Find an NPT chart that shows threads per inch and drill size by going to the national pipe taper threads page on The Engineering Tool Box web site and scrolling down to the chart. In addition to threads per inch and drill size, this chart includes five other categories of information.
National pipe threads are also known as ANSI/ASME B1.20.1 Pipe Threads, General Purpose. They have a 60-degree thread angle, the truncation of roots and crests are flat, the pitch is measured in threads per inch and the angle between the taper and the center axis of the pipe is 1.7899 degrees. To ensure a leak-free seal, a sealant compound or Teflon tape must be used on national pipe threads.
The number of threads per inch on the chart ranges from 27 to 8. The drill size is measured in inches, and its range on the chart is between 7/16 and 6 11/32. The chart also shows the size of the pipe and the nominal outside pipe diameter, both measured in inches. The outside diameter must be checked for positive identification of thread size, because the pitch is the same on more than one pipe thread size. Three approximate measurements are also included: the length of thread, the number of threads to be cut and the total thread makeup.