← AskMachinist · Guides · Calculator · 🔎 Search
NPT Pipe Threading Basics for Machinists
Guide · Updated October 28, 2026
NPT (American National Standard Taper Pipe Thread, ASME B1.20.1) is a 60° thread cut on a taper of 1 in 16 on diameter, or 3/4 in per foot. The taper makes the joint seal as it tightens, usually with tape or sealant. The half-angle to the centerline is 1°47′24″ (1.7899°).
Check the blank
Mic the pipe or blank OD before threading. The free Gage Block Calculator builds the stack.
Step by step
- Look up the size: Get OD, threads per inch, E0 and L2 from the B1.20.1 table.
- Turn the blank: Turn the OD and taper it, or thread from round pipe stock.
- Set the taper attachment: Set it to 1.790° (3/4 in per foot).
- Set the tool square to the axis: Square the 60° tool to the axis, not to the taper.
- Set the gearbox: Set the threads per inch.
- Cut: Cut with the compound, taking out backlash the same way each pass.
- Gauge: Check with an L1 ring gage or a fitting.
Worked example: 1/2-14 NPT
From the B1.20.1 basic dimensions (Machinery's Handbook and the MachiningDoctor NPT chart): pipe OD 0.840 in, 14 TPI (P = 0.07143 in), pitch diameter at the pipe end E0 = 0.75843 in, hand-tight engagement L1 = 0.320 in, effective thread length L2 = 0.5337 in, thread height h = 0.05714 in (0.8P). The pitch diameter grows by 1/16 of the length: over L1, 0.320 ÷ 16 = 0.020 in, so E1 = 0.75843 + 0.020 = 0.77843 in, which matches the table.
Why square to the axis
B1.20.1 defines the thread form with its bisector perpendicular to the axis, not to the taper. Set the tool with a center gage against the work axis.
Gauging
The tolerance is one turn large or small from basic. A ring gage should go on hand-tight to within one turn of its notch.
Common problems
Leaks: wrong taper or torn threads. Fitting goes on too far: thread too small. Thread lean: tool squared to the taper.
Tips
- Taper: taper attachment.
- Gage: center gage.
- Threads: threads on a lathe.
Worked example: 1-11-1/2 NPT engagement
From ASME B1.20.1, 1-11-1/2 NPT has a hand-tight engagement L1 of 0.400 in, which is 0.400 × 11.5 = 4.6 threads, and a common tap drill of 1-5/32 in (1.1563 in). Over L1 the diameter changes 0.400 ÷ 16 = 0.025 in because of the 1:16 taper.
Wrench make-up adds roughly 3 turns past hand-tight for sizes up to about 1 in. If a fitting goes in hand-tight 6 threads instead of 4.6, the internal thread is about 1.4 threads deep, which is (1.4 ÷ 11.5) ÷ 16 ≈ 0.008 in oversize on diameter.
FAQ
What is the NPT taper?
1 in 16 on diameter, 3/4 in per foot.
What is the half-angle?
1°47′24″, or 1.7899°.
What TPI is 1/2 NPT?
14.
What is the OD of 1/2 in pipe?
0.840 in.
How is NPT gauged?
With L1 plug and ring gages, within one turn.
Related guides and calculators
- How to Cut a Taper with a Taper Attachment
- How to Use a Center Gage (Fishtail)
- How to Cut Threads on a Lathe
- Thread Gage Basics
- How to Measure a Taper
- How to Use a Threading Die and Die Stock
- How to Use a Form Tool on a Lathe
- How to Use a Pipe Tap (NPT)
- How to Thread Pipe with a Hand Pipe Die (NPT)
- All free machinist calculators
- NPT vs BSPT pipe thread comparison
- Pipe Schedule Chart: OD and Wall Thickness
- JIC and ORB Hydraulic Fitting Sizes
- O-Ring Size and Gland Squeeze Basics
Build your stack in seconds
The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.
Questions? Ask a machinist.
General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.