Tag: NPT

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National Pipe Thread (NPT) Tap Drill Size

Below is the National Pipe Thread – NPT Tap Drill Size Chart.  It tells us what size hole must be drilled to begin tapping the tapered NPT thread.

Standard NPT Tap Drill Sizes
NPT Size
 Tap Drill Size (in.)
Decimal Equivalent (in.)
1/16 – 27
“C”
0.242
1/8 – 27
“Q”
0.332
1/4 – 18
7/16
0.438
3/8 – 18
9/16
0.562
1/2 – 14
45/64
0.703
3/4 – 14
29/32
0.906
1.0 – 11•1/2
1•9/64
1.141
1•1/4 – 11•1/2
1•31/64
1.484
1•1/2 – 11•1/2
1•23/32
1.719
2.0 – 11•1/2
2•3/16
2.188
2•1/2 – 8
2•39/64
2.609
3.0 – 8
3•15/64
3.234

Metric Tap Drill Sizes
Standard UNC Tap Drill Sizes
Standard UNF Tap Drill Sizes

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National Pipe Thread (NPT) Sizes / Dimensions

What is the outer diameter of a pipe thread?  Pipes are odd in that their size is based roughly on the inner diameter of the pipe.  So when you have a National Pipe Thead – NPT fitting, the thread size is actually much larger than the name implies.


Pipe thread sizes

Nominal
NPT Size		 Outer
Diameter
inch (mm)		 Threads
per inch		 Pitch
of Thread
inch (mm)
1/16“ 0.3125 in (7.94 mm) 27 0.03704 in (0.94082 mm)
⅛” 0.405 in (10.29 mm) 27 0.03704 in (0.94082 mm)
¼” 0.540 in (13.72 mm) 18 0.05556 in (1.41122 mm)
3/8″ 0.675 in (17.15 mm) 18 0.05556 in (1.41122 mm)
½” 0.840 in (21.34 mm) 14 0.07143 in (1.81432 mm)
¾” 1.050 in (26.67 mm) 14 0.07143 in (1.81432 mm)
1″ 1.315 in (33.40 mm) 11½ 0.08696 in (2.20878 mm)
1¼” 1.660 in (42.16 mm) 11½ 0.08696 in (2.20878 mm)
1½” 1.900 in (48.26 mm) 11½ 0.08696 in (2.20878 mm)
2″ 2.375 in (60.33 mm) 11½ 0.08696 in (2.20878 mm)
2½” 2.875 in (73.02 mm) 8 0.12500 in (3.175 mm)
3″ 3.500 in (88.90 mm) 8 0.12500 in (3.175 mm)
4″ 4.500 in (114.30 mm) 8 0.12500 in (3.175 mm)
5″ 5.563 in (141.30 mm) 8 0.12500 in (3.175 mm)
6″ 6.625 in (168.27 mm) 8 0.12500 in (3.175 mm)
10″ 10.750 in (273.05 mm) 8 0.12500 in (3.175 mm)
12″ 12.750 in (323.85 mm) 8 0.12500 in (3.175 mm)
14″ OD 14 in (355.60 mm) 8 0.12500 in (3.175 mm)
16″ OD 16 in (406.40 mm) 8 0.12500 in (3.175 mm)
18″ OD 18 in (457.20 mm) 8 0.12500 in (3.175 mm)
20″ OD 20 in (508.00 mm) 8 0.12500 in (3.175 mm)
24″ OD 24 in (609.60 mm) 8 0.12500 in (3.175 mm)
Definition of pipe thread acronyms
NPT National Pipe Thread
FPT Female Pipe Thread
MPT Male Pipe Thread
PTF SAE short taper pipe thread – dryseal
NPTF American National taper pipe thread for dryseal pressure-tight joints
NPSM American National straight pipe thread for mechanical joints
NPSI American National straight intermediate pipe thread
GHT Garden Hose Thread
NST Fire Hose Thread
BSPT British Standard taper pipe thread (metric dimensions)