What this guide covers: how pipe thread sizes work, common standards (NPT, BSP/BSPT/BSPP), threads-per-inch (TPI) and major diameters, tap & drill sizes for pipe threads, identification tips, and practical selection advice — with a compact, printable chart for common sizes.

Basics: What “pipe thread size” actually means

Pipe thread sizing is confusing at first because the nominal pipe size (e.g., 1/2″, 3/4″) does NOT equal the measured outside diameter (OD) of the threaded section. Instead, nominal size refers roughly to the pipe’s internal bore — you must use a table or chart to match the nominal label to actual OD and threads-per-inch (TPI).

Key fact: For tapered pipe threads (like NPT and BSPT) the threads themselves provide the seal when tightened; for parallel threads (BSPP, NPSM) a gasket or O-ring usually seals the joint.

Standards overview: NPT (ASME), BSP (ISO/BS), NPS

The most common thread standards you’ll encounter are:

• NPT (National Pipe Taper) — ANSI/ASME B1.20.1: widely used in North America; tapered at 1:16 and specified by nominal pipe size, TPI and major diameter.
• BSP (British Standard Pipe) — BSPT (taper) & BSPP (parallel): common in Europe, Asia and many international markets; thread form is Whitworth-based (55°) and uses different pitch/TPI than NPT.
• NPS / NPSM / NPTF: related US series (straight vs tapered / dryseal variants) used for specific mechanical joints or fuel-tight seals.

NPT thread sizes chart (common sizes)

The table below summarizes the most frequently used NPT (tapered) sizes with nominal size, typical outside diameter (OD) and threads per inch (TPI). Use this chart to identify sizes and to select appropriate taps and plugs.

Source tables and dimensions for NPT are defined in ASME/ANSI B1.20.1 and common engineering cheat-sheets; cross-check OD and TPI for positive identification.

BSP thread sizes chart (BSPT & BSPP)

BSP threads use a Whitworth 55° profile and different pitches. BSPT is tapered (R) and BSPP is parallel (G). Below are common BSP nominal sizes, TPI/pitch, and approximate major diameters.

BSP pitch and major diameters vary slightly by standard; refer to British Standards or manufacturer charts when precise tolerances are required.

Pipe tap & drill sizes — practical guidance

Cutting pipe threads is not the same as cutting fastener threads. For tapered threads (NPT/BSPT) you use specialized pipe taps (taper or bottoming taps) and recommended drill sizes that create the correct internal profile allowing taper engagement. Below are practical rules:

• Tapping order: For tapered holes use a taper tap first (3–5 turns in, back off), then a plug/bottoming tap to reach full depth if required.
• Drill size: Use a drill size chart specific to the pipe thread series (NPT tap-drill lists are widely available). For example, many suppliers publish NPT tap drill sizes that match each nominal size.
• Thread form & sealing: NPT forms a seal by metal-to-metal interference of taper — use correct taper and apply sealant; BSPP (parallel) usually requires a washer or O-ring for sealing.

Example — typical NPT tap-drill selections (illustrative)

• 1/8″ NPT — drill ≈ 11/64″ (8.0 mm) (confirm with specific tap chart)
• 1/4″ NPT — drill ≈ 7/32″ (5.5 mm) (confirm)
• 1/2″ NPT — drill ≈ 27/64″ (10.7 mm) (confirm)

Always verify drill sizes with your tap supplier or technical datasheet — formulas vary with tap form and intended thread engagement. See common tap charts for accurate values.

How to identify unknown pipe threads (step-by-step)

When you encounter an unknown threaded fitting, follow these practical steps:

1. Measure Outside Diameter (OD) — use calipers and record to 0.01 mm or 0.001″.
2. Count Threads Per Inch (TPI) — or measure pitch in mm (1/pitch gives TPI roughly).
3. Compare OD + TPI to a chart — NPT, BSP, and metric series have characteristic OD/TPI pairs (see the tables above and published charts).
4. Check thread angle — Whitworth (BSP) uses 55° thread form; NPT uses 60° (unified) form — a profile gauge helps.
5. Test fit — if possible, try a known-good fitting or gauge. Use caution to avoid damaging a critical fitting.

MisolGroup — thread cutting tool manufacturer

MisolGroup (founded 2005) is a Chinese manufacturer specializing in high-performance thread cutting tools, including pipe taps. Their capabilities include full in-house production, vacuum heat treatment, and advanced CNC manufacturing to produce taps in HSS grades like M35, DV3, and M2 — engineered for demanding materials (stainless, carbon steel, cast iron, non-ferrous alloys). Misol offers:

• Carbide Taps: Designed for high-speed applications and hard materials such as stainless steel and titanium.
• Nut Taps: Used for producing threads inside nuts or deep holes.
• Pipe Taps: Ideal for threading pipe fittings that require airtight or watertight seals.
• Spiral Flute Taps: Best for blind holes; they efficiently evacuate chips upward.
• Spiral Point Taps: Suitable for through-holes where chips can exit forward.
• Straight Flute Taps: Used for general-purpose threading in a variety of materials.

Why this matters: using purpose-built pipe taps from a trusted manufacturer like MisolGroup reduces tap breakage, improves thread quality, and ensures correct engagement/seal for tapered threads — critical in plumbing, hydraulic, and aerospace applications.

Summary table — quick reference

References

• ASME / ANSI B1.20.1 — National Pipe Taper threads (NPT) (overview & dimensions).
• NPT thread chart & dimensions (engineering & chart PDFs).
• British Standard Pipe charts (BSPT / BSPP) and Whitworth thread form.
• NPT tap/drill practical charts and supplier guidance.
• MisolGroup — thread tool manufacturing & product categories (carbide taps, pipe taps, spiral flute, etc.). (User-provided insertion)