Pipe Thread Size Charts in MM/Inches (Dimensions, Types, Standards, Calculation)

Among the many types of threads, those used to connect and join piping systems – such as water supply lines and pipe fittings – are known as pipe threads. However, the types and naming conventions of pipe threads can be somewhat complex, often leading to confusion. Using pipe threads from different standards can result in leaks, malfunctions, or even damage. To ensure safe and proper connections, it’s important to thoroughly understand and verify the types, standards, and names of the pipe threads being used. 

So here we will cover the basics of pipe threads, the main standards used globally, detailed size charts in both millimeters and inches, and how to measure and calculate thread dimensions accurately.

What is a Pipe Thread?

A pipe thread is a screw-type connection used to join pipes and fittings. These threads can form a mechanical connection or provide a seal that prevents leaks of air, water, oil, gas, and other fluids.

Pipe Thread Types

Pipe threads fall into two main categories:

Tapered Pipe Threads

Tapered threads are commonly used in piping connections where leak prevention is critical. The thread diameter decreases along the axis (tapered), and as the threads are tightened, surface contact increases, improving the seal and airtightness.

Applications: High-pressure systems like plumbing, gas, and hydraulic systems.

Common Examples:

• R (ISO 7/JIS male tapered thread)
• Rc (ISO/JIS female tapered thread)
• NPT (National Pipe Tapered) – Used in the U.S. and Canada.
• BSPT (British Standard Pipe Tapered) – Common in the UK and other countries.

Parallel Pipe Threads

Parallel threads are used for mechanical joints, especially in high-pressure applications. The diameter remains constant, and the threads run parallel to the axis. To prevent leakage, they are typically used with O-rings or gaskets.

Applications: Low-pressure systems, machinery, and industrial systems.

Common Examples:

• G (ISO 228/JIS parallel thread, both male and female)
• PF (Old JIS designation for parallel threads)
• Rp (Parallel female thread used with male tapered R thread)
• NPS (National Pipe Straight) – U.S. standard.
• BSPP (British Standard Pipe Parallel) – British equivalent.

Pipe Thread Size Chart (Standards & Dimensions)

Pipe threads are governed by several regional and international standards:

What Are Pipe Thread Standards?

Pipe thread standards are formalized systems used to define the shape, size, and function of threads used on pipes and fittings. They ensure that threaded parts fit together properly and can reliably create a mechanical connection and/or pressure-tight seal.

Each standard varies in terms of:

• Thread geometry (angle and form)
• Taper or parallel profile
• Pitch (distance between threads)
• Diameter sizing system (inch or metric)
• Sealing method

These standards ensure compatibility across fittings, valves, and piping systems. However, each standard has its own dimensions and tolerances, meaning threads from different standards may look similar but are not interchangeable. 

So, below we will list some major internationally used pipe thread standards and the commonly used pipe thread size tables in these standards. Each specification includes both inch and metric (mm) dimensions, along with thread characteristics like thread angle (typically 55° or 60°), taper (tapered or parallel), and sealing method (interference fit or O-ring/gasket).

1. NPT (National Pipe Thread Standard)

• Region: North America (USA, Canada)
• Type: Tapered (external and internal)
• Thread Angle: 60° (Unified thread form)
• Seal Type: Threads seal by interference fit; typically requires PTFE (Teflon) tape or pipe dope to prevent leakage.
• Standard: ASME B1.20.1
• Applications: Widely used in plumbing, compressed air systems, natural gas lines, and industrial piping.
• Applicability & Compatibility: Widely used in North America for pressure-tight connections. Not compatible with BSP or ISO threads due to angle and pitch differences. 

NPT Pipe Thread Size Chart (Inch & MM dimensions)

2. NPTF (National Pipe Taper Fuel)

• Region: North America
• Type: Tapered (Dryseal)
• Thread Angle: 60°
• Seal Type: Metal-to-metal seal achieved by tighter thread tolerances; does not require additional sealant.
• Standard: ASME B1.20.3
• Applications: Used where leak-proof joints are essential without sealants, such as in fuel systems, lubrication, and high-pressure hydraulic lines.
• Applicability & Compatibility: Similar to NPT but with tighter tolerances for leak-free connections.

NPTF Pipe Thread Size Chart (Inch & MM Dimensions)

3. BSP (British Standard Pipe)

• Region: United Kingdom, Europe, Asia, Australia, and globally
• Thread Angle: 55° (Whitworth thread form)
• Variants:
  • BSPT (British Standard Pipe Tapered): Tapered male threads for pressure-tight joints (similar to ISO 7-1 R threads).
  • BSPP (British Standard Pipe Parallel): Parallel threads that require a washer or O-ring for sealing.
• Seal Type: BSPT seals via thread interference; BSPP requires separate sealing element.
• Standards: BS 21, ISO 7-1 (BSPT), ISO 228-1 (BSPP)
• Applications: Common in water piping, oil and gas systems, and hydraulic equipment.
• Applicability & Compatibility: The most common standard outside North America.

BSP Pipe Thread Size Chart (Inch & MM Dimensions)

4. ISO Pipe Threads

• Region: Global (international standard)
• Thread Angle: 55° (Whitworth form)
• Variants:
  • ISO 7-1 (R, Rc, Rp Threads): Tapered (R = male, Rc = female, Rp = parallel female). Used for pressure-tight joints.
  • ISO 228-1 (G Threads): Fully parallel threads used in mechanical connections where sealing is done via O-rings or gaskets.
• Seal Type:
  • ISO 7-1: Thread interference
  • ISO 228-1: Gasket or O-ring
• Applications: Used globally across fluid transport, gas lines, and pneumatic systems.
• Compatibility: ISO 7-1 R threads are equivalent to BSPT; ISO 228-1 G threads are equivalent to BSPP.

G (ISO 228) Pipe Thread Size Chart (MM & Inch Dimensions)

5. Metric Pipe Threads (ISO 261 / ISO 724)

• Region: Global (especially Europe and Asia)
• Type: Available in parallel and tapered forms
• Thread Angle: 60° (metric thread form)
• Seal Type: Requires O-rings, sealing washers, or face seals; not pressure-tight on threads alone.
• Standards: ISO 261 (coarse pitch), ISO 724 (basic profile dimensions)
• Applications: Used in precision machinery, automotive, hydraulics, and general engineering systems.
• Compatibility: Metric threads are specified in terms of pitch (distance between threads) and diameter. Commonly used in industrial applications worldwide.

Metric Pipe Thread Size Chart (Inch & MM Dimensions)

6. API Pipe Threads (American Petroleum Institute)

• Region: Global, especially in oil & gas industries
• Type: Tapered
• Standard: API 5B (covers thread forms for casing, tubing, and line pipes)
• Seal Type: Thread interference; may also use compound sealants
• Applications: Drilling equipment, oilfield piping, casing and tubing, wellheads
• Special Features: Designed for extreme pressure and harsh environments

API Pipe Thread Size Chart (MM & Inch Dimensions) – EUE (External Upset End) / NUE (Non-Upset End) / LTC (Long Thread Casing)

7. JIS Threads (Japanese Industrial Standard)

• Region: Japan (also used in parts of Asia)
• Thread Angle: 55°
• Variants:
  • PT Threads: Tapered (same as R threads in ISO 7-1)
  • PF Threads: Parallel (same as G threads in ISO 228-1)
• Seal Type:
  • PT: Thread interference
  • PF: Requires gasket or O-ring
• Applications: Widely used in hydraulics, pneumatics, and general industrial piping in Japan
• Compatibility: Functionally equivalent to BSPT (PT) and BSPP (PF), but markings may differ

JIS Pipe Thread Size Chart (MM & Inch Dimensions)

JIS threads are closely related to BSP threads (British Standard Pipe), often used in Japanese hydraulic, pneumatic, and plumbing systems.

JIS B0202 Parallel Pipe Thread Dimensions (Similar to BSPP / ISO 228)

JIS B0203 Tapered Pipe Thread Dimensions (Similar to BSPT / ISO 7-1)

8. SAE Straight Pipe Thread (Society of Automotive Engineers)

• Region: North America and International
• Type: Parallel (Straight thread – SAE J514 or J1926)
• Thread Angle: 60°
• Seal Type: O-ring face seal (ORFS) or O-ring boss (ORB) for leak-proof hydraulic connections
• Applications: Used in automotive, aerospace, hydraulic, and fuel systems
• Advantages: Excellent for high-pressure, vibration-resistant systems; reusable without damaging threads

SAE Straight Thread Size Chart (MM & Inch Dimensions) – ORB / O-Ring Boss

9. DIN 2999 (German Standard)

• Region: Germany, Europe
• Type: Tapered (and parallel)
• Seal Type: Thread sealant or gaskets
• Status: Superseded by EN 10226 and ISO 7-1, but still referenced in legacy systems
• Applications: Plumbing, compressed air, industrial fittings
• Compatibility: Interchangeable with BSPT/BSPP and ISO equivalents

DIN Pipe Thread Size Chart (MM & Inch Dimensions)

DIN 2999 / DIN ISO 7-1 – Tapered Pipe Threads (Similar to BSPT)

DIN ISO 228 / DIN 259 – Parallel Pipe Threads (Similar to BSPP)

10. R Threads (ISO 7-1)

• Region: Global (ISO standard)
• Type: Tapered (male)
• Seal Type: Thread interference (metal-to-metal seal)
• Standard: ISO 7-1
• Applications: Used in pressurized water, gas, and air pipelines
• Compatibility: Interchangeable with BSPT and JIS PT threads

R Pipe Thread Size Chart (MM & Inch Dimensions) – Tapered External Thread – ISO 7-1

Differences in Pipe Thread Standards & Dimensions

How to Identify (Determine) Pipe Thread Type?

Identifying the correct pipe thread type is an essential first step when working with plumbing or industrial fittings. Threads vary by standard, region, shape, and sealing method, and using the wrong type can lead to leaks or failed connections.

Here’s a step-by-step breakdown to help you determine the thread type:

1. Check the Thread Shape

Look at the crest (top) and root (bottom) of the threads. The threads with a rounded profile and a 55° angle are likely BSP (British Standard Pipe) or ISO. And threads with a flatter crest and a 60° angle are usually NPT/NPTF (National Pipe Thread) or Metric.

2. Determine the Thread Taper

Tapered threads become narrower toward the end and are designed to seal by tightening, and they are common in NPT, NPTF, BSPT, and ISO R threads. Straight (parallel) threads have a uniform diameter, can be found in BSPP (G threads), NPS, and Metric fittings. You can test taper by placing calipers at both ends of the thread and comparing the measurements.

3. Identify Thread Gender

• Male threads: external, visible on the outside of the fitting.
• Female threads: internal, located inside the fitting or port.

4. Consider Regional Use

• North America: NPT/NPTF is most common.
• Europe/Asia/Australia: BSP (G or R) and metric threads are standard.
• Japan: JIS threads (PT/PF) are functionally equivalent to BSP but labeled differently.

5. Application Clues

Equipment origin can indicate thread type. For example, Italian appliances often use BSP, while American systems use NPT.

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How to Measure & Calculate Pipe Thread Sizes?

Measuring pipe threads can be confusing – especially since the nominal size doesn’t always match the physical diameter. Here’s how to accurately measure and determine thread size:

1. Measure the Outer or Inner Diameter

For male threads, measure the outside diameter (OD) of the thread. For female threads, measure the inside diameter (ID). You can use a caliper for better accuracy.

2. Count the Threads per Inch (TPI) or Measure the Pitch (mm)

TPI is used in imperial thread standards like NPT and BSP, and pitch (in millimeters) is used in metric threads. Use a thread gauge for accurate measurement.

3. Compare with Standard Charts

Once you’ve measured the thread diameter and pitch/TPI, compare your values with a pipe thread size chart for the matching standard. For example, a thread that measures ~0.84″ OD and has 14 TPI is likely a 1/2″ NPT.

4. Understand Nominal Size vs Actual Size

The nominal size refers to the approximate internal diameter of the pipe—not the thread OD. For instance, a “1/2 inch” pipe thread has an actual outer diameter of 0.84 inches.

5. Use the Right Tools

Calipers (for diameter), thread pitch gauge (for TPI or metric pitch), reference charts (NPT, BSP, ISO, Metric, JIS, etc.)

By checking the thread profile, measuring the diameter and pitch, and using charts for comparison, you can accurately identify the pipe thread type and size. This ensures better compatibility, fewer leaks, and less frustration during installation.