Determine Pipe Thread Size

Here's a handy chart for determining pipe thread size. You can find thread sizes for both male and female joints.
According to plumbingsupply.com:

"Two common pipe thread sizes exist, the tapered National Pipe Thread (NPT) and the straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM). The tapered threads are for joining and sealing, the straight threads are only for joining."