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 |