

How to use the Rule of 72 Calculator

  1. Choose Years to double or Rate needed.
  2. Enter the yearly Return in percent, or the number of years.
  3. Press Calculate. The rule of 72 estimate appears next to the exact figure.

How it works

years ≈ 72 ÷ rate
rate ≈ 72 ÷ years
exact years = ln 2 ÷ ln(1 + rate ÷ 100)
exact rate = (2^(1 ÷ years) − 1) × 100

72 is used because it divides evenly by many common rates and is close to the exact result between about 6% and 10%.

Examples

  • At 8%: 72 ÷ 8 = 9 years. The exact answer is 9.01 years.
  • At 6%: 12 years by the rule, 11.90 exactly. At 12%: 6 years, 6.12 exactly.
  • To double in 10 years you need about 7.2% a year (7.18% exactly).

It works for debt and inflation too

The rule applies to anything that compounds. A credit card at 24% doubles what you owe in about 3 years if nothing is paid. Inflation of 3% halves the buying power of cash in about 24 years.

Limitations

  • The shortcut drifts at high rates: at 30% it says 2.4 years against an exact 2.64.
  • Assumes a steady yearly return with compounding once a year.

Frequently asked questions

What is the rule of 72?

A quick way to estimate doubling time: divide 72 by the yearly percentage return.

How accurate is it?

Within about a few percent for rates between 6% and 10%. It gets less accurate at very high or very low rates.

What about the rule of 70 or 69?

They are variations. 69.3 is closest for continuous compounding; 72 is easier to divide in your head.

Often used together with the Rule of 72 Calculator.