The Rule of 70

There’s a simple rule of thumb that gives the time it will take for something subject to compound interest to double in value. The time is 70 divided by the interest rate. So if the rate is 5% per year, it will double in 70/5 = 14 years.

This works because, as we discussed earlier, the effect of compound interest can be calculated by P’ = P * e^(r*t). 
In this case, P’/P = 2, 
so 2 = e^(r*t), 
ln(2) = r*t, 
t = ln(2)/r, 
which is approximately 
t = 0.69/r. 
r in that equation is in the decimal form. If we want the rate as a whole number R we can multiply top and bottom by 100 to get 69/R, 
but if you’re doing a quick mental check, 70 is close enough.