Compound interest
See how a starting amount, regular deposits, and time could grow your money.
Saving & investingSaving & investing
Rule of 72 calculator
When could your money double?
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Compare the famous quick estimate with the exact constant-rate doubling time.
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THE MATH, WITHOUT THE MYSTERY
What’s actually happening here?
The Rule of 72 is a mental shortcut for estimating doubling time. Divide 72 by the annual growth rate written as a percentage.
For an effective annual return r written as a decimal, the exact mathematical time is ln(2) / ln(1 + r). The result is a theoretical fractional-year crossover; the first completed annual compounding period is the next whole year.
THE FORMULA
Rule of 72: years ≈ 72 / annual percentage rate
A small example
At 8%, the Rule of 72 gives 9 years. The exact constant-rate answer is about 9.006 years—close, but not identical.
Further reading
SEC Investor.gov · Compound interest calculatorGOOD QUESTIONS
A few things worth knowing.
Does the rule work at 0% or a negative rate?
No. Money does not double through growth at a nonpositive constant rate. Use the compound-interest calculator to explore those scenarios.
Is doubling time a guarantee?
No. It is math for a fixed rate. An investment’s actual returns may vary or be negative.