Measuring national pipe thread (NPT) taper fittings correctly ensures a tight-threaded pipe joint. The tapered threads of the fitting do not directly correspond to the size of the pipe entering a threaded joint. A 6 mm (1/4 inch) diameter NPT fitting will measure larger than the pipe diameter. The size of the fitting threads accounts for the thickness of the pipe wall minus the material removed from the pipe during thread cutting.
Align the 2.5 cm (1 inch) mark of a tape measure with one inside edge of the NPT fitting. Pull the tape measure across the opening of the fitting. Note the measurement to the other side of its other inside edge. Align and measure the distance across the narrowest part of the threads, if measuring a threaded NPT fitting.
Round up the dimension measured across the fitting opening to 1.5 mm (1/16 inch).
Select a thread gauge from a gauge set that matches the diameter of the inside dimension of the NPT fitting.
Set the threaded end of the gauge into the end of the threaded fitting. Turn the gauge clockwise to thread it into the fitting. Stop turning when the top of the gauge sits evenly with the front face of the fitting. Exchange the thread gauge if it will not sit evenly with the face of the fitting.
Read the thread-per-inch count stamped on the top of the gauge. Divide this number by 2.5 to get the thread be centimetre.
Find the inside fitting dimension on an NPT chart. Move across the chart to the TPI count to determine the exact size of the NPT fitting.