Exact Doubling Time
9.01 yrs
Rule of 72 Calculator
Quickly estimate years required to double an investment.
Years to Double (Rule of 72)
9.0 yrs
Rule of 114 (Triple)
14.3 yrs
Rule of 144 (Quadruple)
18.0 yrs
Doubled Value
$20,000.00
How to use the Rule of 72 Calculator
The Rule of 72 Calculator is free and runs entirely in your browser — no sign-up, and nothing you enter leaves your device. It opens pre-filled with a realistic example, so you can see how it works before replacing any figure with your own; the results update as you type. Press Calculate to refresh the result panel, or Reset to return to the example.
The inputs it asks for:
- Annual Interest Rate — a percentage (enter 6 for 6%).
- Principal (optional, for reference) — a dollar amount.
The formula
The Rule of 72 is a mental-math shortcut for compound growth: divide 72 by the annual percentage rate to approximate the years needed to double.
Years to double ≈ 72 ÷ rate(%)- rate
- the annual growth (or interest) rate, in percent
The exact answer is ln(2) ÷ ln(1 + r); 72 is chosen because it divides cleanly by many rates and tracks the exact figure closely for everyday rates (roughly 5–12%). The companion Rule of 114 triples a sum and 144 quadruples it.
Worked example
Using the example values — Annual Interest Rate 8%, Principal (optional, for reference) $10,000.00 — the Rule of 72 Calculator returns a Years to Double (Rule of 72) of 9.0 yrs. It also reports Exact Doubling Time (9.01 yrs), Rule of 114 (Triple) (14.3 yrs), Rule of 144 (Quadruple) (18.0 yrs).
Prefer your own numbers? Change any field above and this recomputes instantly.
Key terms
- Doubling time
- How long it takes a sum to grow to twice its size at a steady rate.
- Compound growth
- Growth that itself earns growth, producing an exponential (not straight-line) curve.
- Rule of 114 / 144
- The same shortcut for tripling (114) and quadrupling (144).
Frequently asked questions
Why 72 and not 70 or 71?
70 is actually more accurate for continuous compounding, but 72 has more whole-number divisors (2, 3, 4, 6, 8, 9, 12…), which makes the mental arithmetic easier.
How accurate is it?
Very close in the everyday 6–10% range. It drifts at very high or very low rates, where the exact doubling-time figure (also shown) is worth using.
Does it work for inflation or debt?
Yes — the same math tells you how fast inflation halves your purchasing power, or how fast a debt balance doubles at a given interest rate.