Rubber Washer Size Chart | Sizing Reference

A rubber washer reference matches inner diameter to bolt size — SAE, USS, and metric standards cover plumbing, automotive, and industrial sealing needs.

Without a reliable rubber washer size chart, matching the inner diameter to a bolt is guesswork — and guesswork under pressure means leaks. SAE, USS, and metric standards each specify ID/OD pairs for every common fastener size, from tiny M3 seals in electronics to large 1-inch washers on heavy machinery. This guide breaks down what each standard covers, gives a quick reference table for the most common metric sizes, and explains how to pick the right washer on the first try.

SAE, USS, and Metric — What Each Standard Means

Three washer sizing systems cover nearly every application. SAE (Society of Automotive Engineers) washers use a narrow outer diameter and fit tight spaces like automotive panels and machinery housings. USS (United States Standard) washers have a wider OD for better load distribution on softer materials like wood or thin metal. Metric washers follow the M-series bolt sizing and are standard on imported vehicles, European plumbing, and most industrial equipment built outside the US.

All three share the same core logic: the washer’s inner diameter must match the bolt’s outer diameter closely enough that it can’t slip off, while the outer diameter must be wide enough to spread the clamping force without exceeding the available clearance. The ASME B18.22M standard defines these dimensions for both metric and inch washers, and ASME/ANSI B18.21.1-2009 covers the Type A plain washers that include SAE and USS series.

Metric Rubber Washer Size Chart

The table below covers the most commonly used metric rubber washer sizes from M3 through M12 — the range you’ll encounter most often in automotive, HVAC, and medium plumbing work. All dimensions are in millimeters.

For SAE inch sizes, the same ID-matching rule applies: a 1/4-inch bolt takes a washer with a 9/32-inch ID and a 5/8-inch OD at 1/16-inch thickness. USS sizes use the same ID but a wider OD — for example, a 1/4-inch USS washer has a 3/4-inch OD instead of 5/8-inch. Always check whether your application calls for SAE or USS clearance before buying.

Which Rubber Washer Size Should You Choose?

Three checks get you the right washer every time. First, match the ID to your bolt diameter — it should slide on without force but not spin freely. A washer with an ID that’s too large will spin on the bolt and fail to seal. Second, confirm the OD fits the space you have and gives enough bearing surface to distribute the load. If the OD is too small, the clamping force concentrates in one spot and causes leaks. Third, pick the standard: SAE for tight clearances, USS for general use where a wider grip helps, and metric for imported parts and international equipment.

Thickness matters more than most people think. Thin washers (0.5 to 1.0 mm) work for light sealing and low vibration. Medium thickness (1.6 to 2.0 mm) handles standard plumbing and automotive joints. Heavy washers (2.5 mm and up) are for high-pressure or high-vibration environments where the seal needs to hold under constant force. Rubber material also affects performance — EPDM handles weather and water well, Nitrile resists oil and fuel, and If you need a specific inch size like a 1 1/2-inch sealing washer, our best 1 1/2 rubber washer picks cover the top options tested for that size.

FAQs

Can I use an SAE washer with a metric bolt?

It’s not recommended. SAE washers are sized in fractions of an inch while metric bolts use millimeter diameters, so the ID fit will be loose or too tight. A metric bolt in an SAE washer often shifts under load, breaking the seal. Always match the washer standard to the bolt standard.

What rubber material is best for outdoor use?

EPDM (ethylene propylene diene monomer) is the standard choice for outdoor and water-exposed applications. It resists UV, ozone, and temperature changes from -40°F to 250°F without cracking or hardening. Nitrile is better for oil and fuel contact but degrades faster in direct sunlight.

How tight should a rubber washer seal feel?

A rubber washer should compress slightly when the fastener is torqued — you’ll see a thin bead of rubber squeeze out at the edge. If the washer stays flat with no compression, it’s too thick or too hard. If it bulges severely or tears, it’s too thin or over-torqued.