FAQ / How to Measure an O-Ring

How to Measure an O-Ring

Measuring an O-Ring correctly is essential to ensure a proper seal and prevent leaks. There are two key dimensions you need to measure:

  • Inside Diameter (ID): The distance across the centre of the O-Ring, from one inside edge to the other.

  • Cross Section (CS): The thickness of the rubber ring.

Together, these dimensions identify the correct O-Ring size.

If the O-Ring is in good condition

The easiest way to measure an O-Ring is with a digital Calliper or Vernier Calliper.

  1. Lay the O-Ring on a flat surface without stretching it.

  2. Measure the Inside Diameter (ID).

  3. Measure the Cross Section (CS) (the thickness of the rubber).

  4. Record both measurements in millimetres.

Example:

  • Inside Diameter (ID): 24.99mm

  • Cross Section (CS): 3.53mm

The correct O-Ring size would be 25 Ă— 3.53mm.

If the O-Ring is broken

If the O-Ring has snapped:

  1. Lay both pieces together without stretching them.

  2. Measure the total inside circumference.

  3. Calculate the Inside Diameter, or contact our team for assistance.

  4. Measure the cross-section as normal.

If the O-Ring is stretched or worn

Rubber O-Rings can expand over time, especially after prolonged use.

If the O-Ring appears stretched, flattened or damaged:

  • Measure the groove or housing if possible.

  • Refer to the original equipment manual.

  • Contact us with your measurements and application details; we'll help identify the correct replacement.

Metric or Imperial?

O-Rings are available in both metric and imperial (BS) sizes.

Many hydraulic and pneumatic systems use British Standard (BS) sizes, while modern machinery often uses metric O-Rings.

If you're unsure which standard you need, see our conversion table here or provide the measurements, and we'll help identify the correct size.

Browse our range of O-Rings and O-Ring Cord here.