72

From the blog financial wizard, T. Rowe Rowe:

Using the number 72, you can determine how long it will take for money to double at various interest rates.

Simply divide the interest rate into 72. For instance, if you’re getting a 6% interest rate on an investment – divide 6 into 72 = 12 years.

And conversely, say you want your money to double in 10 years. Simply divide 10 into 72 = 7.2%.