Flat Washers
Washers & Retaining Rings · 11 specs
Direct Answer
Standard flat washers distribute fastener clamp load and prevent surface embedding, standardized under DIN 125 (ISO 7089 Form A metric without chamfer) and ASME B18.22.1 (Type A Narrow, Regular, and Wide series in imperial sizes).
Source: DIN 125-1:1990 / ISO 7089:2000 / ASME B18.22.1 · Last reviewed Aug 28, 2026
Specs
| Governing Standards | DIN 125 Form A (ISO 7089) / ASME B18.22.1 Type A |
| M3 Flat Washer | ID: 3.2 mm | OD: 7.0 mm | Thickness: 0.5 mm |
| M4 Flat Washer | ID: 4.3 mm | OD: 9.0 mm | Thickness: 0.8 mm |
| M5 Flat Washer | ID: 5.3 mm | OD: 10.0 mm | Thickness: 1.0 mm |
| M6 Flat Washer | ID: 6.4 mm | OD: 12.0 mm | Thickness: 1.6 mm |
| M8 Flat Washer | ID: 8.4 mm | OD: 16.0 mm | Thickness: 1.6 mm |
| M10 Flat Washer | ID: 10.5 mm | OD: 20.0 mm | Thickness: 2.0 mm |
| M12 Flat Washer | ID: 13.0 mm | OD: 24.0 mm | Thickness: 2.5 mm |
| 1/4" ASME Type A | ID: 0.281 in (7.1 mm) | OD: 0.625 in (15.9 mm) | Thickness: 0.065 in (1.65 mm) |
| 3/8" ASME Type A | ID: 0.406 in (10.3 mm) | OD: 0.812 in (20.6 mm) | Thickness: 0.065 in (1.65 mm) |
| 1/2" ASME Type A | ID: 0.531 in (13.5 mm) | OD: 1.062 in (27.0 mm) | Thickness: 0.095 in (2.41 mm) |
Standard Metric (DIN 125 / ISO 7089) & Imperial Flat Washer Dimensions
| Nominal Bolt Size | Metric DIN 125 Inside Dia (d_1) | Metric Outside Dia (d_2) | Metric Thickness (h) | ASME 1/4–1/2" ID (d_1) | ASME OD (d_2) | ASME Thickness (h) |
|---|---|---|---|---|---|---|
| M3 / #4 | 3.2 mm | 7.0 mm | 0.5 mm | 0.125" (3.2 mm) | 0.312" (7.9 mm) | 0.036" (0.9 mm) |
| M4 / #8 | 4.3 mm | 9.0 mm | 0.8 mm | 0.188" (4.8 mm) | 0.438" (11.1 mm) | 0.049" (1.2 mm) |
| M5 / #10 | 5.3 mm | 10.0 mm | 1.0 mm | 0.219" (5.6 mm) | 0.500" (12.7 mm) | 0.049" (1.2 mm) |
| M6 / 1/4" | 6.4 mm | 12.0 mm | 1.6 mm | 0.281" (7.1 mm) | 0.625" (15.9 mm) | 0.065" (1.65 mm) |
| M8 / 5/16" | 8.4 mm | 16.0 mm | 1.6 mm | 0.344" (8.7 mm) | 0.688" (17.5 mm) | 0.065" (1.65 mm) |
| M10 / 3/8" | 10.5 mm | 20.0 mm | 2.0 mm | 0.406" (10.3 mm) | 0.812" (20.6 mm) | 0.065" (1.65 mm) |
| M12 / 1/2" | 13.0 mm | 24.0 mm | 2.5 mm | 0.531" (13.5 mm) | 1.062" (27.0 mm) | 0.095" (2.41 mm) |
| M16 / 5/8" | 17.0 mm | 30.0 mm | 3.0 mm | 0.656" (16.7 mm) | 1.312" (33.3 mm) | 0.095" (2.41 mm) |
| M20 / 3/4" | 21.0 mm | 37.0 mm | 3.0 mm | 0.812" (20.6 mm) | 1.469" (37.3 mm) | 0.134" (3.40 mm) |
Why Flat Washer Inner Diameters Are Always Oversized
A washer inner diameter (d_1) is deliberately manufactured larger than the nominal fastener diameter:
- Fillet Clearance: Underneath the head of a bolt or cap screw, the transition from shank to head features a small transition fillet radius (e.g. 0.4 to 0.8 mm). If the washer hole matched the nominal diameter exactly (e.g. 6.0 mm on an M6 bolt), the washer would ride up on the fillet, preventing flat bearing contact and inducing high bending fatigue stresses under the head.
- ASME Narrow vs Wide Series: ASME B18.22.1 distinguishes between Narrow (SAE) washers (small OD for tight clearance counterbores) and Wide (USS) washers (large OD for softer materials like wood and sheet metal).