Rule of 72 Calculator
The mental-maths shortcut for compound growth: divide 72 by your rate to estimate the doubling time.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut for compound growth: divide 72 by an annual interest or growth rate, and the answer is roughly how many years it takes to double. It works because of how compound interest math behaves — the precise constant is closer to 69.3, but 72 divides evenly by more everyday numbers, which is why it's the version everyone actually remembers and uses. It's an estimate for a quick gut-check, not a substitute for a full compound interest projection like the ISA & Compound Interest Calculator.
Frequently asked questions
How accurate is the Rule of 72?
Very close for rates between roughly 6% and 10% — within a few weeks of the exact compound-interest answer. It drifts slightly more at very low or very high rates, where 69.3 (the precise constant) would be more accurate.
Why 72 specifically?
72 divides evenly by more small numbers (2, 3, 4, 6, 8, 9, 12) than the mathematically precise 69.3, making it easier to calculate in your head — the small accuracy trade-off is why it's a rule of thumb, not an exact formula.
Does the Rule of 72 work for debt too?
Yes — the same math shows how fast a debt balance doubles at a given interest rate if left unpaid, which is a sobering way to look at high-APR credit card debt.
Related searches
- rule of 72 calculator
- rule of 72
- how long to double my money
- doubling time calculator
- investment doubling time
- rule of 70
- double investment
- years to double