A rubber washer is described by three numbers: outer diameter (OD), inner diameter (ID) and thickness. Get any one wrong and the washer won't seal, won't fit the bolt, or won't survive the load. Here's how to read and measure each.
The three dimensions
Inner diameter (ID)
The hole in the middle. The ID must clear the bolt, screw or pipe it slips over — ideally a snug fit with no more than 1/32" of slop. Too tight and you'll tear it forcing it on; too loose and it slides and leaks.
Outer diameter (OD)
The overall width. The OD needs to be large enough to cover the surface you're sealing or spreading load across, but small enough to sit flat inside the counterbore or against the mating face.
Thickness
How much rubber sits between the two surfaces. Thickness controls how much the washer compresses and seals. Thin washers (1/16"–3/32") are best for flat sealing and shimming; thicker washers (1/4" and up) absorb vibration and take up gaps.
How to measure an existing washer
- Lay the washer flat and measure edge-to-edge across the widest point for OD.
- Measure straight across the hole for ID.
- Stand it on edge (or stack a few) to read thickness with calipers.
- Round to the nearest standard fraction — our washers are sized in 1/16" steps.
Choosing thickness by job
| Job | Suggested thickness |
|---|---|
| Flat seal under a bolt head | 1/16" – 1/8" |
| Shim / spacer | 1/16" – 1/4" |
| Vibration & noise damping | 1/4" + |
| Gap filling | 3/8" – 1/2" |
Not sure which material to pair with your size? Read the rubber washer material guide, or jump straight to the full catalog and filter by dimension.