Rule of 72 Calculator
Use the Rule of 72 to estimate how long it takes to double your money at a given interest rate, or what rate you need to double in a given time.
The Rule of 72 is a quick mental math shortcut for estimating how long it takes an investment to double — or how long before inflation cuts your purchasing power in half.
Doubling Time Formula:
Years to Double = 72 / Annual Interest Rate (%)
Rate Needed to Double in N Years:
Rate = 72 / Years
Worked examples:
Savings account at 4.5% APY: Years to double = 72 / 4.5 = 16 years
Stock market (historical 10% average): Years to double = 72 / 10 = 7.2 years
Inflation at 3%: Years until purchasing power halves = 72 / 3 = 24 years
Credit card at 24% APR: Your debt doubles in 72 / 24 = 3 years if unpaid
Why 72 works: The mathematically exact formula is: t = ln(2) / ln(1 + r) = 0.6931 / ln(1 + r)
At 8%, this gives: 0.6931 / 0.07696 = 9.01 years. The Rule of 72 gives: 72 / 8 = 9.0 years. Remarkably accurate.
Accuracy range: The rule works best between 3% and 25%. Below 3%, use 69. Above 25%, use 74.
Rule of 114 (tripling): Use 114 instead of 72 to estimate when money triples. Rule of 144 (quadrupling): Use 144 to estimate quadrupling time.
Practical reference:
| Rate | Doubles in |
|---|---|
| 2% | 36 years |
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
| 12% | 6 years |
| 18% | 4 years |
| 24% | 3 years |
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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