Flat Washers
What are Flat Washers?
Flat washers are thin, round discs with a centered hole used under a bolt head or nut to spread clamping force, protect surfaces, and support consistent torque. They help disperse load, reduce friction, protect surfaces, and prevent loosening. They are the most common washer style for general assembly work.
What are flat washers used for?
- Spreading load to reduce surface denting and finish damage
- Improving torque repeatability and joint preload
- Providing light shimming or spacing for alignment
- Covering slightly oversized holes when the correct outside diameter is selected
Flat washers vs fender washers
Choose a standard flat washer when typical load distribution and clearance are sufficient. Choose a fender washer when you need a much larger outside diameter to bridge oversized or slotted holes or to support thin sheet materials.
What size flat washer do I need? inch and metric
- Match the nominal washer size to your fastener size, for example 1/4 in or M6
- Confirm the inner diameter slides over the shank without binding
- Select the outside diameter and thickness for the load you need to spread
- Available in both inch and metric ranges to pair with US and metric hardware
Materials and finishes
18-8 and 304 stainless steel
General corrosion resistance and a clean appearance for indoor and light outdoor use.
316 stainless steel
Enhanced corrosion resistance for marine, salt, and harsh chemicals; pair with 316 fasteners when possible.
Zinc plated steel
Economical carbon steel with light protective coating for dry, indoor environments.
Hot dip galvanized steel
Thick zinc layer suited to outdoor structural work; match with galvanized bolts and nuts.
Brass and silicon bronze
Conductive and corrosion resistant options often chosen for electrical and marine applications.
Aluminum
Lightweight with natural corrosion resistance; useful on aluminum structures to limit galvanic mismatch.
Black oxide steel
Uniform, low-glare appearance for indoor assemblies; protection depends on proper oiling.
Installation tips
- Place the washer under the rotating component, either the nut or the bolt head, to protect the surface
- Use one washer per bearing surface unless you are deliberately stacking for shimming
- Re-check torque after initial run-in if the joint compresses soft materials
- Keep materials compatible in wet areas to limit galvanic corrosion
Applications
- General fabrication, equipment assembly, and maintenance
- Automotive and machinery service where surface protection is important
- Electrical panels, HVAC brackets, and plumbing or fixture mounting
- Woodworking, framing anchors, and light structural connections
Compatibility and best practices
- Match washer hardness and coating to the fastener grade and environment
- Use stainless or galvanized options for wet or outdoor settings
- Verify clearance around the washer outside diameter to avoid interference
- Add a locking method for vibration if needed, such as a lock washer or threadlocker
FAQs about Flat Washers
Flat washers spread clamping force, protect surfaces, and help achieve consistent torque and preload in a bolted joint.
Match the nominal size to your bolt or screw, ensure the inner diameter fits the shank, and choose an outside diameter that provides enough load spread.
Use 316 in marine, salt, or aggressive environments; 304 (18-8) is fine for general indoor and light outdoor use.
Yes. SAE typically has a smaller OD and tighter fit; USS has a larger OD for greater load distribution.
Often one is sufficient under the rotating side; use two if both surfaces need protection or if specified by a standard.
It is possible, but consider galvanic corrosion outdoors. Matching materials across the joint is usually safer.
Its extra-large OD bridges oversized or slotted holes and spreads load over thin sheet materials.
Black oxide is mainly cosmetic and needs oiling; it is not a heavy-duty corrosion barrier for outdoor exposure.
You can for minor shimming, but dedicated shims or spacers are better for large stand-offs or precision alignment.
Yes, they are conductive and commonly used in electrical work where conductivity and corrosion resistance matter.
Use the fastener’s recommended torque. The washer helps distribute load but does not change the torque spec unless a specific standard requires it.
Not by themselves. Use a lock washer, thread locker, or prevailing-torque nut if vibration is a concern.