Bankers in Renaissance Italy needed a shortcut for compound growth, and the one they found still works: divide 72 by your interest rate and you get the years needed to double your money. At 7 percent, $10,000 becomes $20,000 in about 10.3 years. The math behind it is logarithmic, but the rule itself fits on a napkin -- which is exactly why it survived five centuries.

This calculator runs the rule on your numbers and shows its work. Enter the annual interest rate and your current amount, and it reports the Rule-of-72 estimate for doubling, the doubled amount, and -- because quadrupling is just doubling twice -- the years to quadruple. It also computes the exact doubling time from the logarithmic formula, so you can see the rule's error: at 7 percent the exact answer is 10.24 years versus the rule's 10.29, a rounding error you can ignore.

The rule moonlights as a lie detector. A promoter promising to double your money in three years is promising 24 percent annual returns -- possible in crypto, absurd in anything regulated. It also works in reverse on inflation and debt: at 18 percent credit card APR, your balance doubles in four years if you pay nothing. Memorize 72; it audits every financial claim you will ever hear.

With the pre-filled defaults — Annual Interest Rate 7%, Current Amount $10,000 — the Rule of 72 Calculator returns Years to Double (Rule of 72) 10.3, Doubled Amount $20,000.00, Years to Quadruple 20.6, Quadrupled Amount $40,000.00, Exact Years to Double 10.24. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.