Size guide · ID × CS

Size & measurement guide for O-rings

Read the dimensions. Record the right units. Find your next size. Start from what you have in front of you: the old ring, two measurements, or a standard number.

Shop by size system

Go directly to the O-ring catalog

AS568 O-ringsBrowse inch-series dash sizes from -001 to -475. DIN / ISO metric O-ringsBrowse metric ID ? cross-section sizes in millimetres. All exact-size small packsCompare listed size, material, hardness and pack options.
I have the old ring

Measure it

Two caliper readings are enough. We work out the closest standard size in four systems and the grades we list. Calipers or a ruler and a flat table Open the measuring tool I know ID × CS

Size finder

Enter the inside diameter and cross-section in mm or inches, then compare grades, packs and prices at that exact size. e.g. 20 × 2.5 mm or 0.984 × 0.139 in Find by size I have a standard number

Cross-reference

AS568 dash number, JIS B 2401 or BS 1806 code, or Common metric dimensions: get ID × CS and nearby size references. ISO designation lookup is not provided by this Common metric library. e.g. -214 · P22 · BS 207 Look up a code
O-ring dimensions: ID across the full inside diameter, CS across the cord Front view of an O-ring. The ID arrow runs from inner edge to inner edge across the hole. The cross-section view shows the two cut cords; CS is the thickness of one cord. A dotted outer-edge line is an optional check: inside diameter + 2 × cross section. FRONT VIEW ID inside diameter Outer check · optionalCROSS-SECTION CS CS = thickness of the cord ID Outer check = ID + 2 × CS
Measure the ring in its relaxed state. ID runs from inner edge to inner edge; CS is the thickness of the cord.

How to measure

ID and CS

ID Inside diameter
The opening across the relaxed ring, measured from inner edge to inner edge.
CS Cross section
The thickness of the ring body. Measure the round section, not the opening.

mm or inches?

Metric sizes are usually recorded as ID × CS in millimetres. AS568 uses inch-based nominal sizes and dash numbers. Keep the units shown on your drawing or measuring tool. A converted dimension alone does not identify a standard.

Keep one unit throughout your record. 1 inch = 25.4 mm.

Measuring an existing ring

Enter your readings
  1. 01

    Clean and identify

    Remove the ring, clean it and note the equipment it came from.

  2. 02

    Let it relax

    Place it on a flat surface before measuring. Avoid pulling it into shape.

  3. 03

    Measure cross section

    Use a caliper or pin micrometer at three points. Keep all readings.

  4. 04

    Measure inside diameter

    Measure without stretching. A cone gauge can help with repeatability.

  5. 05

    Record the unit

    Write mm or in beside every dimension. Keep the original readings.

  6. 06

    Add the conditions

    Note the known material, hardness, fluid and operating temperature.

Reference & engineering tools

More ways to pin down the size

Size chartsAS568, metric, JIS and BS 1806 in mm and inches. Popular sizesThe common dash and metric sizes, by series. CalculatorsCord length, stretch, weight, mm ⇄ inch. Groove designSqueeze and gland fill for a chosen size.

Before you order

Common measurement errors

Four mistakes that turn a correct ring into the wrong order.

A used ring can change size

Swelling, compression set and wear change the dimensions. Treat a used-ring measurement as a starting point for review, not the original specification.

Stretching the ring while measuring

Caliper jaws can pull the opening wider. Use the smallest stable reading without forcing the ring.

Mixing units or rounding too early

Keep your measured precision and one unit per record. A 2.60 mm and a 2.65 mm cross section are different sizes.

Using groove dimensions as ring dimensions

Groove dimensions and O-ring dimensions are related but are not interchangeable. If you only have the groove or a drawing, include it in your request.

OXXRING · Measurement worksheet

Worksheet

Your measurement worksheet

One record per ring. Fill it in, print it, or continue with this specification.

The ring

Unit

For a quote request optional

What carries into the next step? Dimensions, units, material and hardness go into the size search. A review request also carries your quantity, reference, ring condition and service notes in this browser session. Nothing is uploaded or saved to an account until you submit a request.

Used-ring measurements are a starting point for review. Confirm the original specification before ordering.

Download size reference charts

Use these PDFs as reference tables, then confirm the actual ID, cross section, material and hardness on the product or drawing before ordering.

A size-chart match identifies geometry only. It does not by itself approve material, compound or application suitability.

Standards and fundamentals

Need more context than a size chart? Use the standards reference and fundamentals guide alongside the Finder. Keep the original drawing or measured dimensions as the governing input.