What is the Rule of 72?

Divide 72 by the annual rate in percent to estimate the years needed to double your money. At 8%, 72 ÷ 8 ≈ 9 years.

How is the exact doubling time calculated?

For n compounding periods per year, years = ln 2 ÷ (n × ln(1 + r/n)). With continuous compounding, years = ln 2 ÷ r.

When is the Rule of 70 or 69.3 better?

69.3 (≈ 100 × ln 2) is exact for continuous compounding. 70 works well for low rates, while 72 is most accurate around 6–10% with annual compounding.

Does it work for inflation?

Yes. 72 ÷ inflation rate estimates how long it takes prices to double, or the purchasing power of money to halve.

Yes. Everything is calculated locally.

Rule of 72 Compound Interest Calculator

Estimate how long an investment takes to double with the Rule of 72, Rule of 70 and Rule of 69.3, and compare with the exact doubling time.

100% free
Private in browser
No signup
No watermarks

Annual interest / return rate (%)

Compounding

Initial investment ($)

Rule of 72 estimate

9 years

Rule of 70 estimate

8.75 years

Rule of 69.3 estimate

8.66 years

Exact doubling time

9.01 years

Rule of 72 error

-0.01 years

Value when doubled

$20,000.00

Rate this tool

Be the first to rate — helps others and improves this page.

About Rule of 72 Compound Interest Calculator

The Rule of 72 is a quick way to estimate doubling time: 72 ÷ annual return. Enter a rate and compounding frequency to compare the Rule of 72, 70 and 69.3 with the exact doubling time, and see how far off the shortcut is.

How to use Rule of 72 Compound Interest Calculator

Enter the annual interest rate or expected return.
Choose how often interest compounds.
Compare the rule-of-thumb estimates with the exact doubling time.

Key features

Rule of 72, 70 and 69.3
Exact doubling time for any compounding
Shows the approximation error
Continuous compounding option
Runs in your browser

Frequently asked questions

What is the Rule of 72?: Divide 72 by the annual rate in percent to estimate the years needed to double your money. At 8%, 72 ÷ 8 ≈ 9 years.
How is the exact doubling time calculated?: For n compounding periods per year, years = ln 2 ÷ (n × ln(1 + r/n)). With continuous compounding, years = ln 2 ÷ r.
When is the Rule of 70 or 69.3 better?: 69.3 (≈ 100 × ln 2) is exact for continuous compounding. 70 works well for low rates, while 72 is most accurate around 6–10% with annual compounding.
Does it work for inflation?: Yes. 72 ÷ inflation rate estimates how long it takes prices to double, or the purchasing power of money to halve.
Is my data private?: Yes. Everything is calculated locally.

Related In-Depth Guides

Learn practical workflows, real-world examples, and expert tips in our comprehensive guides:

Home Renovation & DIY · 8 min read