Rule of 72 Calculator
The Rule of 72 is the fastest piece of mental maths in finance: divide 72 by the annual return to estimate how many years money takes to double. 12% doubles in about 6 years; 8% in about 9; a 6% FD needs 12.
This calculator runs the rule in both directions — years to double at a given rate, and the rate needed to double within a given number of years — and shows the mathematically exact answer next to the approximation.
How it works
- Rule estimate: years ≈ 72 ÷ rate; inverted, rate ≈ 72 ÷ years.
- Exact answer: years = ln(2) ÷ ln(1 + rate) — the rule is a close approximation of this in the 6–12% range.
Frequently asked questions
Why 72?
The exact doubling constant near typical returns is about 69.3 (from ln 2 ≈ 0.693). 72 became the rule because it is almost as accurate in the 6–12% range and divides cleanly by 2, 3, 4, 6, 8, 9 and 12 — perfect for mental arithmetic.
How accurate is it?
Within about a tenth of a year for returns between 6% and 12%. It drifts at the extremes — at 2% the rule says 36 years vs an exact 35; at 30% it says 2.4 vs an exact 2.64. The calculator always shows both.
Is there a rule for tripling or quadrupling?
Yes — the Rule of 114 for tripling and the Rule of 144 for quadrupling, used exactly the same way. At 12%, money triples in about 9.5 years and quadruples in about 12 (which is just doubling twice).
Does the rule work for SIPs?
No — it applies to a lumpsum growing at a compound rate. A SIP's rupees each have different investment periods, so its 'doubling' has no single clean rule; use the SIP calculator to project it properly.
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This tool is an educational estimate, not investment or tax advice. Rates and rules change — verify current figures and consult a professional before acting.