SAE vs USS Washers: What's the Difference and Why it Matters

When assembling bolted connections, the humble washer often goes overlooked. But this seemingly simple component can make a big difference in the integrity and longevity of your design. Among the most common washer types you'll encounter in the U.S. are SAE (Society of Automotive Engineers) and USS (United States Standard) flat washers. Although they may look similar at first glance, they serve different purposes and choosing the wrong one could compromise your project.

In this blog, we’ll dive into the key differences between SAE and USS washers, and help you determine which is right for your application.

What Is a Washer, Anyway?

A washer is a thin, typically disk-shaped plate with a hole in the middle, used to distribute the load of a threaded fastener like a bolt or nut. Washers also help prevent damage to the mating surface, reduce vibration, and can serve as spacers or locking mechanisms.

Flat washers—the kind used most often—come in standardized dimensions. This is where SAE and USS come in.

SAE Washers: Precision for Automotive and Tight-Tolerance Applications

SAE washers were standardized by the Society of Automotive Engineers, and as you might guess, they’re primarily used in the automotive industry. These washers are characterized by:

Smaller outer diameter (OD)
Smaller inner diameter (ID) compared to USS washers of the same bolt size
Tighter manufacturing tolerances

Because of their tighter fit, SAE washers are ideal for applications where precision is important, such as in machinery, engines, and automotive assemblies. They help center the washer on the bolt and ensure that the load is distributed exactly where it's needed.

Typical use cases:

Fine Threaded Bolts

Automotive and aerospace assemblies

High-performance equipment

USS Washers: Broad Load Distribution for General Use

USS washers were originally standardized by the United States Standard (predecessor to ANSI) and are commonly used in structural and general-purpose applications. Compared to SAE washers, USS washers have:

Larger outer and inner diameters
Looser tolerances

This larger footprint makes USS washers better for distributing loads over a wider surface area, which is especially useful when working with softer materials or in less precise setups.

Typical use cases:

Construction and structural applications
Wood or softer base materials
Coarse-threaded bolts

Side-by-Side: SAE vs. USS Washer Dimensions

Here’s a quick comparison using a 1/2" bolt size as an example:

Note: Exact dimensions can vary by manufacturer, but this shows the general trend—USS washers are wider and thicker.

Which One Should You Use?

Here’s a quick guide:

Final Thoughts

Washers are small, but choosing the right type can help avoid big problems. Whether you're building a car engine or a steel bridge, understanding the difference between SAE and USS washers ensures your connections are secure, reliable, and fit for purpose.

Have questions or thoughts about washer selection? Use the Contact Us link to further the conversation on getting the right washer for your application!