Size picker · Hex bolt · ASME B18.2.1 / BS 1768
UNC and UNF hexagon bolt sizes.
The imperial hex with a plain shank under the head. Pick a size and length for the spanner size, head height and how much of it is thread — and pick UNC or UNF, because the diameter alone does not tell you the thread.
1 Pick a diameter
2 Pick a length
Choose a diameter first.
Your browser could not draw the 3D view. The dimensions beside it are unaffected.
Drawn to proportion from the standard dimensions. Illustration, not a photograph of the item.
Selected
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| Sym | Dimension | Value |
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Job stopped? Ring 01327 877577 and we'll check the stock while you're on.
Read the full ASME B18.2.1 / BS 1768 size chart
| Size | 1/2" | 5/8" | 3/4" | 1" | 1 1/4" | 1 1/2" | 1 3/4" | 2" | 2 1/4" | 2 1/2" | 2 3/4" | 3" | 3 1/4" | 3 1/2" | 3 3/4" | 4" | 4 1/2" | 5" | 5 1/2" | 6" | 7" | 8" |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1/4 | ||||||||||||||||||||||
| 5/16 | ||||||||||||||||||||||
| 3/8 | ||||||||||||||||||||||
| 7/16 | ||||||||||||||||||||||
| 1/2 | ||||||||||||||||||||||
| 5/8 | ||||||||||||||||||||||
| 3/4 | ||||||||||||||||||||||
| 7/8 | ||||||||||||||||||||||
| 1 |
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.
Bolt or set screw?
A bolt has a plain shank; a set screw is threaded to the head. Everything on this chart carries a shank where the length allows one: the thread is twice the diameter plus 1/4 inch (plus 1/2 inch on bolts over 6 inches long) — the same rule in ASME B18.2.1 and BS 1768. A bolt short enough that the rule exceeds its length comes fully threaded, which is why the short ones look like set screws.
The spanner follows the standard, not the thread: a 1/2 inch bolt takes a 3/4 inch AF spanner.
UNC or UNF
Most diameters exist in both. A 1/2 inch is 13 threads per inch coarse and 20 fine, and a mismatched pair strips rather than announcing itself — count the threads or bring the old one in.
Finishes worth knowing about
Imperial hex bolts are listed in self-colour high-tensile and stainless. If you want bright zinc, the imperial hex the trade zinc-plates is the fully-threaded set screw — that is what the suppliers actually carry, so that is what the chips here offer.
Where these figures come from
Across flats and head height are ASME B18.2.1, cross-checked against the BS 1768 unified bolt table — the two agree to the thousandth. The lengths come from what our own book and our five fastener suppliers actually list, so a length shown is a length someone stocks.
Working in metric instead?
See the metric hex bolt chart for DIN 931, or the UNC and UNF full nut chart for the nut to go on these.
Not sure which you need?
Bring the old one in, or send us a photo. We are on Baird Close in Daventry and we still keep imperial hex on the shelf.
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.