10 calculations to know

For 2% interest, the "rule of 72" tells you that it takes 72/2 = 36 years for money to double; in fact, it takes 70/2 = 35 years.

For 5% interest, the "rule of 72" says that it takes 72/5 = 14.4 years; in fact, it takes 71/5 = 14.2 years.

For 8% interest, the "rule of 72" says that it takes 72/8 = 9 years, and is correct.

For 11% interest, the "rule of 72" says that it takes 72/11 = 6 + 6/11 years; in fact, it takes 73/11 = 6 + 7/11 years.

For 14% interest, it takes 74/14 years instead of 73/14 years.

Even when you get up to 20% interest, the approximation is close -- it predicts 3.6 years, where the correct value is 3.8 years.