Why Not 70?

72 is not the only doubling shortcut. For very low rates, 70 is actually closer to the true 69.3 — which is why economists often use the Rule of 70 for things like inflation and population growth. Some textbooks even teach 69 for continuously compounded rates.

The trade-off is convenience: 70 and 69 are more precise, but 72 divides evenly by far more everyday rates, so most people keep using it.

Want to use it today? Two quick moves:

• Check any rate: divide 72 by it to see the doubling time — a 6% fund doubles your money in 12 years, an 18% card doubles your debt in 4.
• Compare two options: the one with the shorter doubling time wins. That is how a small fee or a slightly higher rate quietly changes your outcome.