Rule of 72 Calculator

Divide 72 by the growth rate to get doubling time: 8% doubles money in about 9 years, 6% in 12. It works because ln(2) ≈ 0.693 and 72 is conveniently divisible — accurate within a few percent for rates between 2% and 15%.

The rule also measures erosion: 3% inflation halves purchasing power in 24 years; a 12% credit-card... doubles your debt in 6. Any steady percentage — user growth, energy use, fees — obeys the same clock.