Thread Standards
Thread standards are the most common source of confusion when selecting couplings, manifolds, and fittings. Here are the major standards — structure, dimensions, and how to identify them.
Identify Your Thread in 3 Steps
① Parallel vs Tapered
If the thread diameter stays constant along its length it is parallel (G, UNF, Metric); if it narrows toward the end it is tapered (R, NPT). Tapered threads seal on the thread itself; parallel threads seal with an O-ring, washer, or gasket.
② Measure OD and Pitch
Measure the male thread outside diameter (Ø) with calipers and check the pitch with a pitch gauge. Inch standards (G/R/NPT/UNF) use TPI (threads per inch); metric threads use pitch in mm.
③ Match the Tables Below
Compare your measured Ø against each standard. Example: a male OD of ~13 mm and parallel → likely G 1/4"; ~13.6 mm and tapered → likely R 1/4".
G Thread (BSPP)
Cylindrical (parallel) pipe thread per ISO 228-1, commonly called BSP/BSPP (e.g. G 1/4"). It does not seal on the thread — sealing uses a washer or O-ring (ISO 1179). The de-facto standard for fluid equipment in Europe and Asia.
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| G 1/8" | 9.6 | 8.0 |
| G 1/4" | 13.0 | 10.0 |
| G 3/8" | 16.5 | 10.0 |
| G 1/2" | 20.8 | 12.0 |
| G 3/4" | 26.3 | 12.0 |
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| G 1/8" | 8.75 | 7.4 |
| G 1/4" | 11.8 | 11.0 |
| G 3/8" | 15.25 | 11.4 |
| G 1/2" | 19.0 | 15.0 |
| G 3/4" | 24.5 | 16.3 |
R / Rc Thread (BSPT)
Conical (tapered) pipe thread per ISO 7-1, commonly called BSPT and widely used in Korea and Japan. Male threads are marked R; female threads Rc (tapered) or Rp (parallel) — e.g. R 1/4". Sealed with PTFE tape or sealant.
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| R 1/8" | 10.2 | 7.4 |
| R 1/4" | 13.6 | 11.0 |
| R 3/8" | 17.2 | 11.0 |
| R 1/2" | 21.7 | 15.0 |
| R 3/4" | 27.1 | 16.3 |
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| Rc 1/8" | 8.3 | 7.4 |
| Rc 1/4" | 11.0 | 11.0 |
| Rc 3/8" | 14.5 | 11.4 |
| Rc 1/2" | 18.0 | 15.0 |
| Rc 3/4" | 23.5 | 16.3 |
NPT Thread (US Standard)
US tapered pipe thread per ANSI/ASME B1.20.1 (e.g. 1/4" NPT). Its 60° thread angle is NOT compatible with 55° BSPT. The standard for North American equipment.
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| 1/8" NPT | 10.5 | 6.7 |
| 1/4" NPT | 14.0 | 10.2 |
| 3/8" NPT | 17.5 | 10.4 |
| 1/2" NPT | 21.8 | 13.6 |
| 3/4" NPT | 27.1 | 13.9 |
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| 1/8" NPT | 8.5 | 6.9 |
| 1/4" NPT | 11.0 | 10.0 |
| 3/8" NPT | 14.5 | 10.3 |
| 1/2" NPT | 18.0 | 13.6 |
| 3/4" NPT | 23.0 | 14.1 |
Metric Thread
Parallel thread per ISO 68 / ISO 724, designated M + diameter × pitch (e.g. M16×1.5). Used across European and Korean industrial equipment; sealed with an O-ring or bonded seal.
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| M10×1.25 | 9.8 | 9.0 |
| M12×1.5 | 11.85 | 13.0 |
| M14×1.5 | 13.85 | 13.0 |
| M16×1.5 | 15.85 | 8.81 |
| M22×1.5 | 21.85 | 15.7 |
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| M16×1.5 | 14.5 | 9.0 |
UNF Thread (Unified Fine)
US parallel fine thread per ISO 68 / ANSI B1.1, designated diameter-TPI (e.g. 9/16"-18 UNF). Widely used with SAE O-ring Boss (ORB) ports.
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| 7/16"-20 UNF | 11.0 | 13.3 |
| 7/8"-14 UNF | 22.0 | 15.5 |
| Designation | Ø Diameter (mm) | Thread Length L (mm) |
|---|---|---|
| 7/16"-20 UNF | 9.8 | 11.5 |
| 9/16"-18 UNF | 12.9 | 12.7 |
| 3/4"-16 UNF | 17.5 | 14.3 |
| 1 1/16"-16 UNF | 26.8 | 19.0 |
| 1 5/16"-12 UNF | 33.1 | 19.0 |
Hose Connection (Hose Barb)
A push-on hose connection rather than a thread. Standard barbs require a separate hose clamp after insertion; nut-cap style barbs (stream-line type) provide reusable, clamp-free retention.
| Hose ID | Thread Length L (mm) |
|---|---|
| 6.3 mm (1/4") | 18.0 |
| 8 mm (5/16") | 18.0 |
| 10 mm (3/8") | 21.0 |
| 13 mm (1/2") | 21.0 |
| 18 mm (5/8") | 21.0 |
| Hose ID | Thread Length L (mm) |
|---|---|
| 5 × 8 mm | 15.0 |
| 6.5 × 10 mm | 17.0 |
| 8 × 12 mm | 19.0 |
| 9.5 × 13.5 mm | 21.0 |
| 11 × 16 mm | 25.0 |
OD Comparison of Common Sizes — G vs R vs NPT (male)
The three standards most often confused in the field, compared by male outside diameter (Ø). Just 1–2 mm separates them.
| Size | G (BSPP) Ø | R (BSPT) Ø | NPT Ø |
|---|---|---|---|
| 1/8" | 9.6 mm | 10.2 mm | 10.5 mm |
| 1/4" | 13.0 mm | 13.6 mm | 14.0 mm |
| 3/8" | 16.5 mm | 17.2 mm | 17.5 mm |
| 1/2" | 20.8 mm | 21.7 mm | 21.8 mm |
| 3/4" | 26.3 mm | 27.1 mm | 27.1 mm |
Note: table values are representative nominal dimensions (mm). For precise identification use pitch and thread gauges; for product-level drawing dimensions, contact the IMMERSEKOOL engineering team.
Need couplings or fittings in your thread standard?
G, R/Rc, NPT, UNF, metric — we build and supply DLC / immersion-cooling couplings and manifolds to your requested standard.