How fast does your money double?

The Rule of 72 gives a quick mental estimate: years ≈ 72 ÷ rate. For precision this calculator uses the exact doubling equation — and shows the inflation-adjusted truth too.

—
Rule of 72 estimate
—
Exact value
—
After 3% inflation
—
Quadruple time
Why 72? At modest rates, ln(2)/ln(1+r) ≈ 0.693/r, and 69.3 rounds to 72 — a number with lots of divisors, making mental math easy. The approximation is best between 4% and 12%.
Math behind it Doubling time = ln(2) / ln(1 + r). Needed rate = 2^(1/years) − 1. Example at 8%: exact = 9.01 years, rule of 72 = 9.0. At 3% inflation, real doubling takes ln(2)/ln(1.08/1.03) ≈ 17.3 years.
What does the inflation-adjusted number mean? It shows how long until your money doubles in purchasing power, not nominal digits. Nominal growth minus inflation is the return that actually compounds your real wealth.