M20 Washer Dimensions & Size

M20 washer dimensions are standardized for extreme-load structural and mechanical applications, including heavy bridge construction, industrial steel framing, power plant equipment, and heavy vehicle axle assemblies. Specifically designed for fasteners with a 20 mm nominal diameter, these components ensure stable load distribution and protect mating surfaces from damage during high-torque tightening. This chart covers Form A plain washers, which feature a flat surface without a chamfer (unlike Form B).

The M20 size is a heavy-duty industrial standard available in three series: the Normal Series (DIN 125A / ISO 7089) for standard load distribution, the Large Series (DIN 9021 / ISO 7093) for a significant increase in surface area, and the Small Series (DIN 433 / ISO 7092) for tight radial clearances. Based on these standards, M20 washers typically feature a hardness of 200 HV or 300 HV (Vickers Hardness), providing the structural integrity required to prevent “cupping” or deformation under the massive clamping forces of grade 8.8, 10.9, or 12.9 bolts.

Note: While these values reflect standard manufacturer datasheets, actual dimensions and tolerances may vary based on material (e.g., Stainless Steel vs. Hot-Dip Galvanized Steel) and manufacturing class. Always verify with the official product datasheet for critical applications.

Washer dimension diagram showing inner diameter, outside diameter, and thickness
M20 Standard Series Washer – DIN 125A/ISO 7089
Dimension Size (mm)
Min Max
Inner Diameter 21 21.33
Outside Diameter 36.38 37
Thickness 2.7 3.3
M20 Large Series Washer – DIN 9021/ISO 7093
Dimension Size (mm)
Min Max
Inner Diameter 22 22.52
Outside Diameter 58.1 60
Thickness 3.4 4.6
M20 Small Series Washer – DIN 433/ISO 7092
Dimension Size (mm)
Min Max
Inner Diameter 21 21.33
Outside Diameter 33.38 34
Thickness 2.7 3.3
  • Inner Diameter – The diameter of the hole, designed to clear the bolt/screw thread.
  • Outside Diameter – The total width of the washer, determining the load distribution area.
  • Thickness – The height of the washer, providing the spacing and structural support.
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Frequently Asked Questions

What does the “M20” designation mean on a washer?

The “M” stands for Metric, and the number represents the nominal thread diameter of the screw or bolt. An M20 washer is designed specifically for fasteners with a 20 mm nominal major diameter.

What is the difference between Form A and Form B washers?

The difference lies in the outer edge finish. Form A washers have a square, flat edge and are the most common. Form B washers include a 30° to 40° chamfer (beveled edge) on one side, according to DIN 125B and ISO 7090 standards.

When should I use a Large Series (DIN 9021) M20 washer?

The Large Series (DIN 9021 / ISO 7093) features a wide outside diameter of 60 mm. It is used when fastening into softer materials like structural timber, composite panels, or thin metal plates where you need to spread the clamping force over a maximum surface area to prevent the bolt head from sinking into or pulling through the material.

What does the HV rating mean on a washer?

HV stands for Vickers Hardness. For M20 fasteners, a rating of 200 HV or 300 HV is standard for steel and stainless steel. This ensures the washer maintains its flatness and resists indentation even when subjected to the extreme torque levels required for structural mechanical joints.

What is the difference between Small Series (DIN 433) and Normal Series?

The Small Series (DIN 433 / ISO 7092) has a narrower outside diameter (34 mm) compared to the Normal Series (37 mm). It is specifically intended for use with Socket Head Cap Screws or in recessed counterbores where a standard-width washer would be too large to fit the available space.

In which materials are M20 washers commonly available?

M20 washers are most frequently produced in A2 (304) and A4 (316) Stainless Steel for corrosion resistance. They are also widely available in Hot-Dip Galvanized Steel for outdoor construction and Zinc-Plated Steel for general industrial hardware.