Washer sizes.

The 12 or 12.5 problem

An M6 form A washer is 12mm across if it is ISO 7089 and 12.5mm if it is BS 4320. Same bore, same job, different disc. It happens again at M8 (16 or 17) and M10 (20 or 21), and then the two series agree from M12 up. UK wholesale “form A” usually means the BS figure, most import stock is the ISO one, and both are on shelves — so the chart above prints both wherever they differ. If the washer has to clear a counterbore or sit inside a channel, measure the one in your bin before ordering a thousand more.

Which form do you need

Form A is the everyday flat washer: normal diameter, normal thickness. Form B is the BS 4320 light pattern — the same disc as the BS form A but thinner, between a half and two thirds, for stack-ups where a full-thickness washer pushes the bolt length up a size. The BS table runs M6 to M39; we can get M3, M4 and M5 but nobody publishes figures for them, so those rows ask.

The other three are all about spreading the load wider. Form C is BS 4320's large-diameter washer at form A's thickness, 28mm across at M12 where the form A is 24. Form G is wider again and made in black mild steel, 36mm at M12. A penny washer (repair washer, ISO 7093 / DIN 9021) is about three bolt-diameters across, 37mm at M12, for soft material, thin sheet or a hole that has been opened out.

One trap in the penny table worth knowing: the bore steps up past M16. An M18 penny washer has a 20mm hole and an M20 a 22mm one — more clearance than the form A beside them — so a penny washer sits looser on its bolt than the flat washer you just took off.

Form C, form G or a penny — which wide washer

All three spread the load wider than a form A. Two things tell them apart: the size of the disc, and the size of the hole.

Form C keeps the form A bore. An M12 takes 13mm, the same as the flat washer already on the bolt, so you get the wider face and nothing else changes.

Form G is drilled looser — 14mm at M12, 11 at M10, 9 at M8, bigger than the form A and the form C at every size in the chart. That clearance is why it exists, for oversized and slotted holes in steelwork where the bolt is never going to land dead centre.

A penny is the widest disc: 37mm at M12 against the form G's 36 and the form C's 28. Its bore follows the form A up to M16, then steps up to match the form G exactly.

Which means the comparison changes with size, and this is the part to get right. Up to M16 the bore decides. At M8 a form G and a penny are the same disc, 24mm across and 2mm thick, and the only difference is a 9mm hole against 8.4 — so if you have measured the clearance you need there, a penny will not stand in for a form G.

From M18 up the bores are identical and what is left is thickness. An M20 form G and an M20 penny are both a 22mm hole in a 60mm disc; the form G is 5mm thick and the penny 4mm. If you are buying at those sizes for clearance alone, the penny does the same job.

“Form B” means two different washers

Worth knowing before you order a box. BS 4320 form B is the light one: an M12 is 1.6mm thick. DIN 125-B — now DIN EN ISO 7090 — is not the same product at all; it is DIN 125-A with a chamfer on it, so an M12 is 2.5mm, the full form A thickness. Same two words, 56% apart on the one figure that matters.

The form B column above is the BS washer. If you are matching a stack height or a counterbore, say which one you need and we will read the label rather than the name.

DIN 125-C and DIN 9021 — what those labels mean

Two more labels to watch. They mislead in opposite directions.

Plenty of form C stock is labelled DIN 125-C. There is no such standard: DIN 125 covers forms A and B and stops there. It is trade shorthand for the BS 4320 form C in the chart above, and that is what is in the box.

The other way round, plenty of stock sold as form G is labelled DIN 9021, and DIN 9021 is the penny washer. Two different washers under one name, 1mm apart in the bore at M12.

The trade splits them by material more often than not, and so do our own suppliers: in steel — zinc, galvanised or self-colour — a form G is the BS 4320 washer above, 14mm bore and 36mm across at M12. In stainless the same words usually get you a DIN 9021 penny at 13 and 37. So if it has to be stainless and the clearance matters, tell us the hole you need and we will confirm the bore before it goes out.

No thread, no grade — but not no spec

A washer has no thread and no property class, but it does have a hardness: the standards grade steel washers at 140HV, 200HV and 300HV, and under a high-tensile bolt the harder washer is the right partner — a soft washer simply embeds and the joint loses preload. The picker keeps to dimensions; ask us about hardness, imperial (BS 3410) sizes, spring and shakeproof patterns, or anything else the chart does not show.

Not sure which you need?

Bring the old one in, or send us a photo. We are on Baird Close in Daventry with washers from M2 to M60 across the common finishes.