Size picker · Part threaded · DIN 931
Hex bolt sizes.
A bolt has a plain unthreaded shank under the head. Pick a diameter and a length and you get across flats, head height, pitch — and the threaded length, which is the figure that decides whether the shank lands where you need it.
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 |
|---|
Job stopped? Ring 01327 877577 and we'll check the stock while you're on.
Read the full DIN 931 size chart
| Size | 25 | 30 | 35 | 40 | 45 | 50 | 55 | 60 | 65 | 70 | 73 | 75 | 80 | 85 | 90 | 95 | 99 | 100 | 110 | 115 | 120 | 125 | 126 | 127 | 130 | 135 | 140 | 145 | 150 | 160 | 165 | 170 | 175 | 180 | 190 | 200 | 210 | 220 | 230 | 240 | 250 | 260 | 270 | 280 | 290 | 300 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| M3 | ||||||||||||||||||||||||||||||||||||||||||||||
| M4 | ||||||||||||||||||||||||||||||||||||||||||||||
| M5 | ||||||||||||||||||||||||||||||||||||||||||||||
| M6 | ||||||||||||||||||||||||||||||||||||||||||||||
| M8 | ||||||||||||||||||||||||||||||||||||||||||||||
| M10 | ||||||||||||||||||||||||||||||||||||||||||||||
| M12 | ||||||||||||||||||||||||||||||||||||||||||||||
| M14 | ||||||||||||||||||||||||||||||||||||||||||||||
| M16 | ||||||||||||||||||||||||||||||||||||||||||||||
| M18 | ||||||||||||||||||||||||||||||||||||||||||||||
| M20 | ||||||||||||||||||||||||||||||||||||||||||||||
| M22 | ||||||||||||||||||||||||||||||||||||||||||||||
| M24 | ||||||||||||||||||||||||||||||||||||||||||||||
| M27 | ||||||||||||||||||||||||||||||||||||||||||||||
| M30 | ||||||||||||||||||||||||||||||||||||||||||||||
| M33 | ||||||||||||||||||||||||||||||||||||||||||||||
| M36 | ||||||||||||||||||||||||||||||||||||||||||||||
| M39 | ||||||||||||||||||||||||||||||||||||||||||||||
| M42 | ||||||||||||||||||||||||||||||||||||||||||||||
| M45 | ||||||||||||||||||||||||||||||||||||||||||||||
| M48 |
Bolts start at longer lengths than setscrews: a short part-threaded bolt leaves too little plain shank to be worth having, so the trade switches to the full-thread DIN 933 setscrew below each diameter's floor. 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.
Why the threaded length matters
The plain shank is the working part of a bolt. It carries shear, it locates the joint, and it is what stops two plates sliding against each other under load. Thread sitting in that hole instead does none of those things well — it is a smaller effective diameter with a sharp helix cut into it.
So the number to check is not the bolt length, it is the threaded length. Too much thread and the shank never reaches through the joint. Too little and the nut runs out before it tightens.
| Bolt length | Threaded length | M12 works out at |
|---|---|---|
| Up to 125 mm | 2 × diameter + 6 | 30 mm |
| 126 to 200 mm | 2 × diameter + 12 | 36 mm |
| Over 200 mm | 2 × diameter + 25 | 49 mm |
The picker works this out for whichever size you select, so you do not have to.
Bolt or setscrew?
If you want thread all the way to the head, that is a setscrew — DIN 933 — and the sizes are on the hex setscrew page. Setscrews are usually the right answer for a through-bolted joint in clearance holes where nothing needs locating.
Bring the old one in
Quickest way through any of it is to put the part on the counter at Baird Close. If we can see it, we can usually match it there and then.
More size charts
Every size, drawn to scale.
Hex setscrews
DIN 933 · full 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.