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NPT Thread Chart

Find NPT size, threads per inch, pitch, and actual pipe outside diameter, then check detailed taper-thread dimensions and measurement guidance.

NPT Thread Size Chart

24 NPT sizes: 1/16 through 24 OD. NPT uses a 60° thread form and a 1:16 taper on diameter. For sizes through 12, the nominal designation is not the pipe outside diameter; the large 14 OD through 24 OD series uses outside diameter in the designation.

The complete chart is available below in inches. JavaScript adds synchronized inch/millimeter switching.

The compact chart fits typical phone widths. If your browser zoom makes it wider, scroll the chart region horizontally.

General-purpose NPT nominal size, threads per inch, actual pipe outside diameter, and axial thread pitch.
NPT size TPI OD Pitch
1/16 27 0.3125 in 0.03704 in
1/8 27 0.4050 in 0.03704 in
1/4 18 0.5400 in 0.05556 in
3/8 18 0.6750 in 0.05556 in
1/2 14 0.8400 in 0.07143 in
3/4 14 1.0500 in 0.07143 in
1 11.5 1.3150 in 0.08696 in
1-1/4 11.5 1.6600 in 0.08696 in
1-1/2 11.5 1.9000 in 0.08696 in
2 11.5 2.3750 in 0.08696 in
2-1/2 8 2.8750 in 0.12500 in
3 8 3.5000 in 0.12500 in
3-1/2 8 4.0000 in 0.12500 in
4 8 4.5000 in 0.12500 in
5 8 5.5630 in 0.12500 in
6 8 6.6250 in 0.12500 in
8 8 8.6250 in 0.12500 in
10 8 10.7500 in 0.12500 in
12 8 12.7500 in 0.12500 in
14 OD 8 14.0000 in 0.12500 in
16 OD 8 16.0000 in 0.12500 in
18 OD 8 18.0000 in 0.12500 in
20 OD 8 20.0000 in 0.12500 in
24 OD 8 24.0000 in 0.12500 in

Pipe OD is the standardized outside diameter associated with the pipe size. The actual external thread diameter changes along the taper, so pipe OD is not interchangeable with pitch diameter measured at an arbitrary thread location.

Print scope: NPT size chart, dimensional definitions, measurement diagram, detailed dimensions, and source/method notes.

How NPT taper and reference planes are measured

NPT tapered external thread measurement diagram Schematic external NPT thread showing the 60 degree thread form, 1 in 16 taper on diameter, pipe end E0 plane, hand-tight E1 plane, and L1 hand-tight engagement length. 60° NPT thread form · 1:16 taper on diameter E0 · pipe end E1 · hand-tight plane L1 hand-tight engagement 1:16 taper 60° diameter

Schematic, not to scale. NPT taper is 1 in 16 measured on diameter, equivalent to 3/4 in of diameter change per foot of axial length. Because the thread is tapered, measured diameter depends on where the measurement is taken.

Detailed NPT reference dimensions

E0 is pitch diameter at the pipe end, E1 is pitch diameter at the hand-tight plane, L1 is hand-tight engagement length, and L2 is effective external thread length.

This detailed dimension table is wide. Scroll horizontally within the table region to view all columns.

Detailed general-purpose NPT dimensions in the selected display unit.
NPT size TPI Pipe OD E0 E1 L1 L2
1/16 27 0.3125 in 0.27118 in 0.28118 in 0.1600 in 0.2611 in
1/8 27 0.4050 in 0.36351 in 0.37360 in 0.1615 in 0.2639 in
1/4 18 0.5400 in 0.47739 in 0.49163 in 0.2278 in 0.4018 in
3/8 18 0.6750 in 0.61201 in 0.62701 in 0.2400 in 0.4078 in
1/2 14 0.8400 in 0.75843 in 0.77843 in 0.3200 in 0.5337 in
3/4 14 1.0500 in 0.96768 in 0.98887 in 0.3390 in 0.5457 in
1 11.5 1.3150 in 1.21363 in 1.23863 in 0.4000 in 0.6828 in
1-1/4 11.5 1.6600 in 1.55713 in 1.58338 in 0.4200 in 0.7068 in
1-1/2 11.5 1.9000 in 1.79609 in 1.82234 in 0.4200 in 0.7235 in
2 11.5 2.3750 in 2.26902 in 2.29627 in 0.4360 in 0.7565 in
2-1/2 8 2.8750 in 2.71953 in 2.76216 in 0.6820 in 1.1375 in
3 8 3.5000 in 3.34062 in 3.38850 in 0.7660 in 1.2000 in
3-1/2 8 4.0000 in 3.83750 in 3.88881 in 0.8210 in 1.2500 in
4 8 4.5000 in 4.33438 in 4.38712 in 0.8440 in 1.3000 in
5 8 5.5630 in 5.39073 in 5.44929 in 0.9370 in 1.4063 in
6 8 6.6250 in 6.44609 in 6.50597 in 0.9580 in 1.5125 in
8 8 8.6250 in 8.43359 in 8.50003 in 1.0630 in 1.7125 in
10 8 10.7500 in 10.54531 in 10.62094 in 1.2100 in 1.9250 in
12 8 12.7500 in 12.53281 in 12.61781 in 1.3600 in 2.1250 in
14 OD 8 14.0000 in 13.77500 in 13.87262 in 1.5620 in 2.2500 in
16 OD 8 16.0000 in 15.76250 in 15.87575 in 1.8120 in 2.4500 in
18 OD 8 18.0000 in 17.75000 in 17.87500 in 2.0000 in 2.6500 in
20 OD 8 20.0000 in 19.73750 in 19.87031 in 2.1250 in 2.8500 in
24 OD 8 24.0000 in 23.71250 in 23.86094 in 2.3750 in 3.2500 in

The inch values are the canonical build dataset. Metric values are exact 25.4 mm/in conversions shown with display rounding.

How do you identify an NPT thread?

  1. Measure the male thread or pipe outside diameter with calipers.
  2. Find the nearest listed pipe OD in the chart rather than treating the nominal size as a physical diameter.
  3. Check threads per inch with a thread-pitch gauge.
  4. Confirm the thread is tapered rather than parallel.
  5. Do not assume NPTF or BSPT is interchangeable just because a nominal size looks similar.

Worked example: an outside diameter near 0.840 in is consistent with 1/2 NPT, whose expected pitch is 14 TPI. Verify the 14-TPI pitch and taper before final identification.

Why the names change above 12: conventional NPT sizes through 12 use nominal pipe-size designations that differ from actual OD. The large-size series is labeled 14 OD, 16 OD, 18 OD, 20 OD, and 24 OD because those designations use outside diameter directly.

NPT is not the same as NPTF or BSPT

High-level identification differences. Use the governing standard for production acceptance.
Thread familyTaper / angleWhat this page covers
NPT1:16 taper on diameter; 60° thread formPrimary chart and dimensions
NPTFDryseal family with separate crest/root requirementsNot a production-acceptance table for NPTF
BSPT / RSeparate pipe-thread family with a 55° thread formNot covered by the NPT dimensions above

This chart does not establish pressure rating, material compatibility, sealing performance, or inspection acceptance. A dimensional match is not a substitute for the applicable product, piping, or inspection specification.

Sources, formulas, and reference scope

Standard basis: ASME B1.20.1-2013 (R2018), with dimensional verification against FED-STD-H28 Section 7 and independent formula checks. This page does not reproduce ASME tables, figures, or layout.

Displayed range
24 general-purpose NPT sizes, 1/16 through 24 OD
Canonical unit
Inches
Metric conversion
Exact 25.4 mm per inch; display rounded
Pitch
Calculated as 1 ÷ TPI
E0 check
D − (0.05D + 1.1)p
L2 check
(0.8D + 6.8)p
E1 check
E0 + 0.0625L1
Excluded
NPTF, BSPT/R, NPSC, NPSM, NPSL, NPTR, pressure ratings, tolerances, gaging acceptance
  • Pipe Size Chart — nominal pipe size, OD, schedule, wall thickness, and inside diameter.
  • Tap Drill Chart — UNC, UNF, metric, and shop drill-size reference.
  • Screw Size Chart — numbered screw diameters and identification reference.

NPT Thread Chart FAQ

Why does 1/2 NPT measure about 0.84 inch?

The 1/2 designation is nominal, not the physical outside diameter. The associated pipe outside diameter is 0.840 in.

What does 1/2-14 NPT mean?

It identifies nominal 1/2 NPT with 14 threads per inch.

What is the NPT taper?

NPT has a 1:16 taper measured on diameter, equal to 3/4 in of diameter change per foot of axial length.

Can outside diameter alone identify NPT?

No. OD can narrow the likely nominal size, but you should also verify thread pitch, taper, and the thread family.

Is NPT the same as NPTF?

No. NPTF is a separate Dryseal thread family with different crest/root requirements. This chart is for general-purpose NPT.

Is NPT interchangeable with BSPT?

No. BSPT is a different thread family and uses a 55° thread form. Similar-looking nominal sizes are not proof of interchangeability.

What size drill do I use before tapping an NPT thread?

There is not one universal pre-drill value for every tapping method. Recommended starting holes can vary with tap manufacturer, material, thread depth, and whether a taper reamer is used. Use a tooling-specific recommendation or the Tap Drill Chart, then verify the NPT tap manufacturer’s instructions.

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