While the thread angle of both MIP and NPT threads are the same, they both come in varying diameters and sizes. If you don’t already know your fitting’s pitch, or amount of threads per inch (TPI), it’s easy to measure it yourself. Use a string to find the circumference of your fitting. Then, divide it by pi, or 3. 14159, to find the decimal equivalent. Simply use a conversion chart to find the thread dimensions.
Generally, tapered fittings make a tighter seal than straight fittings. [8] X Research source
While these are the sealants most commonly used with each fitting, NPT threads can be sealed with pipe dope, and MIP fittings can be wrapped in teflon tape.
When connecting your pipes, use a MIP fitting with a FIP fitting, and an NPT thread with a matching FNPT thread. While NPT and MIP threads can be used together, they are not compatible with other common fitting types like BSPT, British Standard Pipe Taper, or BSPP, British Standard Pipe Parallel. [11] X Research source
