Size picker · Hex socket drive · ASME B18.3
UNC and UNF socket cap screw sizes.
The imperial socket cap screw, still the standard on American plant, older British machinery and anything built to an inch drawing. Pick a size and length for the head diameter, head height and the Allen key it takes — and pick the thread, because most imperial sizes come in both UNC and UNF.
1 Pick a diameter
2 Pick a length
Choose a diameter first.
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Selected
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Job stopped? Ring 01327 877577 and we'll check the stock while you're on.
Read the full ASME B18.3 size chart
| Size | 1/16" | 3/32" | 1/8" | 5/32" | 3/16" | 1/4" | 9/32" | 5/16" | 3/8" | 7/16" | 0.469" | 1/2" | 9/16" | 5/8" | 11/16" | 3/4" | 13/16" | 7/8" | 1" | 1 1/8" | 1 3/16" | 1 1/4" | 1 5/16" | 1 3/8" | 1 1/2" | 1 5/8" | 1 3/4" | 1 7/8" | 2" | 2 1/8" | 2 1/4" | 2 3/8" | 2 1/2" | 2 3/4" | 3" | 3 1/4" | 3 1/2" | 3 3/4" | 4" | 4 1/4" | 4 1/2" | 4 3/4" | 5" | 5 1/4" | 5 1/2" | 6" | 6 1/2" | 7" | 7 1/2" | 8" | 8 1/2" | 9" | 9 1/2" | 10" | 10 1/2" | 11" | 12" |
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The range shown is what fastener wholesalers commonly list — not a claim about what the standard mandates, and not a stock list. In-between lengths (85, 95, 170…), lengths past 300mm and odd diameters like M7 are made too, and availability varies by finish and grade — ask us for anything you can't see.
UNC or UNF — the one thing to get right
An imperial diameter is not one thread. A 1/4 inch cap screw is 20 threads per inch in UNC and 28 in UNF, and the two do not interchange — a UNF screw will start in a UNC hole and then bind. Coarse (UNC) is the general-purpose thread and what most stock is; fine (UNF) holds better under vibration and is common on hydraulic and engine work. If you are matching an existing screw, count the threads over an inch, or bring it in and we will gauge it.
The gauge sizes — #0 through #12 — are numbers, not measurements. A #10 is 0.190 inch across the thread, which is why it looks like a size between 3/16 and 1/4: it is.
Where these figures come from
Head diameter, head height and socket size are ASME B18.3, the 1960 series, cross-checked against two independently published tables before anything printed here. The lengths on the chart are not a standard series — they are the sizes our own stock and our five fastener suppliers actually list, so a length shown is a length that exists.
We hold the common sizes in 12.9 self-colour and A2 stainless. Sizes past 2 inch, BSW and BSF threads, and the Whitworth range are all available to order — ask us.
Working in metric instead?
See the metric socket cap screw sizes chart for DIN 912, or the UNC and UNF hexagon set screw chart if you want a hex head rather than a socket drive.
Not sure which you need?
Bring the old one in, or send us a photo. We are on Baird Close in Daventry and we keep imperial socket screws on the shelf — which is less common than it used to be.
More size charts
Every size, drawn to scale.
Hex setscrews
DIN 933 · full thread
Hex bolts
DIN 931 · part thread
Socket cap screws
DIN 912
Low head cap screws
DIN 7984
Button head screws
ISO 7380-1
Flanged button heads
ISO 7380-2
Countersunk socket screws
DIN 7991
Grub screws
DIN 913–916 · five points
Nuts
DIN 934 · nyloc · thin · Stover · flange
Washers
ISO 7089 · BS 4320 · penny
Not sure what the markings on the head mean? Bolt grades explained covers 8.8, 10.9, 12.9, stainless A2 and A4, and the imperial SAE grades.
All of it comes off the shelf at Baird Close — see the full fasteners & fixings range, or every size guide.