If your phone has a calculator, use that. Want to find the decimal equiv. of 5/16"? Type in 5 divided by 16 and hit enter. The answer is in decimal form, try it. It's a handy thing to remember, and faster than looking in the Black Book.
Eventually you memorize all common decimals however.
1/32 - .03125
1/16 - .0625
1/8 - .125
1/4 - .25
Etc.