Rule of 72 calculator (time to double your money)
Find out how long an investment takes to double, triple or quadruple at a given interest rate.
A compound-interest approximation: the higher the rate, the less exact (but great for mental math).
How the calculation works
The rule of 72 is a shortcut to estimate, in your head, how many years an amount takes to double under compound interest — no calculator.
years to double ≈ 72 ÷ rate (%/yr)
At 8%/yr, it doubles in 72 ÷ 8 = 9 years. At 12%, 6 years. It works in reverse too: to double in 6 years you need 72 ÷ 6 = 12%/yr.
Variations
- To triple, use 114.
- To quadruple, use 144.
It works for debt too
At 10%/month, credit-card debt doubles in ~7 months — compounding on the wrong side.
Examples
- At 6%/yr: 72 ÷ 6 = 12 years to double.
- Doubling in 8 years needs 72 ÷ 8 = 9%/yr.
Frequently asked questions
Why 72?
Because 72 has many divisors (2, 3, 4, 6, 8, 9…), making mental math easy, and it approximates compound-interest math well.
Does it account for inflation and taxes?
No. For purchasing power, use the real rate; for net returns, subtract tax on gains.
How accurate is it?
Very good between 5% and 12%. At high rates the exact (logarithmic) value drifts a bit — but for a mental estimate, it works.
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Updated on June 30, 2026 · by Rafael Rossi · Methodology & sources