Metric vs Imperial Fasteners: How to Tell Them Apart
Metric fasteners are sized in millimetres: an M8 bolt has an 8 mm thread. Imperial fasteners are sized in inches and come in several thread families, mainly UNC, UNF, BSW, BSF and BA. Metric and imperial threads never interchange, even where the sizes look almost the same.
Almost everything built in the UK today is metric. Imperial threads live on in American equipment, older British machinery, classic vehicles and electronics, which is why repairs so often turn up a bolt that nothing in the toolbox fits. This guide shows how to tell the systems apart, why they cannot be mixed, and the sizes we stock in each.
Metric and imperial at a glance
| Metric | Unified (UNC, UNF) | British (BSW, BSF) | BA | |
|---|---|---|---|---|
| Sized in | Millimetres | Inches | Inches | BA number |
| Written as | M8 × 30 | 1/4" UNC × 1", 10-24 UNC | 1/4" BSW × 1" | 4BA × 1/2" |
| Thread spacing | Pitch in mm | Threads per inch (TPI) | Threads per inch (TPI) | Pitch in mm |
| Thread angle | 60° | 60° | 55° | 47.5° |
| Where you find it | Almost everything new in the UK and Europe | American and Canadian equipment and vehicles | Older British machinery, buildings and classic vehicles | Electronics, instruments, clocks and models |
How to tell which one you have
- Check the head. Stainless metric bolts are usually marked with the grade and strength, such as A2-70 or A4-80. A head marked only 18-8 or 316 is usually American, so UNC or UNF.
- Measure the diameter. Use a caliper across the outside of the thread. A whole number of millimetres, just under size (7.8 to 7.9 mm for M8), points to metric. A reading near a fraction of an inch, such as 6.3 mm for 1/4" or 9.5 mm for 3/8", points to imperial.
- Check the thread spacing. A thread gauge settles it. Without one, lay a ruler along the thread: measure across 10 peaks for a metric pitch in mm, or count the peaks in one inch for threads per inch.
- Think about where it came from. A new European product is metric. An American machine is UNC or UNF. A 1960s British car or a Victorian gate is likely Whitworth (BSW) or BSF. A radio or model engine may be BA.
- Test gently. Try a nut you know by hand only. If it runs freely, you have a match. If it binds after a turn or two, stop: forcing it will damage both threads.
Why metric and imperial never mix
Several imperial sizes are within a fraction of a millimetre of a metric size, and that is exactly what makes them dangerous. The diameter is close enough for a nut to start, but the threads are spaced differently, so the flanks soon clash. The nut then jams, strips, or sits on a few damaged threads that cannot take the load.
| Metric | Looks like | Diameter | Thread spacing | Interchangeable? |
|---|---|---|---|---|
| M6 | 1/4" UNC | 6.00 mm v 6.35 mm | 1.00 mm v 1.27 mm (20 TPI) | No |
| M8 | 5/16" UNC | 8.00 mm v 7.94 mm | 1.25 mm v 1.41 mm (18 TPI) | No |
| M10 | 3/8" UNC | 10.00 mm v 9.53 mm | 1.50 mm v 1.59 mm (16 TPI) | No |
| M12 | 1/2" UNC | 12.00 mm v 12.70 mm | 1.75 mm v 1.95 mm (13 TPI) | No |
| M16 | 5/8" UNC | 16.00 mm v 15.88 mm | 2.00 mm v 2.31 mm (11 TPI) | No |
| M20 | 3/4" UNC | 20.00 mm v 19.05 mm | 2.50 mm v 2.54 mm (10 TPI) | No |
The worst pairs are M8 and 5/16" UNC, and M16 and 5/8" UNC. Their diameters are almost identical, so a mismatch is easy to miss until the joint fails. British threads add another trap: Whitworth threads have a rounder 55° profile, so even a 1/4" BSW and a 1/4" UNC with the same 20 threads per inch do not seat properly together.
UNC and UNF number sizes
Small Unified screws use a number instead of a fraction. 10-24 UNC means a No. 10 screw (0.190", 4.83 mm) with 24 threads per inch. The higher the number, the thicker the screw. A dash means that thread exists but we do not stock it.
| Size | Diameter | UNC (TPI) | UNF (TPI) | Hex key (socket cap) | Closest metric |
|---|---|---|---|---|---|
| No. 0 | 0.060" / 1.52 mm | – | 80 | 0.050" | M1.6 |
| No. 1 | 0.073" / 1.85 mm | 64 | 72 | 1/16" | M2 |
| No. 2 | 0.086" / 2.18 mm | 56 | 64 | 5/64" | M2 |
| No. 3 | 0.099" / 2.51 mm | 48 | – | 5/64" | M2.5 |
| No. 4 | 0.112" / 2.84 mm | 40 | – | 3/32" | M3 |
| No. 5 | 0.125" / 3.18 mm | 40 | – | 3/32" | M3 |
| No. 6 | 0.138" / 3.51 mm | 32 | – | 7/64" | M3.5 |
| No. 8 | 0.164" / 4.17 mm | 32 | 36 | 9/64" | M4 |
| No. 10 | 0.190" / 4.83 mm | 24 | 32 | 5/32" | M5 |
Fractional imperial sizes: UNC, UNF, BSW and BSF
From 1/4" upwards, imperial bolts are sized as a fraction of an inch, followed by the thread family. The numbers in the thread columns are threads per inch for the sizes we stock. Coarse (UNC, BSW) is the everyday choice; fine (UNF, BSF) is used where vibration or fine adjustment matters.
| Size | Diameter | UNC | UNF | BSW | BSF | Spanner (UNC/UNF hex) | Closest metric |
|---|---|---|---|---|---|---|---|
| 1/8" | 3.18 mm | – | – | 40 | – | – | M3 |
| 3/16" | 4.76 mm | – | – | 24 | – | – | M5 |
| 1/4" | 6.35 mm | 20 | 28 | 20 | 26 | 7/16" | M6 |
| 5/16" | 7.94 mm | 18 | 24 | 18 | 22 | 1/2" | M8 |
| 3/8" | 9.53 mm | 16 | 24 | 16 | 20 | 9/16" | M10 |
| 7/16" | 11.11 mm | 14 | 20 | 14 | – | 5/8" | M10 or M12 |
| 1/2" | 12.70 mm | 13 | 20 | 12 | – | 3/4" | M12 |
| 9/16" | 14.29 mm | 12 | 18 | – | – | 13/16" | M14 |
| 5/8" | 15.88 mm | 11 | 18 | 11 | 14 | 15/16" | M16 |
| 3/4" | 19.05 mm | 10 | 16 | 10 | – | 1.1/8" | M20 |
| 7/8" | 22.23 mm | 9 | 14 | 9 | 11 | 1.5/16" | M22 |
| 1" | 25.40 mm | 8 | 12 | 8 | 10 | 1.1/2" | M24 |
| 1.1/8" | 28.58 mm | 7 | 12 | 7 | – | 1.11/16" | M27 or M30 |
| 1.1/4" | 31.75 mm | – | – | 7 | 9 | – | M33 |
| 1.3/8" | 34.93 mm | – | – | – | 8 | – | M36 |
| 1.1/2" | 38.10 mm | 6 | 12 | 6 | – | 2.1/4" | M39 |
| 1.3/4" | 44.45 mm | 5 | – | – | – | – | M45 |
| 2" | 50.80 mm | 4.5 | – | – | – | – | M52 |
The closest metric size is given to help you picture the size, not as a replacement. Whitworth hex heads take Whitworth spanners, which are marked with the bolt size (for example 1/4W) rather than the width across the flats.
BA threads
British Association (BA) threads are small, fine threads defined in millimetres, used in electronics, instruments, clocks and model engineering. The smaller the BA number, the bigger the screw, so 0BA is the largest. BA screw lengths are given in inches.
| Size | Diameter | Pitch | Threads per inch | Closest metric |
|---|---|---|---|---|
| 0BA | 6.0 mm | 1.00 mm | 25.4 | M6 |
| 2BA | 4.7 mm | 0.81 mm | 31.4 | M5 |
| 3BA | 4.1 mm | 0.73 mm | 34.8 | M4 |
| 4BA | 3.6 mm | 0.66 mm | 38.5 | M3.5 |
| 6BA | 2.8 mm | 0.53 mm | 47.9 | M3 |
| 8BA | 2.2 mm | 0.43 mm | 59.1 | M2 |
| 10BA | 1.7 mm | 0.35 mm | 72.6 | M1.6 |
| 12BA | 1.3 mm | 0.28 mm | 90.9 | M1.2 |
Inch to mm: lengths
Imperial screw lengths are in inches. Our product titles write mixed numbers with a dot, so 1.1/2" means one and a half inches (38.1 mm), and 2.3/4" means two and three quarter inches (69.9 mm).
| Inches | mm | Inches | mm | Inches | mm |
|---|---|---|---|---|---|
| 1/8" | 3.2 | 3/4" | 19.1 | 2.1/2" | 63.5 |
| 3/16" | 4.8 | 7/8" | 22.2 | 2.3/4" | 69.9 |
| 1/4" | 6.4 | 1" | 25.4 | 3" | 76.2 |
| 5/16" | 7.9 | 1.1/4" | 31.8 | 3.1/2" | 88.9 |
| 3/8" | 9.5 | 1.1/2" | 38.1 | 4" | 101.6 |
| 7/16" | 11.1 | 1.3/4" | 44.5 | 4.1/2" | 114.3 |
| 1/2" | 12.7 | 2" | 50.8 | 5" | 127.0 |
| 5/8" | 15.9 | 2.1/4" | 57.2 | 6" | 152.4 |
Reading an imperial product title
- 5/16" UNC X 1.1/4" Hex Set Screw: a 5/16" Unified coarse thread (18 threads per inch), one and a quarter inches long.
- 10-32 UNF X 1/2" Socket Cap Screw: a No. 10 screw with 32 threads per inch, half an inch long. It takes a 5/32" hex key.
- 3/8" BSW Hex Full Nut: a Whitworth nut for a 3/8" BSW bolt, 16 threads per inch.
- 4BA X 1/2" Slot Cheese Head: a 4BA screw (3.6 mm), half an inch long.
Common questions
Is 1/4" the same as M6?
No. A 1/4" bolt is 6.35 mm, against 6 mm for M6, and the thread spacing is different. An M6 nut will not go on a 1/4" bolt, and a 1/4" nut will be loose and weak on an M6 bolt.
Can I use a metric nut on a UNC bolt?
No. Always match the nut to the bolt's thread family and size: UNC with UNC, BSW with BSW, metric with metric at the same pitch.
Which system should I use for a new project?
Metric. It is the UK and European standard, parts are easier to find, and the size picker in our fastener size guide gives the spanner, hex key and drill sizes for every metric thread. Use imperial when you are repairing or matching something that already uses it.
What is BA used for?
Electronics, instruments, clocks, model engineering and older British electrical fittings. It is still the right choice when you are repairing or restoring equipment built with it.
Do you stock imperial stainless fasteners?
Yes. We stock over 1,500 imperial fasteners in A2 and A4 stainless steel, from No. 0 UNF to 2" UNC, plus Whitworth, BSF and BA. Shop imperial sizes, or read the size guide for how to measure what you have.
