A Guide to Common Metric Flat Washer Dimensions

When it comes to flat washers, the good news is that regardless of where you buy them or who makes them, the dimensions should be almost exactly the same. This is because all metric flat washers are classed within certain Forms, and each form has the same dimensions regardless of manufacturer.

The only slight possible difference is that there is a manufacturing tolerance allowance of 1mm above or below the dimension stated in the form table. The thickness of the washer should not vary away from the thickness stated in the table because all washers are made from standard sheets of metal that are themselves standardised in thickness.

Form A is the most commonly used type of metric flat washer and they are available in a wide range of sizes and finishes. They are commonly used with hex nuts, machine screws and socket screws as well as many other types of fastener.

Form B is the same diameter as a Form A washer but the thickness is smaller -the washer is thinner.

Form C washers have a slightly large overall diameter than Form A & B washers, as you can see by comparing the same sized washer in the table below. This means that they are particularly suitable for use with materials that are softer than hard metal (such as wood and plastic), as they give a wider spread of load and reduce the potential for damage.

If you need a washer that is larger still then choose a repair washer.

Often called penny washers or mudguard washers, repair washers have an overall outside diameter that is larger than standard flat washers.

This increased surface area spreads the force (and hence the tension load) that is applied over a much greater area when the nut is tightened, making them ideal for use on weak or damaged surfaces.

The following table shows the exact dimensions of all our Form A & B Washers (don’t forget to make an allowance for the 1mm +/- tolerance in diameter as stated above).

As with all of our fasteners, please note that it is the responsibility of the user to assess the suitability of the product in question.

FORM A & B WASHERS	FORM C WASHERS