Quick answer: a "3/8 rubber washer" normally means a 3/8" inside diameter, the hole for a 3/8" bolt or stud. We list 148 sizes with a 3/8" hole, with outside diameters from 9/16" to 4" and thicknesses from 1/32" to 1/2", each offered in seven materials. Pick the OD by coverage, the thickness by the job, and the material by heat, chemicals and weather. Browse the 3/8" ID rubber washers collection.
3/8" fasteners are a step up from the light 1/4" class. They show up on trailer and truck hardware, brackets and frames, machinery feet, gutters and downspout straps, exhaust and heat shields, and plenty of plumbing and HVAC fittings. A rubber washer here typically seals a bolt hole against water, cushions a bracket against vibration, or stops two metals touching.
3/8" bolt, 3/8" tube or 3/8" OD?
- 3/8" bolts and studs (3/8"-16, 3/8"-24): use a 3/8" ID. It fits snugly and seals around the shank.
- M10 bolts are 10 mm (0.394"), slightly larger than 3/8" (0.375"). A 3/8" ID washer has to stretch to fit and may tear when it is forced on. Use a 7/16" ID or see M10 rubber washers.
- M8 bolts (0.315") are smaller, so a 3/8" ID fits loosely. See M8 rubber washers.
- 3/8" OD washers are small parts with a 1/8" or 3/16" hole. We list 14 of them, for example the 3/8" OD x 1/8" ID x 1/16".
- Tubing and hose: a washer slipped over a tube needs an ID that matches the tube's outside diameter, which is not always the nominal size. Measure the tube.
Every 3/8" ID size we list
Every OD is listed in the thicknesses shown and in seven materials: neoprene, EPDM, nitrile, silicone, natural rubber, Viton and fiber.
| OD with a 3/8" hole | Thicknesses listed |
|---|---|
| 9/16" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 5/8" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 11/16" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 3/4" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 13/16" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 7/8" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 15/16" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 1" | 1/32", 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 1-1/8" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 1-1/4" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 1-3/8" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 1-1/2" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 1-5/8" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 1-3/4" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 2" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 2-1/4" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 2-1/2" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 2-3/4" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 3" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 3-1/2" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
| 4" | 1/16", 3/32", 1/8", 3/16", 1/4", 3/8", 1/2" |
The full rubber washer size chart shows every combination in inch and mm, if you need to compare 3/8" with the neighbouring 5/16" and 7/16" holes.
How to choose the OD for a 3/8" bolt
For a 3/8" bolt clamping metal to metal, an OD of about 7/8" to 1" is a common, balanced size. It gives a good sealing ring around the hole without overhanging. Move up to 1-1/4" to 2" when the washer bears on soft material such as wood, plastic, rubber matting or fiberglass, when it has to bridge a slotted or oversized hole, or when it is the cushion in a vibration mount. Make sure the OD will fit the flat area around the hole.
How to choose the thickness
For sealing a flat joint, 1/16" to 1/8" is usually best: enough rubber to fill surface roughness, not so much that it squeezes out. For damping vibration or cushioning a bracket, 3/16" to 1/4" gives more give. For spacing parts apart, choose the thickness that equals the gap. If you're unsure, the sealing vs spacing washers guide walks through it. Remember a worn washer measures thinner than it did when new.
How to choose the material
| Material | Usual choice for |
|---|---|
| Neoprene | All-purpose sealing and cushioning, indoors and out |
| EPDM | Water, steam, sunlight and ozone; not petroleum oils |
| Nitrile | Oil, fuel and grease |
| Silicone | High and low temperatures, food-contact equipment |
| Viton | Aggressive chemicals, fuels and high heat |
| Natural rubber | Shock and vibration absorption |
| Fiber | Firm, dry gasketing on fittings |
Most 3/8" applications outdoors (trailers, gutters, roofing brackets) are best served by EPDM or neoprene. Engine-bay and machinery bolts that see oil call for nitrile, or Viton where it is also hot. Compare them in the material comparison.
3/8" washers ready to ship (October 2026)
These 3/8" ID and 3/8" OD sizes had at least one material in stock when this guide was updated. Stock changes, so check the product page for current availability.
| OD | ID | Thick | In stock in | |
|---|---|---|---|---|
| 1" | 3/8" | 1/32" | Neoprene | View size |
| 1-1/2" | 3/8" | 1/16" | Neoprene | View size |
| 1-3/8" | 3/8" | 3/16" | Neoprene | View size |
| 1-1/4" | 3/8" | 1/16" | Neoprene | View size |
| 1-1/8" | 3/8" | 1/8" | Neoprene | View size |
| 1" | 3/8" | 1/8" | Neoprene | View size |
| 1" | 3/8" | 1/16" | Neoprene | View size |
| 7/8" | 3/8" | 1/16" | Neoprene | View size |
| 13/16" | 3/8" | 1/8" | Neoprene | View size |
| 13/16" | 3/8" | 3/32" | EPDM | View size |
| 3/4" | 3/8" | 1/16" | Neoprene, Silicone | View size |
| 5/8" | 3/8" | 3/32" | EPDM | View size |
| 3/8" | 3/16" | 3/32" | EPDM | View size |
| 3/8" | 3/16" | 1/16" | Silicone | View size |
| 3/8" | 1/8" | 3/32" | EPDM | View size |
| 3/8" | 1/8" | 1/16" | Neoprene | View size |
Tips for a better seal on a 3/8" bolt
- Put a flat metal washer on top of the rubber washer under the nut or bolt head. It spreads the force evenly and stops the rubber twisting as you tighten.
- Tighten until the rubber just starts to bulge at the edge. Overtightening splits thin washers and squeezes thick ones out of shape.
- Clean paint flakes, rust and old sealant off both faces first.
FAQ
What size rubber washer fits a 3/8" bolt?
A 3/8" ID washer. A 7/8" or 1" OD at 1/16" to 1/8" thick suits most sealing jobs.
Can I use a 3/8" rubber washer on an M10 bolt?
It's a tight stretch, because M10 is slightly larger than 3/8". A 7/16" ID fits better.
Are 3/8" rubber washers sold in bulk?
Yes. Any size can be ordered in quantity. See buying rubber washers in bulk.
What's the most common 3/8" rubber washer?
There's no single standard, but 3/8" ID washers in the 7/8" to 1" OD range at 1/16" to 1/8" thick cover most general sealing and anti-rattle jobs.