Tooling & Production May 2004

"Shop Talk with Steve Rose"

The Author, Steve Rose

Pipe Taper Dimensions

     In the world of manufacturing you’ll find many companies producing parts with National Pipe Taper (NPT) threads. In those shops, a part print may require an NPT features such as 1-11.5 NPT. The print may specify a nominal diameter and the programmer is required to calculate the features of the thread.

     What is an NPT thread? This is a tapered thread where the diameter changes by 3/4” per foot of length. This means that the diameter increases or decreases by 0.750” for each foot in length. Because NPT threads are not generally 1 foot long, we must calculate the length of the thread in relationship to the diameter required.

     Using trigonometry the NPT angle can be calculated. Figure 1 shows a triangle where one side is 12” and the other is 0.750”. Use the tangent rule to calculate the NPT angle of 1.7899 degrees per side.

TAN-1 = 0.375 ๗ 12 = 1.7899˚
     Note: The tangent of 1.7899˚ is 0.03125, an easy number to remember. Also it is helpful to remember that the American, British and metric pipe taper angles are all the same, 1.7899 degrees.

     Knowing the NPT angle value, you can now calculate the thread diameter and lengths for any part. When calculating the diameter value, make sure that the resulting thread has the correct thread truncation, the size of the flat on the thread. This value can easily be overlooked. Always measure the flat to ensure the correct width is produced.

External Thread Truncation
Width of Resultant Flat
TPI Max Min
27 0.0014 0.0041
18 0.0021 0.0057
14 0.0027 0.064
11.5 0.0033 0.0073
8 0.0048 0.0090

     To create the flat on the thread, the part is often pre-turned to produce a taper prior to machining the thread. Extra care must be taken to turn the correct pre-thread diameter to avoid excessive truncation.

     Now let’s use the NPT angle we calculated above to determine the unknown diameter on this piece. Note, the sketch is not to scale. In this example, calculate the unknown diameter as shown for this external NPT thread.

     First sketch the triangle and label the sides. The angle is the NPT angle of 1.7899 degrees and the thread length is 0.7935”. Use trigonometry to calculate the side opposite.

     Side opposite = 0.7935 x TAN 1.7899
     Side opposite = 0.0248

     Remember this value is one side, double the value and subtract from 1.050 to determine diameter A.

1.050 – (2 x 0.0248) = 1.0004 = diameter A

     This next example shows an internal NPT thread. Again, we know the NPT angle, use this value to calculate the thread length.

     First draw the triangle and label the sides. Calculate the radius value (0.9889 – 0.9543) ๗ 2 = 0.0173, this is the value of the side opposite.

     Use the NPT angle and the side opposite to calculate the side adjacent (the thread length).
     Side adjacent = 0.0173 ๗ TAN 1.7899
     Adjacent = 0.5536”