NPT Pipe Thread Chart

Notes on This Chart

• NPT dimensions are per ASME B1.20.1. Major diameters are measured at the large end of the external thread.
• Pitch diameters are at the gauge plane (E1) — the reference point at the end of hand-tight engagement.
• Tap drill sizes are approximate starting points. The actual hole size may vary depending on material and tap manufacturer recommendations.
• All NPT threads have a taper of 3/4″ per foot (1°47′) on diameter.
• NPT and NPTF share the same basic dimensions, but NPTF has tighter crest/root tolerances for dryseal capability.
• BSPT dimensions are per ISO 7/1 (BS 21). BSPT uses a 55° thread angle vs. NPT’s 60°.
• NPT and BSPT are not interchangeable despite similar nominal sizes. Mixing them will leak.

How to Use This NPT Pipe Thread Chart

Finding the right pipe thread dimensions is straightforward once you know what to look for:

1. Identify the pipe size. NPT sizes are called out by nominal pipe size (NPS), not actual diameter. A “1/2 NPT” fitting does not have a 1/2″ outside diameter — it has a 0.840″ major diameter.
2. Find the row in the chart. Use the search box or filter tabs to locate your pipe size and thread standard (NPT, NPTF, or BSPT).
3. Read the dimensions. The major, minor, and pitch diameters tell you the thread geometry. The tap drill size tells you what drill to use before tapping an internal pipe thread.
4. Check the engagement length. This is the hand-tight engagement length (L1). You will tighten 3–5 additional turns past this point with a wrench.
5. Apply sealant. For NPT threads, always use PTFE tape or pipe dope. For NPTF (dryseal), sealant is optional but recommended for critical applications.

1/4 NPT Thread Dimensions — Quick Reference

1/4″ NPT is one of the most widely used pipe thread sizes. You will find it in pneumatic fittings, hydraulic ports, instrument connections, and fluid power manifolds. Here are the exact specifications:

Tip: When drilling for a 1/4″ NPT tap, use a 7/16″ drill bit (0.4375″). Drill deep enough to accommodate the full effective thread length plus the vanish threads at the bottom. For a through hole, drill all the way through and tap from one side.

NPT vs BSPT: Which Pipe Thread to Use

NPT and BSPT are both tapered pipe thread standards, but they are not interchangeable. Here is a side-by-side comparison:

When to use NPT: Any project destined for the North American market, plumbing per US codes, pneumatic/hydraulic systems in US-made equipment.

When to use BSPT: Export products, European/Asian equipment, systems that must comply with ISO standards.

NPT Thread Sealant Guide

• PTFE tape (Teflon tape). The most common sealant. Wrap 3–5 turns clockwise (looking at the thread end) around the male threads. Use yellow tape for gas lines, white tape for water and general purpose, green tape for oxygen systems.
• Pipe dope (thread compound). Brush it onto the male threads. Better than tape for large pipe sizes and high-vibration applications. Some compounds are rated for specific fluids or temperatures.
• Anaerobic thread sealant. Liquid sealants like Loctite 567 or Permatex that cure in the absence of air between metal surfaces. Good for high-pressure applications and permanent connections.
• NPTF (dryseal) threads. Designed to seal without sealant by crushing the thread crests and roots. However, sealant is still recommended for critical applications as insurance against micro-leaks.
• Do not use tape on NPTF threads in fuel/hydraulic systems — tape fragments can contaminate the fluid and clog filters or valves.