O-Ring Sizes Explained: ID, OD & CS — What They Mean and How to Measure Them
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Choosing the right O-ring starts with understanding its size. Whether you’re replacing a worn seal or building something new, every O-ring is defined by three simple measurements:
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ID – Inner Diameter
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OD – Outer Diameter
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CS – Cross-Section (thickness)
These three numbers determine whether an O-ring will seal correctly or fail prematurely — so getting them right matters.
Below is a simple guide to what each measurement means, how to measure them accurately, and how to find the size you need.
What Is the ID (Inner Diameter)?
The ID is the distance across the inside of the O-ring. This is usually the most important measurement because it must match the groove or the shaft the O-ring sits around.
How to measure ID
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Lay the O-ring flat.
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Use a ruler or caliper to measure straight across the centre of the hole.
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Measure inner edge to inner edge.
🔗 Tip: If you're working with imperial sizes, our Imperial O-Ring Gauge makes this extremely quick.
👉 View O-Ring Gauge → O Ring Measuring Gauge Tool - BS1806, AS568 Range – Seal with Selo
What Is the OD (Outer Diameter)?
The OD is the measurement from the outside edge to the opposite outside edge.
How to measure OD
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Measure the O-ring’s full outside width, directly across the centre.
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If you know ID and CS, you can calculate OD using:
OD = ID + (2 × CS)
You won’t always need the OD unless the application has a physical space limitation.
What Is the CS (Cross-Section)?
The CS is the thickness of the O-ring — the width of the rubber if you cut it and looked at the circular cross-section.
How to measure CS
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Use calipers for best accuracy.
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Gently close them on the thickness of the O-ring (don’t compress too hard).
CS is vital for determining compression and sealing performance.
Metric vs Imperial O-Ring Sizing
There are two common sizing systems:
Metric O-Rings
Measured in millimetres (mm).
Examples: 10 × 2 mm, 25 × 3.5 mm
Ideal for automotive, industrial, and modern equipment.
Imperial / BS O-Rings
Measured in inches, defined by British Standard (BS1806 / AS568).
Examples: BS007, BS010, BS022
Each BS number corresponds to a fixed ID + CS combination.
We convert the listing titles for BS o-rings to mm.
Which Measurement Matters Most?
All three matter, but in practice:
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ID must match the groove/shaft closely
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CS must match the groove depth
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OD must match the housing (if relevant)
A perfect seal depends on the correct combination.
Tools That Make O-Ring Sizing Easier
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Calipers → for accurate ID/OD/CS
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Imperial O-Ring Gauge → for quick BS number checks
👉 Shop O-Ring Gauge →O Ring Measuring Gauge Tool - BS1806, AS568 Range – Seal with Selo
Material Matters Too
Once you know the size, choose the correct material:
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NBR → oils, general use
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FKM (Viton®) → heat & chemicals
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Silicone → clean sealing & low compression
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EPDM → water & steam
👉 Shop NBR O-Rings - Nitrile (NBR) O-Rings – Fuel, Oil & Hydraulic Seals | SealWithSelo – Seal with Selo
👉 Shop FKM O-Rings - Buy FKM Rubber O-Rings | Viton® FKM O-Ring Seals – SELO – Seal with Selo
👉 Shop Red Silicone O-Rings - Red Silicone O-Rings – High Temp & Hygienic Seals | SealWithSelo – Seal with Selo
👉 Shop EPDM O-Rings - EPDM O-Rings – Water, Steam & Brake Fluid Seals | SealWithSelo – Seal with Selo