National Pipe Thread (NPT) Size Measured Against a Ruler*
Actual Diameter (Inches)
National Pipe Thread (NPT) (Fractional)
0.405"
1/8"
0.540"
1/4"
0.675"
3/8"
0.840"
1/2"
*NOTE: A very common misconception is that a 1/4" NPT measures as 1/4 inch on a ruler or tape measurer. However, that is not the case. NPT will measure larger in diameter. Above is a sizing reference to help clarify.