Pipe Taper Dimensions
In the
world of manufacturing youll find many companies producing parts
with National Pipe Taper (NPT) threads. In those shops, a part print
may require an NPT features such as 111.5 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 preturned to produce a
taper prior to machining the thread. Extra care must be taken to
turn the correct prethread diameter to avoid excessive truncation.
Now lets 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
