Thread types and sizes: UNF, metric, BSP and NPT explained

The difference between standard and size, which thread in which industry, and how to choose the right fitting

In practice, thread types are constantly confused. Someone asks for "a UNF fitting" or for "10-32 thread", and means two very different things. This leads to incorrect orders, damaged ports and leaking systems.

The distinction is simple once you see it: a thread standard (UNF, UNC, BSP, NPT, metric) is the family to which the thread belongs. A thread size (10-32, 1/4-28, M6) is the specific dimension within that family. Two threads from the same standard only fit together if the size is also identical.

This article explains all common thread types, shows which standard dominates in which industry, and gives you a practical step-by-step plan for identifying an unknown thread.

⏱ approx. 9 minutes reading time

The core in one sentence

A thread standard is the family (UNF, BSP, NPT, metric). A thread size is the specific member of that family (10-32, 1/4-28, M6). Only when both standard and size are identical do two parts fit together.

Thread standard versus thread size: the crucial distinction

This is the most common source of confusion around thread types. The designation on a fitting contains two types of information at the same time.

A 10-32 UNF and a 1/4-28 UNF belong to the same standard (UNF), but have different sizes. They are therefore never interchangeable, even if the fitting sometimes appears to fit.

The most common mistake

"They are both UNF, so it fits." That is not true. UNF is the family; the size determines whether parts physically work together. A 10-32 nut never fits a 1/4-28 port, and vice versa. Forcing it permanently damages the thread in the instrument.

The five most important thread standards

Below are the five standards you will encounter in process and laboratory technology, with their identifying features and fields of application.

1. Metric (M and MF): the European standard

Metric thread is by far the most widely used standard in Europe and follows ISO 261 and ISO 965. The designation consists of the letter M followed by the outer diameter in millimetres, optionally followed by the pitch. M6 has an outer diameter of 6.00 mm; M6x0.75 is the fine variant with a pitch of 0.75 mm.

Identification: the outer diameter comes out at a round number of millimetres. Flank angle 60 degrees. Parallel.

Application: European mechanical engineering, installation technology, European analytical instruments (some Agilent and Shimadzu HPLC modules, among others, use M6).

2. UNF (Unified National Fine): fine American thread

UNF is an American thread family with a fine pitch. The designation consists of the nominal size followed by the number of threads per inch: 10-32 means size #10 with 32 threads per inch. Sizes within UNF include #4-40, #6-32, 10-32, 1/4-28, 5/16-24 and 3/8-24.

Identification: the outer diameter comes out at an 'illogical' number of millimetres, such as 4.83 or 6.35 mm. Flank angle 60 degrees. Parallel. Fine pitch.

Application: analytical chemistry (HPLC, LC-MS, GC), vibration-sensitive constructions, precision instrumentation, American mechanical engineering.

3. UNC (Unified National Coarse): coarse American thread

UNC is the coarse counterpart of UNF: the same diameters, but fewer threads per inch. This makes the thread deeper and more robust, quicker to assemble and less susceptible to damage, but also less resistant to vibration.

Identification: visibly coarser threads than UNF at the same diameter. Flank angle 60 degrees.

Application: American machines and vehicles, structural engineering, fastenings in softer materials. Rarely used in fluidics applications.

4. BSP (British Standard Pipe): the European pipe thread

BSP is the dominant pipe thread standard outside North America and is often referred to as gas thread in the Netherlands. BSP has two variants that differ fundamentally:

• BSPP (designation G): parallel thread. Seals via a flat gasket, O-ring or bonded seal (Usit ring) on the sealing surface. Never with Teflon tape.
• BSPT (designation R): tapered thread. Seals on the thread itself, with PTFE tape or sealing paste.

Identification: flank angle 55 degrees (Whitworth profile) with slightly rounded crests. The size designation refers to the historical inner diameter of the pipe, not to the outer diameter of the thread. G1/8 has an outer diameter of approx. 9.7 mm.

Application: European hydraulics and pneumatics, process installations, drinking water, instrumentation, gas supply.

5. NPT (National Pipe Taper): the American pipe thread

NPT is the American pipe thread standard and is always tapered (1:16 taper). The thread locks when tightened, so that sealing takes place on the thread itself. For a leak-free connection, a sealant such as PTFE tape is almost always necessary.

Identification: flank angle 60 degrees, clearly tapered profile. Measure the diameter at the fourth or fifth full thread.

Application: North American process equipment, gas and water pipes, petrochemicals, HVAC, pneumatic systems of American make.

Which thread types are used in which industry?

The choice of a thread standard is rarely a technical consideration on the part of the end user. It is determined by the origin of the equipment and the convention within the sector.

Step-by-step plan: how do you identify an unknown thread?

Holding a fitting without knowing which thread it has? Follow these four steps. You will need a calliper and a thread gauge.

Step 1: tapered or parallel?

Hold a ruler or straight edge along the thread. If the diameter visibly tapers, you have NPT or BSPT. If the diameter remains constant, you have metric, UNF, UNC or BSPP. This is the fastest way to halve the number of possibilities straight away.

Step 2: measure the outer diameter

Measure across the thread crests with a calliper. With tapered thread, measure at the fourth or fifth full thread. If you get a round number of millimetres (6, 8, 10 mm), this points to metric thread. If you get an 'odd' number (4.83 or 6.35 mm), you almost certainly have an inch size.

Step 3: determine the pitch or threads per inch

Place the thread gauge on the thread. If the gauge fits seamlessly into the threads without any light showing through, you have found the right size. With metric thread, you read the pitch in millimetres; with inch thread, the number of threads per inch (TPI).

Step 4: check the flank angle

This is the final check and sometimes the decisive one. Metric, UNF, UNC and NPT have a flank angle of 60 degrees. BSP has 55 degrees with slightly rounded crests. If in doubt between two candidates with a similar diameter and pitch, the flank angle settles it.

Practical tip for the lab

Keep a set of thread gauges (metric and inch) near the instruments. Five minutes of measuring prevents a damaged port on an instrument worth tens of thousands of euros. Still in doubt? Send your supplier a photo with a calliper next to the thread.

Interchangeability: what fits and what does not?

The most frequently asked practical question about thread types: does part A fit port B? The table below gives a direct answer for the most common combinations.

Take care with stainless steel ferrules

A stainless steel ferrule deforms permanently on first assembly and sets in the exact position in that specific fitting. Reusing it in a different body results in leakage, even with identical specifications. PEEK ferrules, on the other hand, are reusable and can be repositioned.

Interchangeability per manufacturer in analytical systems

In analytical chemistry, 10-32 UNF has become the de facto standard. The overview below shows which brands are mutually compatible when using 1/16 inch capillary.

Stock strategy for laboratories

Keep PEEK nuts and PEEK ferrules for 10-32 with 1/16 inch capillary in stock. This combination covers the vast majority of common HPLC instruments and saves waiting time in the event of faults.

Frequently asked questions about thread types

What is the difference between UNF and 10-32?

UNF is the thread standard: a family of American threads with a fine pitch. 10-32 is a specific size within that family, namely size #10 with 32 threads per inch. All 10-32 thread is UNF, but not all UNF is 10-32. If someone asks for "UNF fittings", always ask for the exact size.

Does a 10-32 ferrule fit a 1/4-28 fitting?

No. Even though the capillary may have the same outer diameter (for example 1/16 inch), the thread size of the nut differs. A 10-32 ferrule with a 1/4-28 nut does not fit, nor does the reverse. Some ferrules are universal in terms of tube diameter, but the nut and fitting must always have the same thread.

Can I use NPT and BSP interchangeably?

No. Both are tapered pipe threads, but NPT has a flank angle of 60 degrees and BSPT 55 degrees. In some sizes they do engage, but they do not seal and damage each other's thread flanks. Always use an adapter: an NPT/BSP adapter seals on the NPT side with PTFE tape and on the BSP side with an O-ring or bonded seal (Usit ring).

When should and shouldn't I use Teflon tape?

Only use Teflon tape and sealing paste on tapered thread connections: NPT and BSPT. With parallel thread with a face seal (BSPP, metric with O-ring), Teflon tape is unsuitable and actually causes leakage. Never use Teflon tape on HPLC fittings: it introduces dead volume and contamination.

Why is BSP 1/8 inch not equal to 3.2 millimetres?

With BSP, the size designation refers to the historical inner diameter of the pipe, not to the outer diameter of the thread. G1/8 has an outer diameter of approx. 9.7 mm. This is one of the most confusing features of pipe thread standards and a common source of ordering errors.

Which thread does my European machine have?

European mechanical engineering predominantly uses metric thread for mechanical fastenings and BSPP (G thread) for liquid and gas connections. With imported American equipment, you will encounter NPT and UNF. If in doubt, follow the step-by-step plan above or send your supplier a photo with a calliper.

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