Flat Washer Sizes and Uses: A Practical Guide
By Mutual Screw & Supply | Published: 2026-10-18
Category: Buying Guides
Learn how to choose the right flat washer size and material. This guide covers dimensions, uses, and decision rules for steel zinc plated washers.
Quick answer
Choose flat washers by matching the inside diameter to your bolt size and the outside diameter to the load-bearing surface. For general use, a steel zinc plated washer like the #5 x 3/8" OD offers a balance of strength and corrosion resistance.
Why Washer Thickness Matters
Thicker washers distribute load over a larger area and resist crushing under high torque. Thinner washers are lighter and fit tighter spaces but may deform under heavy loads. Always check the washer thickness against the clamping force of your fastener.
Material and Finish Trade-offs
Steel zinc plated washers offer good corrosion resistance for indoor or dry environments. For outdoor or wet conditions, consider stainless steel or hot-dip galvanized washers. Zinc plating can wear off over time, especially with repeated disassembly.
How to Choose a Flat Washer: Decision Rules
- Match the inside diameter (ID): The ID must be slightly larger than the bolt diameter to slide freely. For a #5 bolt (1/8"), use a washer with a .125" ID. A too-small ID will not fit; a too-large ID reduces load distribution.
- Select the outside diameter (OD): The OD should be large enough to spread the load over the bearing surface but not so large that it overlaps adjacent fasteners or interferes with assembly. For a #5 bolt, a 3/8" OD is a common choice.
- Check the thickness: Thicker washers (e.g., .040" and above) are suitable for structural or high-torque applications. Thinner washers are for light-duty or clearance adjustments. If unsure, choose a standard thickness like .040" for general use.
- Consider the environment: Indoor or dry environments: steel zinc plated washers are cost-effective. Outdoor or wet environments: upgrade to stainless steel or galvanized. Zinc plating offers basic corrosion protection but is not marine-grade.
Product facts
- #5 x 3/8" OD Flat Washers | .040" Thick | Steel Zinc Plated (2000 / Pkg.): These flat washers are made from steel with a zinc plated finish and have a .125" ID x .375" OD x .040" thick specification. The .125" ID fits a #5 or 1/8" bolt, while the .375" OD and .040" thickness provide adequate load distribution for light to medium-duty applications.
#5 x 3/8" OD Flat Washers | .040" Thick | Steel Zinc Plated (2000 / Pkg.)
Who should use these washers?
Best for
- General indoor assembly and light to medium-duty fastening
- Users needing a cost-effective washer with basic corrosion protection
Not for
- Outdoor or marine environments where zinc plating may corrode
- High-strength structural applications requiring thicker washers or specific grades
