Rule of 72 Calculator – Doubling Time

The Rule of 72 is mental math for compounding: divide 72 by the annual percentage rate and you get the years needed to double. At 8%, 72 ÷ 8 = 9 years; at 6%, 12 years; at 12%, 6 years. The calculator also shows the exact logarithmic answer — ln(2) ÷ ln(1+r) — so you can see the rule’s accuracy: at 8% the true doubling time is 9.006 years, an error of just 0.07%. The sweet spot is 6–10%, where the rule lands within 1%; below 4% it slightly overstates and above 20% it understates the time. The same trick powers inflation thinking — at 3% inflation, prices double in 72 ÷ 3 = 24 years, meaning cash under the mattress loses half its purchasing power in about a quarter century. Related shortcuts: the Rule of 115 for tripling and the Rule of 69.3 for continuous compounding.

1. 1

   Step 1

   Enter the expected annual return or inflation rate as a percentage.

2. 2

   Step 2

   Read the estimated doubling years from 72 ÷ rate.

3. 3

   Step 3

   Check the exact logarithmic doubling time beside it.

4. 4

   Step 4

   Flip to tripling with the Rule of 115 when needed.

• Applying it to simple interest

• Using dollar amounts instead of %

• Trusting it at extreme rates