Imperial Threads Explained: A Guide to Imperial Thread Sizes

When choosing engineering components, fasteners, knobs, handles, clamps or other threaded parts, selecting the correct thread size is essential. While metric threads such as M6 or M8 are widely used, imperial threads remain extremely common, particularly in machinery, engineering, manufacturing and equipment originating from the UK or USA.

But what does 8-32, 10-32 or 1/4-20 actually mean?

In this guide, we explain imperial threads in simple terms, including how imperial thread sizes are written, what the numbers mean and how they compare with metric threads.

What is an imperial thread?

An imperial thread is a screw thread specified using the inch-based measurement system, rather than millimetres.

Unlike metric threads, where the thread diameter is normally given in millimetres, imperial threads use a combination of:

• A number or fractional inch diameter
• The number of threads per inch (TPI)

For example:

8-32

This means:

• 8 = the screw's nominal diameter
• 32 = 32 threads per inch

The first number can be slightly confusing because sizes such as #8 and #10 are numbered screw sizes, rather than meaning 8 inches or 10 inches.

What does 8-32 mean?

An 8-32 thread is a common imperial Unified Thread Standard (UTS) thread.

The 8 refers to the nominal screw size, while 32 means there are 32 threads along one inch of thread.

An 8-32 screw therefore has:

#8 diameter + 32 threads per inch

The pitch can also be calculated from the TPI:

1 ÷ 32 = 0.03125 inches per thread

So each thread is approximately 0.03125" apart.

An 8-32 thread is commonly found on smaller engineering components, machinery and equipment.

What does 10-32 mean?

A 10-32 thread follows exactly the same principle.

• 10 = #10 screw size
• 32 = 32 threads per inch

Therefore, 10-32 has a larger diameter than 8-32, while both have the same 32 TPI.

This is an important distinction:

8-32 and 10-32 are not the same thread.

Although they both have 32 threads per inch, the diameter is different.

What does the number actually mean?

One of the most common questions about imperial threads is:

"If 8-32 means 8, does that mean the thread is 8 inches?"

No.

The 8 is a numbered screw diameter, part of the standardised imperial screw sizing system.

For smaller screws, imperial sizes are commonly identified using numbers such as:

The larger the number, the larger the nominal diameter.

Once imperial threads reach larger sizes, they are commonly specified using fractional inches, for example:

• 1/4"
• 5/16"
• 3/8"
• 1/2"
• 3/4"

These are then combined with the threads per inch.

For example:

1/4-20

means a nominal diameter of 1/4 inch and 20 threads per inch.

What does TPI mean?

TPI stands for Threads Per Inch.

It tells you how many complete threads are contained within one inch of the threaded section.

For example:

1/4-20 = 20 threads per inch

1/4-28 = 28 threads per inch

Both are 1/4" diameter threads, but they have different thread pitches.

This means they are not interchangeable.

A 1/4-20 component will not correctly fit a 1/4-28 thread.

Coarse and fine imperial threads

Imperial threads are commonly available in different thread pitches.

Two of the most common standards are:

UNC – Unified National Coarse

UNC threads have a relatively coarse thread pitch.

For example:

1/4-20 UNC

has 20 threads per inch.

Coarse threads are commonly used for general-purpose fastening and engineering applications.

UNF – Unified National Fine

UNF threads have more threads per inch and therefore a finer pitch.

For example:

1/4-28 UNF

has 28 threads per inch.

Fine threads can be useful where a finer adjustment or greater thread engagement is required.

Imperial vs metric threads

One of the biggest differences between imperial and metric threads is how the size is specified.

A metric thread such as:

M6 x 1.0

means:

• M = metric
• 6 = 6 mm nominal diameter
• 1.0 = 1.0 mm thread pitch

An imperial thread such as:

1/4-20

means:

• 1/4" = nominal diameter
• 20 = 20 threads per inch

The two systems are different and should not be treated as interchangeable.

For example, an M6 thread may appear visually similar to an imperial thread, but this does not mean the components will fit correctly.

Can imperial and metric threads be used together?

Generally, no.

Although some imperial and metric thread sizes can appear very similar, the diameter and thread pitch are different.

Trying to force an imperial component into a metric thread, or vice versa, can damage:

• The threaded component
• The mating thread
• Nuts and bolts
• Machinery
• Precision equipment

It is therefore important to identify the thread standard before ordering replacement components.

Common imperial thread sizes

Some common imperial threads you may encounter include:

• #6-32
• #8-32
• #10-24
• #10-32
• 1/4-20
• 1/4-28
• 5/16-18
• 5/16-24
• 3/8-16
• 3/8-24
• 1/2-13
• 1/2-20

The first part identifies the diameter and the second part identifies the number of threads per inch.

Why choosing the correct thread matters

For engineering components such as clamp levers, knobs, handles, indexing plungers and other operating parts, the thread is a critical part of the specification.

Two components can look almost identical but have completely different threads.

For example, a component available with:

M6

may also be available with:

1/4-20

While these sizes are relatively close in diameter, they are different thread standards and should not be substituted for one another without confirming compatibility.

Always check the manufacturer's technical specification before selecting a replacement.

How to identify an imperial thread

If you have an existing component but don't know its thread size, there are several ways to identify it.

1. Check the original specification

The easiest and most reliable method is to check the manufacturer's part number, technical drawing or product specification.

2. Measure the diameter

Use a vernier caliper to measure the outside diameter of the threaded section.

Remember that a #8 or #10 screw will not measure exactly 8 mm or 10 mm — these are numbered imperial sizes.

3. Measure the thread pitch

For imperial threads, thread pitch is normally specified as threads per inch (TPI).

A thread pitch gauge is one of the easiest ways to identify this.

4. Check whether the thread is metric or imperial

If you are unsure whether a component is metric or imperial, don't assume based solely on its appearance.

Check the original equipment specification or measure the thread carefully.

Imperial thread terminology at a glance

Imperial threads from Good Hand UK

At Good Hand UK, we supply a wide range of industrial and engineering components, including products available with both metric and imperial thread options.

Whether you require a clamp lever, knob, handle, indexing plunger or another operating component, selecting the correct thread is essential for ensuring compatibility with your existing equipment.

If you are unsure which thread you require, check the product specification or contact our team for assistance.

The correct thread means the correct fit — and the correct fit means clamping with confidence.