How to Measure and Select the Right O-Ring: A Practical Guide for UK Engineers
An O-ring is defined by three measurements: inner diameter (ID), cross section (CS — also called cord diameter), and the outer diameter (OD), which is calculated from the first two: OD = ID + (2 × CS). Every O-ring in every standard from BS 4518 to ISO 3601 is catalogued by these three values. If you can measure ID and CS accurately with vernier calipers, you can identify any O-ring ever made — and order the correct replacement.
This guide explains how to take those measurements, how UK O-ring standards work, how to select the right compound for your application, and what to do when you’re faced with a mystery seal with no part number and no documentation.
The Three Measurements That Define Every O-Ring
Inner Diameter (ID)
The inner diameter is the measurement across the inside of the O-ring — the hole through the middle. For a seal sitting in a groove, this is the dimension that determines whether it will fit correctly over the shaft or into the bore without excessive stretch or compression in the installed position.
Lay the O-ring flat on a clean surface and measure across the inside opening at the widest point. Use vernier or digital calipers — not a steel rule. For very small O-rings (ID under 5mm), a measuring microscope or optical comparator gives more reliable results.
Cross Section / Cord Diameter (CS)
The cross section is the thickness of the O-ring’s cord — measured across the circular profile of the seal itself. This is the measurement most commonly taken inaccurately, because engineers try to measure it with a rule rather than calipers.
Grip a single point of the O-ring between the jaws of calibrated vernier calipers and measure across the cord. Take three measurements at different points around the ring and average them — worn or aged O-rings may be slightly oval rather than perfectly circular in cross section.
Outer Diameter (OD)
The outer diameter is derived, not measured directly: OD = ID + (2 × CS). For a 10mm ID O-ring with a 2mm cross section, OD = 10 + (2 × 2) = 14mm. You can verify this by measuring across the outside of the ring, but the formula is more reliable for worn or deformed seals.
How to measure accurately — tools and technique
Use calibrated digital or vernier calipers with 0.05mm or finer resolution. Before measuring:
- Clean the O-ring — contamination affects reading accuracy.
- Lay the ring on a flat surface for ID measurement.
- Do not stretch or compress the ring during measurement.
- For worn seals: measure at multiple points and note the range. A significantly out-of-round O-ring may have been installed incorrectly or run past its service life — the replacement size is the nominal standard size nearest to your average measurement, not the measured value itself.
Always cross-reference your measured values against the nearest size in the applicable standard. O-ring sizes follow defined series with fixed increments — if your measurement gives 9.72mm ID, the correct replacement is the 9.75mm or 10.0mm standard size, not a custom 9.72mm.
UK O-Ring Standards Explained
BS 1806 — the UK/ISO metric and imperial standard
BS 1806 is the UK’s equivalent of the international ISO 3601 standard and the American AS568 series. It covers a wide range of O-rings in both metric-related and inch-based dimensions. BS 1806 sizes are widely used on imported equipment — particularly from the United States — where the AS568 standard applies. Many hydraulic and pneumatic components from US manufacturers specify BS 1806 / AS568 O-ring sizes.
BS 4518 — metric sizes for UK-designed machinery
BS 4518 is the purely metric standard predominantly used in machinery designed and manufactured in the UK. It specifies O-ring dimensions in millimetres with clearly defined cross-section series (1.5mm, 2.5mm, 3.5mm, 5.0mm, 6.0mm, 8.0mm CS). If you’re working on UK-built equipment — machine tools, process plant, industrial valves — BS 4518 is likely the applicable standard.
Metric vs imperial — which do you have?
This is the most common source of confusion in O-ring replacement. The practical test:
- Take your ID measurement in millimetres. If it corresponds closely to a round metric number (10mm, 12mm, 15mm, 20mm, etc.), it is almost certainly a metric O-ring to BS 4518 or ISO 3601.
- Convert the measurement to inches. If the result is a recognisable fraction (3/8″, 1/2″, 3/4″, etc.) or a standard AS568 dash number dimension, it is likely an imperial or AS568 O-ring.
- Equipment origin is a strong indicator: UK-designed machinery → BS 4518 metric. US equipment → AS568 / BS 1806. German, French, or Italian equipment → ISO 3601 metric.
How to cross-reference standards when replacing a seal
When replacing an O-ring on equipment from an unknown origin, measure the O-ring and the groove. The groove dimensions — width and depth — confirm the nominal O-ring cross section (the groove is typically designed for 15–30% compression of the O-ring cross section). Cross-reference your measured values against BS 4518, BS 1806, and ISO 3601 tables simultaneously to identify the correct standard and size number. Delta Rubber can assist with cross-referencing if you provide your measurements.
O-Ring Material Selection — Which Compound for Which Application?
Material selection determines whether an O-ring seals reliably for years or fails within weeks. The decision is driven by three factors: the sealed medium, the operating temperature, and any regulatory requirements.
Nitrile (NBR) — the general-purpose oil-resistant standard
Nitrile rubber (NBR) is the default O-ring compound for oil, fuel, and hydraulic fluid applications. It accounts for the majority of O-rings used in UK industrial and automotive applications.
Sealed medium: Petroleum oils, mineral oils, hydraulic fluids, petrol, diesel, LPG, grease.
Temperature range: -30°C to +120°C.
Do not use in: steam, ozone-rich environments, aromatic hydrocarbons, ketones.
EPDM — for water, steam and outdoor applications
EPDM rubber (Ethylene Propylene Diene Monomer) is the standard O-ring compound for water, steam, and outdoor-exposed sealing. It offers outstanding ozone and UV resistance — unlike nitrile, it will not surface-crack in outdoor installations.
Sealed medium: Water, steam, dilute acids and alkalis, ozone-rich environments.
Temperature range: -40°C to +130°C.
Do not use in: oils, fuels, or petroleum products — EPDM will swell severely in hydrocarbon contact.
Viton (FKM) — for aggressive chemicals and high temperatures
Viton (fluorocarbon rubber, FKM) is specified where nitrile’s limitations are reached: temperatures above 120°C, aggressive chemicals including aromatic hydrocarbons, chlorinated solvents, and strong acids, or applications requiring the best possible chemical resistance. Viton O-rings are significantly more expensive than NBR but are the correct specification when conditions demand it.
Sealed medium: Aggressive chemicals, aromatic hydrocarbons, chlorinated solvents, fuels at elevated temperature.
Temperature range: -20°C to +200°C continuous, short-term to +230°C.
Silicone — for food, medical and wide temperature ranges
Silicone rubber is the standard for food-contact, pharmaceutical, and medical applications, and for applications requiring a very wide operating temperature range. Silicone is compliant with FDA 21 CFR regulations for food contact when specified in the correct grade. Its mechanical properties are inferior to NBR and FKM — silicone O-rings should not be used in high-pressure dynamic sealing.
Sealed medium: Food, beverages, pharmaceutical fluids, steam (low pressure), air.
Temperature range: -60°C to +220°C.
Neoprene — for refrigerants and moderate chemical resistance
Neoprene (polychloroprene) offers moderate resistance to oils, fuels, and weathering — making it a practical general-purpose O-ring compound for applications that fall between the clear specialisms of nitrile and EPDM. Historically used extensively in refrigerant systems with older CFC and HCFC refrigerants (though HFO and HFC refrigerants in modern systems often require EPDM or specific FKM grades).
Temperature range: -40°C to +120°C.
Material comparison table
| Compound | Oils/Fuels | Water/Steam | Chemicals | Ozone/UV | Temp Range |
|---|---|---|---|---|---|
| Nitrile (NBR) | Excellent | Moderate | Moderate | Poor | -30°C to +120°C |
| EPDM | Poor | Excellent | Good | Excellent | -40°C to +130°C |
| Viton (FKM) | Excellent | Good | Excellent | Excellent | -20°C to +200°C |
| Silicone | Poor | Moderate | Moderate | Good | -60°C to +220°C |
| Neoprene | Moderate | Good | Moderate | Good | -40°C to +120°C |
How to Identify a Mystery O-Ring (No Markings, No Part Number)
This is one of the most common maintenance scenarios: a seal has failed, the equipment has no parts list, and the only reference is the worn O-ring in your hand. Systematic identification solves it every time.
Step 1 — Measure the groove, not just the ring
The groove that housed the O-ring is often a more reliable reference than the worn seal itself. Measure:
– Groove width: This gives you the nominal cross-section size (groove width ≈ 1.3–1.5× the O-ring CS in a correctly designed installation).
– Groove depth: Cross-referencing groove depth with standard O-ring CS dimensions confirms the nominal size.
– Groove diameter: For a shaft groove, the groove diameter gives you the nominal O-ring ID. For a bore groove, the bore diameter and groove depth combine to give the O-ring OD.
Step 2 — Identify the material by appearance and feel
This is imprecise but narrows the field significantly:
– Black, firm, oil-smelling: Almost certainly nitrile (NBR). Nitrile is the default for oil and hydraulic systems.
– Black, slightly softer, odourless, installed in outdoor or steam application: Likely EPDM or neoprene.
– Brown or green, very firm, fuel application: Probably Viton (FKM).
– Translucent grey, white, or red, soft and flexible, food/medical context: Silicone.
A simple test: dab the O-ring with a drop of mineral oil. Nitrile shows no reaction; EPDM will begin to swell with prolonged exposure.
Step 3 — Cross-reference with standard size charts
With your ID and CS measurements, cross-reference against BS 4518, BS 1806/AS568, and ISO 3601 size tables simultaneously. In most cases, a clear standard match will emerge within ±0.2mm. If measurements fall between two standards, consider the equipment origin to resolve the ambiguity.
For complex cross-referencing, the Delta Rubber seals and O-rings team can assist with identification — provide your measured dimensions, groove measurements if available, and the equipment type.
How to Order O-Rings in the UK
Individual seals vs kit packs — when each makes sense
Individual O-rings: When you know the exact size, standard, and compound. Correct for planned maintenance, known replacement schedules, and production component supply. Order directly from Delta’s shop or request a quote for volume orders.
Metric O-ring kits: Assortments of 200–400 O-rings covering the most common BS 4518 metric sizes in nitrile. Standard stock for any engineering workshop that encounters O-ring replacement regularly but cannot predict which sizes will be needed. Eliminates the risk of a production stoppage while waiting for a specific O-ring to arrive.
Imperial / AS568 kits: For workshops maintaining US-origin equipment. Cover the most common AS568 dash numbers in nitrile.
Buying in bulk for stock
For production applications where a specific O-ring is used repeatedly in volume, bulk ordering reduces per-piece cost significantly. Delta supplies O-rings in bags of 50, 100, 500, and 1,000+ for maintenance stock and production use. Request a volume quote via deltarubber.co.uk.
For non-standard seals where no catalogue O-ring fits the application, custom-cut seals from Gaskets Direct provide an alternative: a seal cut to exact dimensions from rubber sheet.
Frequently Asked Questions
How do you measure an O-ring?
Measure the inner diameter (ID) across the inside opening of the O-ring and the cross section (CS) across the cord thickness — both using calibrated vernier or digital calipers. Outer diameter (OD) is calculated: OD = ID + (2 × CS). Do not use a steel rule; for O-rings under 5mm ID, use precision calipers with 0.02mm resolution or better. Measure worn O-rings at multiple points and average the results.
What is the difference between BS 1806 and BS 4518 O-rings?
BS 1806 is the UK equivalent of ISO 3601 and the US AS568 standard, covering O-rings in metric-related and inch-based dimensions — common on imported machinery. BS 4518 is a purely metric standard predominantly used on UK-designed machinery. When replacing O-rings on imported equipment, identify the country of equipment origin to determine the most likely standard.
What does CS mean on an O-ring?
CS stands for Cross Section — also called cord diameter. It is the thickness of the O-ring’s circular cord, measured across the ring profile using vernier calipers. CS is one of the two primary measurements (along with inner diameter) used to identify and order the correct replacement O-ring. The standard cross-section series in BS 4518 are 1.5mm, 2.5mm, 3.5mm, 5.0mm, 6.0mm, and 8.0mm.
What material should I use for an O-ring?
The correct O-ring compound depends on the sealed medium and operating temperature. Nitrile (NBR) is the standard for oil and fuel systems (-30°C to +120°C). EPDM is preferred for water, steam, and outdoor applications (-40°C to +130°C). Viton (FKM) is specified for aggressive chemicals and temperatures above 120°C (-20°C to +200°C). Silicone is used for food contact, pharmaceutical, and wide temperature range applications (-60°C to +220°C).
Can I use nitrile O-rings in water applications?
Nitrile O-rings will function in water applications at ambient temperature, but EPDM is the correct specification for water sealing — particularly in potable water systems, where WRAS-approved EPDM should be specified. Nitrile is not suitable for steam applications and has poor ozone resistance, which limits its outdoor service life.
What if I cannot find a standard O-ring in the size I need?
If your measured dimensions do not correspond to any standard O-ring size, a custom-cut seal is the solution. Gaskets Direct provides CNC-cut seals to exact dimensions from a wide range of rubber compounds — with no tooling charge and no minimum order. This is commonly used for non-standard groove dimensions or seals for equipment manufactured to proprietary specifications.
Getting the right O-ring into service quickly comes down to accurate measurement, correct standard identification, and choosing the right compound for the sealed medium. The Delta Rubber O-ring and seals range covers BS 4518 metric, BS 1806/AS568 imperial, and common ISO 3601 sizes across nitrile, EPDM, Viton, silicone, and neoprene compounds — with same-day despatch on standard sizes from the Delta shop. For non-stan
