Money fundamentals · Growth
The Rule of 72: a quick way to see what time can do.
You don’t need a calculator to understand how money grows. The Rule of 72 is a simple shortcut that estimates how long it takes money to double at a given rate. Once you see it, you’ll never look at time the same way.
How the rule works
Divide 72 by an annual rate of growth. The answer is roughly the number of years it would take money to double at that rate.
- At 4% a year: 72 ÷ 4 = about 18 years to double.
- At 8% a year: 72 ÷ 8 = about 9 years to double.
It’s an approximation, but a surprisingly close one for everyday rates.
Why it changes how you think about time
Notice what happens with doubling. At a higher rate, money doesn’t just grow a little faster; it doubles more times over the same span of years. And the more years you give it, the more doublings you get. That’s why starting early can matter as much as how much you put away.
It works against you, too
The same math applies to debt. A credit card balance at a high interest rate can double surprisingly fast if it isn’t paid down. The Rule of 72 is just as useful for seeing why high-interest debt deserves urgent attention.
Inflation and the rule
You can also use the rule to estimate how quickly rising prices cut purchasing power in half. Divide 72 by the inflation rate, and you’ll see why money sitting still slowly loses ground.
The short version
- Divide 72 by a rate to estimate the years it takes to double.
- More time means more doublings, so starting early matters.
- The rule also shows how fast high-interest debt can grow.
- Returns aren’t guaranteed; the rule is an estimate, not a promise.
Common questions
Is the Rule of 72 exact?
No. It’s an approximation that works best for moderate rates. It’s meant to build intuition, not to predict results.
Can I count on a certain rate of return?
No. Returns aren’t guaranteed, investments can lose value and actual results vary. The examples above are illustrations of the math only.
The rates shown are hypothetical, used only to illustrate the math, and do not represent any specific investment. Rates of return are not guaranteed, investments can lose value, and actual results will vary.
[Insert compliance-approved hypothetical illustration disclosure here.]
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