FAQ - 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 × 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, provide the measurements, and we'll help identify the correct size.