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Screw Size Guide | What Are Screw Thread Sizes?

What is a Screw Thread? 

The thread of a screw is the helical tooth which runs around the outside of the screw shaft. A thread is the defining attribute which gives a screw its fastening capabilities, and it is this feature which distinguishes screws from more rudimentary fasteners such as nails.

Why Doesn’t my Screw Fit?

Incompatibility in screw forms can occur for several reasons. A screw may have an unmatched thread size, thread pitch, or may even be out-of-spec. Screws are manufactured in a wide range of sizes to suit applications both large and small. Choosing the correct screw size is vital to ensure compatibility between a screw and a chosen threaded hole. For more information on measuring screw pitch, see our article on How to Measure Screw Thread Pitch.

How Many Thread Sizes Are There?

Fasteners are manufactured to suit an almost innumerable range of different manufacturing standards, with each of these standards containing its own range of screw sizes. For more information about screws before standardised sizing, see our article on How Is a Machine Screw Made?

The vast majority of machine screws sold in Europe follow the ISO metric thread standard, which is common throughout the world. Many different modern and historical thread standards exist, a handful of the most popular are detailed below:

Thread Type


ISO Metric Screw Thread

Sometimes called an “M” size, ISO metric threads are commonplace throughout the world. The “M” size of an ISO metric thread represents the screw’s nominal size in millimetres. For example, an M3 screw will have a 3mm nominal thread diameter - in reality, the screw would be slightly undersized to ensure compatibility with an M3 (3mm) nut (which, in turn, would also be slightly oversized).

Unified Thread Standard  (UTS)

An imperial thread type introduced shortly after World War II in an effort to harmonise worldwide manufacturing at a time when most countries had their own incompatible thread forms. For a detailed summary of sizes within the UTS standard, visit our UTS Thread Conversion Table. UTS threads come in two main variants:

Unified Coarse Pitch Thread (UNC)

UNC threads are the most common form of imperial thread in Europe. This type of thread is commonly found in legacy projects, and is still widely used throughout the US.

Unified Fine Pitch Thread (UNF)

UNF threads are the fine pitch variant of UNC threads, their finer thread pitch can be visually distinguished from UNC threads.

British Association Thread (BA)

British Association thread is a relatively uncommon thread type which is largely obsolete. BA threads are still used in certain optic devices, but are mostly found in legacy projects.

Trapezoidal Thread (TR)

Trapezoidal Threads are not intended for fastening applications, and are instead designed for use in the manufacture of lead screws. Trapezoidal threads are tapered to a 30° angle to encourage smooth motion in precision actuation applications.

British Standard Whitworth Thread (BSW)

British Standard Whitworth Thread is an archaic and largely redundant imperial screw standard. The BSW thread standard was one of the first thread standard to be mass-produced, and is found in very few modern applications.

Pipe Thread

British Standard Pipe Thread (BSP)

Found commonly throughout Europe and used heavily within the UK, BSP threads are available in tapered and untapered variants. The tapered variety of this thread is designed to achieve an airtight fit by increasing in diameter as it is tightened.

Northern Standard Pipe Thread (NSP)

NSP threads function identically to BSP threads, but are used within North America. Like BSP threads, NSP threads also include both tapered and non-tapered variants.