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Flat washers may be simple components, but selecting the right washer involves more than matching it to a bolt diameter. Inside diameter, outside diameter, thickness, material, finish, and applicable standards can all affect how a washer performs in an assembly.
Whether you're sourcing washers for manufacturing, construction, maintenance, or OEM production, understanding common flat washer sizes and dimensions can help you specify the right component and avoid fit or performance issues.
Browse flat washers →The Three Key Dimensions
Inside diameter (ID)
The diameter of the center hole. Must provide adequate clearance for the mating bolt or screw — slightly larger than the nominal fastener diameter.
Outside diameter (OD)
Determines how much surface area the washer covers. A larger OD distributes load over a greater area — important for softer materials or oversized holes.
Thickness
Affects stiffness, spacing, and load distribution. A washer that's too thin may deform under load. Some applications require hardened washers to a specific specification.
SAE Flat Washer Size Chart
Dimensions vary by standard and washer series. The chart below shows common SAE flat washer dimensions as a general reference.
| Nominal size | Inside diameter | Outside diameter | Thickness |
|---|---|---|---|
| #6 | 5/32" | 3/8" | 3/64" |
| #8 | 3/16" | 7/16" | 3/64" |
| #10 | 7/32" | 1/2" | 3/64" |
| 1/4" | 9/32" | 5/8" | 1/16" |
| 5/16" | 11/32" | 11/16" | 1/16" |
| 3/8" | 13/32" | 13/16" | 1/16" |
| 7/16" | 15/32" | 59/64" | 1/16" |
| 1/2" | 17/32" | 1-1/16" | 3/32" |
| 9/16" | 19/32" | 1-3/16" | 3/32" |
| 5/8" | 21/32" | 1-5/16" | 3/32" |
| 3/4" | 13/16" | 1-1/2" | 9/64" |
| 7/8" | 15/16" | 1-3/4" | 9/64" |
| 1" | 1-1/16" | 2" | 9/64" |
SAE vs. USS Flat Washers
One of the most common sources of confusion when ordering flat washers is the difference between SAE and USS series. Both are used with inch fasteners but have different dimensional characteristics.
SAE Flat Washers
More compact proportions- Smaller outside diameters
- Tighter, more compact proportions
- Less surface coverage
- Machinery, automotive, manufacturing
USS Flat Washers
Greater bearing surface- Larger outside diameters
- Greater bearing surface
- More material around center hole
- Construction and general industrial
What About Fender Washers?
If you need substantially more surface coverage, a fender washer may be appropriate. Fender washers have a much larger outside diameter relative to their inside diameter, spreading load over a larger area. They're commonly used with sheet metal, thin materials, wood, and oversized holes.
How to Choose the Right Flat Washer
Start with the fastener size
Determine the nominal diameter of the bolt or screw. This is your starting point — but nominal size alone isn't enough to fully specify a washer.
Check the inside diameter
Confirm the washer's ID provides the required clearance for the fastener. If working from an engineering drawing, use the specified dimensions rather than nominal descriptions.
Determine the required outside diameter
Consider the material beneath the washer. A larger OD may be needed when the base material is soft, the hole is oversized, or more load distribution is required.
Consider washer thickness
A washer that is too thin may deform under load. Structural or high-load applications may require hardened washers manufactured to a specific specification.
Choose the right material
Carbon steel, stainless steel, brass, aluminum, and nylon each suit different strength, corrosion, temperature, and electrical requirements. Match the material to the environment and surrounding assembly.
Select the appropriate finish
For outdoor or corrosive environments, finish selection significantly affects service life. Options include plain, zinc plated, hot-dip galvanized, black oxide, and specialty coatings.
When to Use Stainless Steel Flat Washers
Stainless steel flat washers are the right call when corrosion resistance is a priority. Common stainless options include 18-8/304 for general outdoor and industrial use, and 316 for marine, coastal, and chemically aggressive environments.
- Outdoor equipment
- Food processing
- High-humidity environments
- Coastal applications
- Equipment requiring a clean appearance
Flat Washers vs. Structural Washers
General-purpose flat washers and structural washers are not interchangeable. Structural bolting applications may require hardened washers that meet specific dimensional and mechanical requirements — for example, ASTM F436 for structural steel connections. Always select washers according to the applicable project specifications.
Browse structural washers →Metric Flat Washer Sizes
Metric washers follow metric dimensional standards and are identified by the nominal metric fastener size. Do not assume a similar-sized inch washer is an appropriate substitute — dimensions and applicable standards differ.
Common Flat Washer Selection Mistakes
What to Provide When Ordering Flat Washers
Frequently Asked Questions
Find the Right Flat Washer for Your Application
Selecting a flat washer isn't simply a matter of finding one that fits over the bolt. Inside diameter, outside diameter, thickness, material, finish, hardness, and dimensional standard can all affect performance.
Blue Ribbon Fastener supplies flat washers, structural washers, fender washers, stainless steel washers, metric washers, and specialty fastening components for manufacturing, construction, OEM, and industrial applications.