A pipe thread size chart gives you the exact outside diameter (OD), threads per inch (TPI), tap drill size, and engagement length for every standard pipe thread size. NPT (National Pipe Thread) is used throughout North America; BSP (British Standard Pipe) is standard in Europe, Asia, and most of the world. Use this reference to identify fittings, select the correct tap drill, and choose the right thread sealant.
All Pipe Thread Standards
| Nominal Size | Standard | TPI | OD (in) | OD (mm) | Tap Drill | Engagement (in) | Engagement (mm) |
|---|
NPT vs BSP — Key Differences
| Property | NPT / NPTF | BSPT (R/Rc) | BSPP (G) |
|---|---|---|---|
| Thread Angle | 60° | 55° (Whitworth) | 55° (Whitworth) |
| Taper | 1:16 (3/4" per foot) | 1:16 (3/4" per foot) | None (parallel) |
| Seal Method | Thread interference | Thread interference | O-ring / bonded seal |
| Sealant Needed? | Yes (PTFE / pipe dope) | Yes (PTFE / pipe dope) | No (O-ring seals) |
| Standard | ASME B1.20.1 | ISO 7-1 / BS 21 | ISO 228-1 / BS 2779 |
| Common Region | North America | Europe, Asia, UK | Europe, hydraulics |
| Interchangeable? | No — NPT and BSP are NOT interchangeable. Thread angles differ (60° vs 55°) and joints will leak under pressure. | ||
Size Comparison — Same Nominal, Different OD
| Nominal | NPT OD (in) | NPT TPI | BSP OD (in) | BSP TPI | OD Match? |
|---|
Thread Sealant Guide
| Sealant Type | Works With | Best For | Notes |
|---|---|---|---|
| PTFE Tape (Teflon) | NPT BSPT | Water, gas, air lines | Wrap 2–3 turns clockwise; use gas-rated (yellow) tape for gas lines. Do not use on BSPP. |
| Pipe Dope (Paste) | NPT BSPT | Steel, iron pipe; plumbing | Brush onto male threads. Provides some lubrication. Choose a compound rated for your media (water, gas, steam). |
| Anaerobic Thread Sealant | NPT BSPT BSPP | Hydraulics, vibrating systems | Loctite 565 / 567. Cures without air in the joint gap. Excellent vibration resistance. Not for oxygen service. |
| O-Ring / Bonded Seal | BSPP | Hydraulic fittings, BSP ports | BSPP requires a flat-face seal; no PTFE tape needed. Choose O-ring material (NBR, Viton) to match fluid and temperature. |
| PTFE + Pipe Dope | NPT | High-pressure lines | Apply PTFE tape first, then a thin coat of compatible pipe dope on top. Belt-and-suspenders approach for critical connections. |
Pipe Fitting Types Reference
How to Use the Pipe Thread Size Chart
This pipe thread size chart covers both NPT (National Pipe Thread) used in North America and BSP (British Standard Pipe) used internationally. Each standard lists nominal size, outside diameter, threads per inch, tap drill size, and engagement length. Use the tab bar to filter by standard, the search box to jump to a specific size, and the unit toggle to switch between inches and millimeters.
Step 1: Identify Your Thread Standard
Before looking up dimensions, identify whether you have NPT or BSP threads. NPT threads have a 60° thread angle and a pronounced taper you can often feel with your finger — the thread gets narrower toward the end. BSP threads have a 55° Whitworth form. BSPT (tapered) looks similar to NPT but the angles won't mate. BSPP (parallel) threads are straight with no taper. If in doubt, use a thread pitch gauge.
Step 2: Find the Nominal Size
Measure the outside diameter of the male threads (or inside of a female fitting) with calipers. Do NOT expect the OD to equal the nominal size — it won't. For example, a 1/2" NPT fitting has an OD of 0.840" (21.34 mm). Use the chart to match your measured OD to the correct nominal size. The pipe thread size chart lists OD in both inches and millimeters for quick matching.
Step 3: Select Tap Drill for New Threads
When threading a new hole for a pipe fitting, drill to the tap drill size listed for the nominal pipe size. For example, a 1/2" NPT tap requires a 23/32" (0.7188") drill. The tap drill size creates the correct diameter for the tap to cut proper threads. Always drill completely through or to full depth before tapping.
Step 4: Apply the Correct Sealant
NPT and BSPT tapered threads require a thread sealant because the taper wedge action alone can still allow seepage. Apply PTFE tape (2–3 wraps clockwise on male threads) or pipe dope before assembly. BSPP parallel threads seal on a flat face with an O-ring or bonded seal — never use PTFE tape on BSPP as it can prevent the O-ring from seating correctly.
Step 5: Use the NPT vs BSP Comparison Tab
Select the "NPT vs BSP" tab to see a side-by-side comparison of thread dimensions for each nominal size. This is useful when sourcing international fittings — while many nominal sizes appear close in OD, the thread profiles are incompatible. The comparison table highlights whether the OD matches (some do) and the TPI differences that make them non-interchangeable.
Frequently Asked Questions
Is this pipe thread size chart free?
Yes, completely free with no account required. All data is built into the page and works offline — nothing is sent to any server.
Is my data private when using this tool?
Yes, all lookups happen entirely in your browser. No search queries or data are transmitted anywhere. The tool works offline once the page loads.
What is the difference between NPT and BSP threads?
NPT (National Pipe Thread) is the North American standard using a 60° thread angle and taper of 1 in 16. BSP (British Standard Pipe) uses a 55° Whitworth thread form. While nominal pipe sizes (like 1/2") refer to the same internal bore, the thread profiles are incompatible — you cannot mix NPT and BSP fittings.
What does nominal pipe size actually mean?
Nominal pipe size is a dimensionless label that roughly corresponds to the inside diameter of the pipe. The actual outside diameter is always larger. For example, a 1/2" NPT pipe has an OD of 0.840" (21.34 mm), not 0.5". This convention dates from when pipe wall thickness was standardized for iron pipe.
What is the difference between BSPT and BSPP?
BSPT (British Standard Pipe Taper) is a tapered thread that seals on the threads themselves, similar to NPT. BSPP (British Standard Pipe Parallel) has straight threads that seal using a bonded seal or O-ring on a flat face. BSPP is common in hydraulic systems; BSPT is used in plumbing and pneumatic lines.
How many turns should I use when assembling NPT fittings?
For NPT connections, hand-tighten first (typically 3–4 turns), then add 2–3 wrench turns for a pressure-tight seal. The taper ensures that the threads wedge together and seal without a separate gasket. Over-tightening can crack fittings, especially in brass or plastic.
Do I need thread sealant on NPT or BSP threads?
NPT threads require sealant (PTFE tape or pipe dope) because the taper seal can still leak without it. BSPP threads use a bonded seal or O-ring and typically do not need tape. BSPT threads also benefit from PTFE tape. Anaerobic thread sealants (like Loctite 565) work on both systems and resist vibration loosening better than tape.
Can I use NPT fittings with BSP threads?
Not safely. NPT (60° angle, 1:16 taper) and BSP (55° angle) threads are not interchangeable. Some nominal sizes share similar OD measurements, so they may appear to fit, but the thread angles won't mate correctly and the joint will leak under pressure. Always use the correct standard.