Standard Flat Washer Sizes
Flat washers distribute load under a bolt head or nut, protect the surface from damage, and span oversized or slotted holes. The two common types in the US are:
- Type A (Narrow) — general purpose, standard series
- Type B (Wide) — larger OD for better load distribution on soft materials
Dimensions per ASME B18.22.1:
| Bolt Size | ID (in) | OD — Type A (in) | OD — Type B (in) | Thickness (in) |
|---|---|---|---|---|
| #6 | 0.156 | 0.375 | — | 0.049 |
| #8 | 0.188 | 0.438 | — | 0.049 |
| #10 | 0.219 | 0.500 | — | 0.049 |
| 1/4″ | 0.281 | 0.625 | 0.734 | 0.065 |
| 5/16″ | 0.344 | 0.688 | 0.875 | 0.065 |
| 3/8″ | 0.406 | 0.812 | 1.000 | 0.065 |
| 7/16″ | 0.469 | 0.922 | 1.250 | 0.065 |
| 1/2″ | 0.531 | 1.062 | 1.375 | 0.095 |
| 9/16″ | 0.594 | 1.156 | 1.469 | 0.095 |
| 5/8″ | 0.656 | 1.312 | 1.750 | 0.095 |
| 3/4″ | 0.812 | 1.469 | 2.000 | 0.134 |
| 7/8″ | 0.938 | 1.750 | 2.250 | 0.134 |
| 1″ | 1.062 | 2.000 | 2.500 | 0.134 |
| 1-1/8″ | 1.188 | 2.250 | 2.750 | 0.134 |
| 1-1/4″ | 1.312 | 2.500 | 3.000 | 0.165 |
| 1-1/2″ | 1.562 | 2.750 | 3.500 | 0.165 |
When to Use Type B (Wide)
- Bolting into wood, plastic, or composite — wider bearing area prevents pull-through
- Oversized or slotted holes — need the extra OD coverage
- Soft metals (aluminum) — prevents the nut from digging into the surface
Material Selection
- Zinc-plated steel — standard for most indoor/general use
- Stainless steel (18-8 / 316) — corrosion resistance, outdoor, marine
- Hot-dip galvanized — structural, outdoor, bridges (thicker coating)
- Nylon/plastic — electrical isolation, prevent galvanic corrosion
Related: Hex Cap Bolt Dimensions | Hex Nut Dimensions