Pipe Thread Size Chart 2026 : Simple Guide + PDF

If you have ever tried to fix a plumbing line, install an air compressor, or replace a pipe fitting, you already know one thing: pipe threads can be confusing. You pick up a pipe marked ½-inch, and it doesn’t look like ½ inch at all. Then one fitting screws in perfectly, while another, also labeled the same size—refuses to fit. I have personally faced this headache while working with hydraulic fittings and even simple home plumbing repairs. After a lot of trial, error, and digging through charts, I finally understood how pipe thread sizing actually works.
So in this post, I am going to walk you through everything in a simple, teacher-like way—no over-technical jargon.

By the end, you will know:

• What pipe threads are
• Why the size printed on a pipe is not its actual physical size
• The difference between NPT, BSP, and other thread types
• How to read and use a pipe thread size chart
• Common pipe thread sizes for plumbing, air systems, gas lines, and more

Let’s begin.

What Are Pipe Threads?

Pipe threads are the spiral grooves cut on the outside (male) or inside (female) of a pipe or fitting. They allow the pieces to screw together and form a mechanical connection as well as, often, a leak-proof seal.

Threads help you:

• Connect two pipes
• Attach fittings (elbows, tees, couplers)
• Connect pipes to valves or equipment
• Seal fluid, air, or gas systems

But the main confusion starts here: in pipe sizing, the measurement printed on the pipe is nominal—not the actual diameter.
That means a “½-inch pipe” doesn’t measure ½ inch anywhere on the pipe!

Why?  Because pipe sizing was standardized more than 100 years ago, based on the inside diameter of older pipes. Even when pipe walls changed in thickness, the size names stayed the same.

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Why Pipe Thread Sizes Are Confusing

There are two reasons people get confused:

1. Nominal pipe size ≠ actual size

For example:

A ½-inch NPT pipe has an outside diameter (OD) of 0.84 inches

A 1-inch pipe has an OD of 1.315 inches

So when someone says “1-inch pipe,” it’s not actually 1 inch anywhere.

2. There are different thread standards

Different countries and industries use different thread types:

NPT (National Pipe Thread) – USA standard

NPTF (Dryseal) – leak-proof threads

BSPP/BSPT – British standard, used worldwide

Metric threads – used in Europe and automotive sectors

Garden hose threads (GHT)

Fire hose threads (NH/NST)

Each type has its own angle, diameter, and shape. That’s why two pipes with the same “size” won’t fit unless they share the same system.

Pipe Thread Size Chart (NPT) – Most Common in USA

Here is the simple, accurate NPT thread chart you can use for plumbing, air compressors, hydraulic fittings, gas pipes, and general use.

📌 NPT Pipe Thread Size Chart

This chart helps answer the most common question:

👉 Why doesn’t my half-inch pipe measure half an inch?
Because the size is only a name, not a measurement.

Pipe Thread Size Chart PDF

Understanding pipe threads is essential for anyone working with plumbing, gas lines, or industrial piping. Sizes can be confusing because the nominal size often differs from the actual diameter. This Pipe Thread Size Chart PDF provides a clear, easy-to-use reference for NPT threads, showing nominal sizes, actual diameters, threads per inch, and common uses to simplify your work.

Pipe_Thread_Size_Chart PDF

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Understand Each Column of the Chart

To make pipe thread charts useful, it’s important to understand what each column represents. Each heading provides specific information about the pipe or fitting. By knowing how to interpret these values, you can quickly select the correct pipe size, avoid leaks, and ensure your plumbing, gas, or industrial system works smoothly.

1. Nominal Pipe Size

The nominal pipe size is simply the label used to identify the pipe. It is a traditional naming system and does not correspond to the actual measurements of the pipe. Understanding nominal size helps you communicate effectively when ordering pipes or fittings and ensures you select compatible components for your project.

2. Actual Outside Diameter (OD)

The actual outside diameter (OD) is the real physical measurement across the outside of the pipe thread. This dimension is critical when selecting fittings because the OD determines whether a male thread can properly fit into a female thread. Always measure OD for accurate, leak-free connections in plumbing and industrial applications.

3. Threads Per Inch (TPI)

Threads per inch (TPI) indicate how many threads are present along one inch of pipe. This measurement determines thread spacing and compatibility between fittings. For example, a ½-inch NPT has 14 TPI while a ¼-inch NPT has 18 TPI. Mismatched TPI can prevent proper assembly and cause leaks or damage.

NPT vs BSP – Don’t Mix Them Up

One of the biggest mistakes beginners make is mixing NPT and BSP threads.

Here’s the difference:

An NPT fitting will almost fit into a BSP fitting but not perfectly. It may screw halfway and then jam.This creates leaks or thread damage. If you work with imports, air tools, or equipment from China/Europe, double-check before connecting.

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How to Identify Pipe Thread Size (Simple Method)

Identifying pipe thread size becomes easy when you follow a clear, step-by-step approach. This practical method is the same one commonly used in workshops, plumbing jobs, and industrial maintenance. By measuring correctly and comparing your results with a chart, you can avoid mismatched fittings, leaks, and unnecessary frustration.

Step 1: Measure Outer Diameter (OD)

Start by measuring the pipe’s actual outside diameter using a caliper for best accuracy, though a ruler also works in simple cases. The OD helps you identify the real physical size of the pipe, which rarely matches the nominal name. For example, an OD of 0.84 inches usually corresponds to a ½-inch NPT pipe.

Step 2: Count the Threads Per Inch (TPI)

Next, determine the number of threads per inch. Use a thread gauge if available, or mark a one-inch section and count the threads manually. TPI is crucial because it ensures proper thread engagement. Even a small mismatch in thread count can cause fittings to jam, leak, or fail under pressure.

Step 3: Match Results With the Pipe Size Chart

Once you have both the OD and TPI measurements, compare them with a verified pipe thread size chart. The chart helps you identify the exact nominal pipe size and corresponding thread type. This prevents guesswork and ensures you select the correct fitting every time, whether you’re working on plumbing, gas lines, or industrial equipment.

Step 4: Identify Thread Type

Finally, determine whether the thread is tapered or straight. Tapered threads become slightly narrower toward the end, indicating types like NPT or BSPT. Straight threads maintain the same diameter along their length, such as BSPP or metric threads. Identifying the correct thread type prevents cross-threading, leaks, and system damage.

Why Tapered Threads Matter

Pipe threads are often tapered, meaning:

The male thread becomes narrower at the end

When you tighten it, it wedges into the female thread

This creates a leak-resistant seal

NPT threads seal through metal-to-metal pressure. That’s why we use:

Teflon tape

Pipe dope

Thread sealant

These help fill microscopic gaps.

Pipe Thread Uses

Pipe thread sizes are designed for specific applications, and knowing where each size is commonly used helps you choose the right fittings. Here’s a clear breakdown of how each size range is used in practical, everyday situations.

1- Small Sizes (1/8″ – 3/8″)

Small pipe thread sizes are ideal for low-flow, precision systems where tight control is needed. These threads connect small instruments, compact lines, and equipment that rely on accurate pressure or fluid movements.

Pressure gauges

Used to connect gauge housings to pressure lines, allowing accurate pressure readings in air, water, or hydraulic systems without leaks or measurement errors.

Air compressor fittings

Commonly used for quick-connect couplers, regulator ports, and airline accessories, ensuring smooth airflow in small pneumatic tools and household compressors.

Refrigeration lines

Used in refrigeration and HVAC systems for connecting capillary tubes, pressure switches, and sensors where small, precise connections are essential.

Water filters

Found on compact filter housings and drinking water systems, these threads enable secure, leak-free connections for safe and continuous water flow.

Small hydraulic systems

Used in light-duty hydraulic applications like foot pumps and compact cylinders, providing reliable sealing under moderate fluid pressure.

2- Medium Sizes (1/2″ – 1″)

Medium pipe thread sizes are the most common in everyday households. They handle general water, gas, and outdoor utility needs and offer the perfect balance of flow capacity and ease of installation.

Faucets

Used for connecting faucets and mixer taps, providing dependable water supply connections for sinks, kitchens, and bathrooms.

Water supply lines

These threads are standard on household plumbing lines, ensuring safe water delivery to appliances like heaters, washing machines, and dishwashers.

Gas pipes

Used for residential gas supply systems, including natural gas and LPG lines, ensuring safe and consistent flow to stoves or heaters.

Garden adapters

Found in outdoor spigots, hose connectors, and irrigation systems, providing strong, weather-resistant connections for gardening tools and water hoses.

3- Large Sizes (1-1/4″ – 4″)

Large pipe thread sizes are built for high-volume, heavy-duty systems where strong flow and durability are crucial. These threads are commonly used in industrial, commercial, and high-pressure environments.

Industrial pipelines

Used for transferring liquids, chemicals, steam, or air in factories, ensuring high-capacity flow and robust system reliability.

Sprinkler systems

Installed in fire sprinkler networks and large irrigation systems, providing strong connections capable of handling high water discharge.

Fire hydrants

Used in municipal firefighting connections, these threads allow quick attachment of hoses and withstand extreme pressure during emergency use.

Water pumps

Found in pumps for agriculture, industry, and commercial buildings, helping move large volumes of water efficiently and safely.

Drainage lines

Used for waste and stormwater systems, offering wide channels to remove water quickly and prevent blockages in large-scale plumbing.

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Tips You Should Always Remember (From Experience)

These practical tips come directly from real workshop situations and home plumbing projects. Following them saves time, prevents thread damage, and ensures leak-free connections. Even experienced technicians rely on these basics because they consistently make installation and troubleshooting much easier.

✔ Always check OD, not the nominal size

Measuring the actual outside diameter removes most sizing confusion because nominal pipe size rarely matches real measurements. When you rely on OD instead of labels, you immediately know the true pipe size and avoid mismatched fittings, incorrect ordering, or wasted installation time.

✔ Never force a fitting

If a fitting doesn’t screw in smoothly by hand, stop immediately. Forcing it can damage threads, ruin the seal, or cause leaks later. Smooth engagement means correct size and thread type. Resistance usually signals an incorrect match, worn threads, or dirty connections.

✔ When in doubt, use a thread gauge

Thread gauges are inexpensive, simple to use, and extremely accurate. They help you quickly check thread pitch and TPI, ensuring the correct match every time. Using one prevents guesswork, avoids fitting mistakes, and is especially helpful when working with unfamiliar or imported parts.

✔ Use sealant on tapered threads

Tapered threads rely on pressure to form a leak-resistant seal, but even perfect tapered fits can leak if not sealed properly. Always apply Teflon tape or pipe dope to fill microscopic gaps and ensure long-term, leak-free performance under pressure.

✔ Parallel threads need a washer

Parallel threads do not seal by tightening alone because they don’t taper. They require a washer, O-ring, or gasket to create a proper seal. This is especially true for BSPP threads, where the sealing surface depends entirely on the added washer.

Extended Pipe Thread Size Chart (NPT + BSP)

Here is a more detailed comparison chart that is helpful for international fittings.

NPT vs BSP Comparison Chart

They look dangerously close—but they do NOT fit perfectly.

How to Know Which Thread System You Need?

Choosing the correct thread system is essential for creating leak-free, secure, and compatible connections. Because different regions and industries use different standards, identifying the right system ensures your fittings match perfectly and prevents damage, frustration, and costly installation mistakes.

1. If You Are in the USA – Use NPT

In the United States, NPT is the standard for plumbing, gas lines, air compressors, and industrial systems. Most American-made tools and fittings follow NPT, so choosing this system ensures proper sealing, easy compatibility, and reliable performance without needing adapters or conversions.

2. If Your Equipment Is From Europe or Asia – Use BSP

Imported tools, pumps, compressors, and valves from Europe or Asia commonly use BSP threads. These fittings look similar to NPT but differ in angle and sealing method. Matching them with BSP ensures accurate tightening, proper sealing, and prevents cross-threading or damaging connections.

3. If You’re Unsure – Measure OD and TPI

When the thread type isn’t clear, measuring the outside diameter and counting threads per inch is the safest approach. These two measurements allow you to compare directly with a thread chart and identify the correct system without guessing or risking mismatched fittings.

4. If the Thread Shape Confuses You – Check Taper vs. Straight

Thread shape immediately reveals the system. Tapered threads become narrower and usually indicate NPT or BSPT. Straight threads maintain the same diameter, meaning BSPP or metric. Identifying this difference helps you choose the correct sealing method and avoid installation problems.

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Final Thoughts

Understanding pipe threads is one of those skills that seems complicated at first—but once you learn the basics, everything becomes easy. I struggled with this years ago, especially when fitting sizes didn’t match what was written on the box. But with charts, measuring, and a little practice, it becomes second nature.

Use the chart included in this guide to quickly identify any NPT or BSP size you’re working with.
And remember:

• Nominal size is just a name
• Actual OD is what matters
• TPI must match
• Thread type must match

Once you follow these rules, you will never mix up pipe threads again.

FAQs: