The ISO Pipe Thread Standard is internationally recognized and widely used in Europe, Asia, and other global markets for fluid, gas, and mechanical connections. It includes two main types of threads:
✔ ISO G Thread (Parallel – BSPP Equivalent) – Straight (parallel) threads requiring a sealing washer or O-ring for leak-proof connections.
✔ ISO R Thread (Tapered – BSPT Equivalent) – Tapered threads that create a mechanical seal through thread interference, similar to NPT.
These threads are standardized under ISO 228-1 (G Thread – Parallel) and ISO 7-1 (R Thread – Tapered).
Key Features of ISO Pipe Threads:
✔ Thread Angle: 55° Whitworth form (Same as BSP, different from NPT’s 60°.)
✔ G Thread (ISO 228-1 – Parallel): Requires a sealing element (gasket or O-ring).
✔ R Thread (ISO 7-1 – Tapered): Forms a pressure-tight seal without additional elements.
✔ ISO Standardization: Widely used globally in hydraulics, air systems, and industrial piping.
ISO G vs. ISO R – Key Differences
| Feature | ISO G Thread (Parallel) | ISO R Thread (Tapered) |
|---|---|---|
| Thread Shape | Straight (Parallel) | Tapered |
| Sealing Method | Requires washer or gasket | Seals through thread interference |
| Common Use | Hydraulics, fittings, low-pressure applications | Gas, oil, and high-pressure applications |
| Standard | ISO 228-1 | ISO 7-1 |
| Thread Angle | 55° | 55° |
Common Applications of ISO Pipe Threads:
- Plumbing & Water Systems (Used for water supply lines and fittings.)
- Hydraulics & Pneumatics (Common in hydraulic systems and air compressors.)
- Oil & Gas Industry (Found in gas pipelines, fuel transfer systems, and pressure vessels.)
- Industrial Equipment & Machinery (Pipes, pumps, valves, and other mechanical fittings.)
- Irrigation & Agricultural Equipment (Water pumps, filters, and sprinkler systems.)
ISO Pipe Standard Thread Sizes & Dimensions
The following tables provide both inch and metric equivalents for major, pitch, and minor diameters, as well as recommended tap drill sizes for ISO G and ISO R threads.
ISO G (Parallel) Thread Dimensions (Inches)
| Nominal Size | Threads Per Inch (TPI) | Major Diameter (in) | Pitch Diameter (in) | Minor Diameter (in) | Tap Drill Size (in) |
|---|---|---|---|---|---|
| 1/16″ | 28 | 0.2611 | 0.2385 | 0.2194 | 0.2180 |
| 1/8″ | 28 | 0.3839 | 0.3605 | 0.3406 | 0.3400 |
| 1/4″ | 19 | 0.5185 | 0.4826 | 0.4530 | 0.4530 |
| 3/8″ | 19 | 0.6560 | 0.6200 | 0.5890 | 0.5890 |
| 1/2″ | 14 | 0.8250 | 0.7700 | 0.7330 | 0.7330 |
| 3/4″ | 14 | 1.0410 | 0.9860 | 0.9490 | 0.9490 |
| 1″ | 11 | 1.3090 | 1.2490 | 1.2060 | 1.2060 |
ISO R (Tapered) Thread Dimensions (Inches)
| Nominal Size | Threads Per Inch (TPI) | Major Diameter (in) | Pitch Diameter (in) | Minor Diameter (in) | Tap Drill Size (in) |
|---|---|---|---|---|---|
| 1/16″ | 28 | 0.2611 | 0.2418 | 0.2270 | 0.2180 |
| 1/8″ | 28 | 0.3839 | 0.3643 | 0.3490 | 0.3400 |
| 1/4″ | 19 | 0.5185 | 0.4946 | 0.4720 | 0.4530 |
| 3/8″ | 19 | 0.6560 | 0.6320 | 0.6100 | 0.5890 |
| 1/2″ | 14 | 0.8250 | 0.8001 | 0.7780 | 0.7330 |
| 3/4″ | 14 | 1.0410 | 1.0161 | 0.9940 | 0.9490 |
| 1″ | 11 | 1.3090 | 1.2841 | 1.2620 | 1.2060 |
ISO G (Parallel) Thread Dimensions (Metric – mm)
| Nominal Size | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| 1/16″ | 6.63 | 6.14 | 5.77 | 5.54 |
| 1/8″ | 9.75 | 9.25 | 8.86 | 8.64 |
| 1/4″ | 13.17 | 12.56 | 11.99 | 11.51 |
| 3/8″ | 16.66 | 16.05 | 15.49 | 14.96 |
| 1/2″ | 20.95 | 20.32 | 19.76 | 18.62 |
| 3/4″ | 26.44 | 25.81 | 25.25 | 23.88 |
| 1″ | 33.24 | 32.59 | 32.03 | 30.63 |
ISO R (Tapered) Thread Dimensions (Metric – mm)
| Nominal Size | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| 1/16″ | 6.63 | 6.14 | 5.77 | 5.54 |
| 1/8″ | 9.75 | 9.25 | 8.86 | 8.64 |
| 1/4″ | 13.17 | 12.56 | 11.99 | 11.51 |
| 3/8″ | 16.66 | 16.05 | 15.49 | 14.96 |
| 1/2″ | 20.95 | 20.32 | 19.76 | 18.62 |
| 3/4″ | 26.44 | 25.81 | 25.25 | 23.88 |
| 1″ | 33.24 | 32.59 | 32.03 | 30.63 |
Related Pipe Thread Standards
- BSP – British Standard Pipe (Widely used in the UK, Europe, and Asia.)
- NPT – National Pipe Thread (Common in North America, with a 60° thread angle.)
- NPS – National Pipe Straight (Straight-thread alternative for mechanical connections.)
Important Notice on Standards Validity
This information is for reference purposes only. Users should consult official sources and updated standards to ensure compliance with current regulations. The validity of standards may change over time, so it is recommended to verify them through recognized engineering and standardization organizations (ISO, ANSI, ASME, etc.).
Check with relevant standardization bodies in your region to confirm whether a specific standard is still valid and applicable. Regulations and industry requirements may vary based on region and application.