How do I know what size washer I need?

If you're trying to match an existing flat washer, or figure out what size washers you need to order, the sizing process can be frustrating.  Although it may seem like there are a limitless number of sizes available, there are also standards for the specs and dimensions of fasteners that manufacturers follow.  The truth is flat washers can be broken down into just a few categories.

Washers provide a smooth load bearing surface between the head of your fastener and the surface of the material being fastened.  The size and grade of the washer should be matched to the type of fastener you're using in order to provide a secure connection.

USS and SAE Flat Washers

The two most popular sizes or standards are USS (United States Standard) and SAE (Society of Automotive Engineers).  A USS washer has a larger outside diameter and fits loosely on the screw or bolt.  This type of washer is the kind you'll find at a hardware store and works well for most construction projects.

An SAE washer has a smaller outside diameter and fits tighter on the fastener.  This type of washers is often used in machine and automotive applications where tolerances are tighter and space limitations may be a factor.  The majority of flat washers fall into these 2 categories.

To determine the size of a washer there are 3 critical dimensions.  The inner diameter (ID), the outer diameter (OD), and the thickness (T).

The size of the washer is referred to as the nominal size, meaning that the washer designation refers to the size of the fastener it was designed to work with. For instance, a 3/8 USS flat washer has an ID of 7/16" and an OD of 1" but it's referred to as a 3/8 washer because it's designed to be used on a 3/8 fastener.  In comparison, the 3/8 SAE flat washer has an ID of 13/32" (1/32" tighter on the bolt) and an OD of 13/16" (3/16" smaller in outside diameter).

Not only should the size of the washer be matched to the size of the fastener, but it should also be a corresponding grade of material.  Washers are not graded the same as a hex screw (Grade 2, 5, 8 etc.) but are classified as unhardened and hardened.  An easy way to determine if the washer you're using is something more than just low carbon steel is to look for a manufacturing stamp on the face of the washer.

Low carbon steel washers are unhardened and often have surface imperfections and thickness variations.  There will be no markings on these washers to determine where they were manufactured.  Higher grade steel washers will be through-hardened and heat-treated and will prominently display a manufacturer's stamp somewhere on the face of the washer.  These washers will typically be much better quality since the manufacturer was proud enough to put their name on it.

F436 Flat Washers

Another popular standard for flat washers is ASTM F436 (American Society for Testing and Materials) and is designed around mechanical and structural standards.

F436 flat washers have a slightly larger ID and OD than the SAE standard because they are designed to be used with similarly matching heavy hex structural bolts and nuts.  These washers will always be hardened and are typically stamped with the corresponding F436 designation.

Although most flat washers you come across will fall into the SAE, USS, and F436 categories, there are 2 more commonly found in hardware stores and every day applications.

Fender Washer and Stainless Steel

Fender washers are flat washers with an oversized outside diameter.  The name comes from the automotive application where they were used to secure the unusually large holes required for proper panel alignment.  They are typically punched from 16 gauge low carbon sheet steel and can be recognized by the proportionally larger outer diameter.  Due to their large diameter and thin composition they don't offer a lot of clamping force and are easily distorted, but they work well to prevent pull-through on soft materials and thin gauge sheet metal.  The size of a fender washer is still referred to by the diameter of the fastener that it is designed to work with but is also qualified by the outside diameter.   A 1/4" fender washer is designed to be used with a 1/4" diameter fastener and if you're looking for one with a 2" outside diameter, it would be referred to as a 1/4 x 2 fender washer.

Lastly, a note about stainless steel flat washers.  Stainless washers can be manufactured to any of the specifications described above but you will often find them with their own standard of dimensions being smaller and thinner than that of the SAE specifications.  Smaller and thinner washers make the price more attractive for higher expense materials such as stainless.  Be sure to confirm the material composition and dimensions when ordering.

We have created a dimensional standards chart for USS and SAE washers you can download here:

If you have any questions about determining the size of your washers or would like to discuss your application we'd be happy to assist.