What is the rule of 72
Updated on June 30, 2026 · by Rafael Rossi
The rule of 72 is one of the handiest financial shortcuts: it estimates, in your head, how many years an investment takes to double with compound interest — no calculator needed.
How to use it
years to double ≈ 72 ÷ rate (%/year)
At 8%/year, money doubles in 72 ÷ 8 = 9 years. At 12%, 6 years. At 6%, 12 years.
It works in reverse
Want to double in a set time? Divide 72 by the years to find the rate needed. To double in 6 years: 72 ÷ 6 = 12%/year.
Variations: triple and quadruple
- To triple, use the rule of 114.
- To quadruple, use 144.
Why 72?
It best approximates the exact (logarithmic) calculation for real-world rates and has many divisors (2, 3, 4, 6, 8, 9, 12), making mental math easy.
Accuracy and limits
It's an approximation: the higher the rate, the less exact. For typical rates (5–12%), the error is small. It also ignores taxes and inflation.
FAQ
Does it work for debt too? Yes. At 10%/month, credit-card debt doubles in ~7 months — showing the (scary) power of compounding against you.