Rule of 72: calculator and explanation
The rule of 72 is a mental shortcut: divide 72 by the annual interest rate and you get the years it takes an amount to double. At 8%, about 9 years. It works the same for savings that grow and for debt that grows: a card at 24% doubles what you owe in 3 years.
Backwards: what rate do I need?
With $10,000 at 8%: becomes $20,000 in ≈9 years, $40,000 in ≈18 and $80,000 in ≈27.
Works the same for debt: at that rate, what you owe doubles in the same time.
Backwards: what rate do I need?
Common rates
| What | Years |
|---|---|
| Savings account (0.5%) | 144 years |
| Treasury bonds (4%) | 18 years |
| Index fund (7%) | 10.3 years |
| Historical stock return (10%) | 7.2 years |
| Auto loan (8%) | 9 years |
| Credit card (24%) | 3 years |
| Cash advance (30%) | 2.4 years |
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Join the waitlistHow it is calculated
The approximation is 72 ÷ rate. At high rates (over 15%) the rule falls short: the real time is longer than 72 ÷ rate.
The inverse answers “what rate do I need to double in N years?”: 72 ÷ N as the approximation, and 2^(1/N) − 1 exactly.
Frequently asked questions
Why 72?
Because ln(2) ≈ 0.693 and, with the adjustment that makes the approximation most accurate around 8%, the round number with the most divisors (2, 3, 4, 6, 8, 9, 12) is 72: easy to divide in your head.
Does it work for inflation?
Yes: at 3% inflation, prices double (and your money loses half its purchasing power) in about 24 years. At 6%, in 12.
What about tripling or quadrupling?
To quadruple, double the time (two doublings). To triple there is the rule of 114 (114 ÷ rate) and to multiply by 10, the rule of 240.
These calculators are educational estimates based on the numbers you enter; they are not a credit offer nor financial, legal or tax advice. Reference rates are reviewed yearly.