Thread type

Thread size is governed by standards, including BSP (British Standard Pipe) and NPT (National Pipe Thread), although there are many other international standards, and sometimes it depends on the industry and/or country of use. Thread size is always measured and formulated on the interior diameter of the conduit and corresponds to a specific number of threads per inch (TPI). The conduit size is measured in metric (mm) or imperial (inches), and proper sizing is critical, as over- or under-sized fittings will either be wholly incompatible or make an inadequate connection or seal.

The table below shows nominal thread sizes with corresponding values.

from Internet