What Are O-Rings? Sizes, Tolerances & Measuring Guide

O-rings are among the most essential sealing components used in countless industries, from automotive to medical devices. Their simple design—a circular ring with a round cross-section—belies their critical role in preventing leaks of liquids and gases. However, to ensure an effective seal, selecting the correct O ring sizes are paramount. This article delves into the fundamentals of O-ring dimensions, how to interpret an O ring sizes chart, and the proper way to measure O rings for your applications.

What Are O-Rings?

An O-ring is a mechanical gasket shaped like a torus, typically made from elastomeric materials. When compressed between two surfaces, it creates a tight seal that prevents fluid or gas leakage. O-rings have a wide range of applications across many industries. In automotive engines and braking systems, O-rings provide oil and heat resistance, contributing to vehicle safety and performance. In electronic devices, O-rings protect internal components from dust and water damage.

The two Critical Dimensions to Understand are:

Inside Diameter (ID): The distance across the inner circle of the O-ring.
Cross Section (CS): The thickness or width of the ring’s circular cross-section.
Together, these define the overall size and fit of the O-ring. The outside diameter (OD) can be calculated using the formula: OD=ID+2×CS

How to Measure O Ring Sizes : A Step-by-Step Guide

Measuring an O-ring accurately is critical when replacing or specifying components. Here’s how to do it, using tools like calipers or cone gauges and referencing an O-ring measurement chart:

1. Select the Right Tool: Digital calipers offer precision for both inside diameter (ID) and cross-section (CS). For high-volume tasks, cone gauges simplify matching to standard sizes.
2. Measure Inside Diameter (ID): Place the O ring flat on a non-deformable surface. Gently measure the inner circle’s diameter, ensuring no stretching. Record in inches or millimeters.
3. Measure Cross-Section (CS): Measure the thickness of the O-ring’s toroidal section. Take multiple readings to account for wear or manufacturing variations.
4. Verify Against Charts: Compare measurements to an O ring size chart (AS568 or metric). For example, an ID of 0.362 inches and CS of 0.103 inches matches AS568-110.
5. Account for Stretch: If the O-ring is used, note potential stretching or compression. Measure in a relaxed state for accuracy.

Why Accurate O Ring Sizing Matters？

Using the correct O-ring dimensions ensures that the seal will perform reliably under pressure, temperature, and chemical exposure. An O-ring that is too small may stretch excessively, leading to premature failure. Conversely, an oversized O-ring might not seat properly, resulting in leaks.

Additionally, the cross-section affects how the O-ring compresses and seals. Different applications may require different cross-section sizes to accommodate pressure and movement.

Metric vs. Standard O-Ring Sizes

AS568 (standard) and metric, each tailored to specific industries and regions. The AS568 standard, widely adopted in North America, uses imperial units (inches) to define O-ring dimensions. This system offers over 400 standardized sizes, cataloged in an O-ring-size-chart, ensuring consistency for applications like automotive or aerospace.
Conversely, metric O-ring sizes cater to international markets, using millimeters under standards like ISO 3601 or JIS B 2401. A metric O-ring equivalent to AS568-012 might have an ID of 9.25 mm and a CS of 1.78 mm.

Below is a sample from such a O ring size chart: