How to identify and measure Washers

Three numbers identify any flat washer: the inside diameter (the hole the bolt passes through), the outside diameter (rim to rim) and the thickness. Washers are named by the bolt they fit, so most of the time you never need to measure at all - this guide shows you the quick path first, then how to measure a loose washer when you have to.

Standard Flat Washer

Inside diameter · Outside diameter · Thickness

Step 1  -  Match the bolt size (the quick path)

Washers are named by the bolt or screw they fit: an M6 bolt takes an M6 washer, M10 takes M10, and so on. If you know your bolt size you already know your washer size - look for it at the start of the product name (e.g. 'M6 Flat Round Washer'). Imperial bolts work the same way (a 1/4" bolt takes a 1/4" washer).

Step 2  -  Measure the inside diameter

Only have a loose washer? Measure across the hole and pick the next size down. The inside is always slightly larger than the matching bolt so it slides on freely: a measurement of roughly 6.5mm means an M6 washer, and roughly 17.25mm means an M16. Typical inside diameters for metric flat washers:

Treat these as a guide only - the exact clearance varies between washer series and batches.

Step 3  -  Check the outside diameter

Measured outside-to-outside across the rim. The outside diameter sets how far the load is spread: standard flat washers suit general work, mudguard (penny) washers have an oversized rim for soft or thin material, and small-OD washers suit counterbored or tight spots. If your old washer has an unusually large or small rim, match that family - not just the bolt size.

Step 4  -  Check the thickness

Measure flat across the face. Thickness varies slightly between batches - washers are not precision-made fasteners, so treat published figures as a guide. Some of our washers are sized by gauge (SWG) instead of millimetres: see the washer gauge to mm sizing chart for the complete 8g to 24g range in mm and inch.

Browse the full range of washers. Still not sure which washer you have? Contact us and we will match it for you.