Flat washers are some of the most ubiquitous components in engineering and construction. Despite their simple appearance, they play a critical role in preserving joint integrity, distributing loads, and preventing surface damage.

Among the various industrial standards, Form A flat washers are the most frequently specified general-purpose washers. Here is a detailed breakdown of what Form A washers are, how they are standardized, and how to select the right one for your project.

What is a Form A Washer?

A Form A washer is a standard-pattern flat washer with a normal thickness and an outside diameter (OD) that is roughly twice the size of the inside diameter (ID).

Unlike specialized washers—such as Form C (large OD/thick) or Form G (extra-large OD)—Form A represents the baseline, mid-range dimension for standard mechanical fastening.

Core Functions

• Load Distribution: Spreads the clamping force of a threaded fastener over a larger surface area to prevent material crushing.
• Surface Protection: Prevents the rotating bolt head or nut from galling, scratching, or scoring the mating component during tightening.
• Hole Bridging: Spans oversized or clearance holes to ensure the fastener head does not pull through the substrate.

Size Matrix: Typical Dimensions

Form A washers are designated by the nominal thread size of the bolt they are meant to fit (e.g., an M6 washer fits an M6 bolt). The actual internal diameter is always slightly larger than the nominal bolt size to allow for seamless clearance.

Below is a reference chart for common metric Form A sizes (standard DIN 125 / ISO 7089 parameters):