Where Does the Flat Washer Go on a Screw?

A Complete Guide to Flat washer Placement, Function, and Best Practices

In mechanical assembly, construction, equipment manufacturing, and even DIY projects, one very common but often overlooked question is:

Where does the flat washer go on a screw?

Although a flat washer is one of the simplest and most widely used fasteners, its installation position has a direct impact on load distribution, joint reliability, surface protection, and service life. This article explains what is a flat washer, where it should be installed, and how to select and use different washer sizes correctly in real-world applications.

1. What Is a Flat Washer?

Before answering where does the flat washer go on a screw, it is important to understand:

What is a flat washer?

A flat washer is a thin, round disc with a central hole. Unlike Spring washers, it does not provide elastic force. Its primary functions include:

1. Distributing clamping force over a larger surface area

2. Preventing damage to the mating surface

3. Providing a smooth bearing surface for fasteners

4. Compensating for minor surface irregularities

Because of these functions, flat washers are widely used in metal, plastic, wood, and composite material assemblies.

2. The Core Rule: Where Does the Flat Washer Go?

 General principle: place the flat washer on the rotating side

In most fastening applications:

• If the screw rotates and the nut or threaded hole is fixed → place the flat washer under the screw head

• If the nut rotates and the screw or bolt head is fixed → place the flat washer under the nut

This rule answers the question where does the flat washer go on a screw from an engineering perspective.

3. Most Common Case: Flat Washer Under the Screw Head

In many assemblies, especially those without a nut, the flat washer is installed between the screw head and the clamped material. Typical examples include:

• Self-tapping screws

• Wood screws

• Screws threaded directly into a tapped hole

• Assemblies where the nut is not accessible or does not rotate

For example, using an m4 flat washer or m5 flat washer under the screw head helps:

• Prevent the screw head from sinking into the material

• Protect painted or coated surfaces

• Reduce localized stress

This is especially important for softer materials such as plastic or aluminum.

4. Bolt and Nut Assemblies: Flat Washer Under the Nut

In standard bolt-and-nut connections, tightening is usually done by rotating the nut while holding the bolt head stationary. In this case, the flat washer should be placed under the nut.

Common examples include:

• m10 washer

• m16 flat washer

• 3 8 flat washer

• 1 3 4 flat washer

Placing the flat washer under the nut reduces friction, protects the surface, and allows for more consistent tightening torque.

5. Spring Washers and Flat Washers Used Together

In applications requiring vibration resistance, spring washers and flat washers are often used together.

The correct installation order is usually:

Screw or nut → spring washer → flat washer → clamped surface

Reasons for this configuration include:

• The spring washer provides anti-loosening force

• The flat washer distributes load and protects the surface

Reversing this order reduces the effectiveness of the spring washer and can lead to joint loosening.

6. Common Flat Washer Sizes and Typical Applications

Small Flat Washers

• m4 flat washer – electronics, light-duty assemblies

• m5 flat washer – enclosures, small machinery

Medium Flat Washers

• m10 washer – general mechanical equipment

• 3 8 flat washer – inch-based systems and North American standards

Large Flat Washers

• m16 flat washer – heavy machinery and steel structures

• 4 inch outside diameter flat washer – structural and load-spreading applications

• 6 inch washer – large-scale construction and special engineering projects

Large-diameter washers are especially important when clamping soft materials or handling high loads.

7. Special Washer Dimensions and Use Cases

Large Inner Diameter Washers

• 1.25 id washer

These are commonly used for:

• Large Bolts

• Sleeve or spacer assemblies

• Non-standard fastening applications

Large Inch-Size Washers

• 1 3 4 flat washer

Frequently used in:

• Heavy equipment

• Infrastructure projects

• North American industrial systems

8. Common Installation Mistakes to Avoid

 Placing the flat washer on the non-rotating side
 Installing a spring washer underneath a flat washer
 Using undersized washers in high-load applications
 Using a flat washer as a substitute for a locking washer

These mistakes can significantly reduce joint strength and reliability.

9. Best Practices for Flat Washer Installation

 Always place the flat washer on the rotating side of the fastener
 Use flat washers when clamping soft or finished surfaces
 Select large outer diameter washers for high-load applications
 Combine flat washers with spring washers when vibration resistance is required
 Match washer size to the fastener (m4, m5, m10, m16, 3/8, 1 3/4, etc.)

10. Final Answer: Where Does the Flat Washer Go on a Screw?

So, where does the flat washer go on a screw?

 The answer is:

On the side of the fastener that rotates during tightening, directly against the surface being protected.

Whether you are using:

• m4 flat washer or m5 flat washer

• m10 washer or m16 flat washer

• 3 8 flat washer or 1 3 4 flat washer

• 1.25 id washer, 4 inch outside diameter flat washer, or 6 inch washer

Correct flat washer placement ensures better load distribution, surface protection, and long-term fastening reliability.

If you want next, I can also help you:

•  Rewrite this into a pure SEO-focused English version

•  Convert it into product category descriptions

•  Create FAQ sections for engineers and buyers

•  Adapt it for Amazon / Alibaba / B2B fastener listings