The O-ring is one of the most important parts in an assembly, and one of the highest-cost if it is incorrect. A seal that is 1/16" off in the inside diameter or one molded in a compound that expands in the fluid it is immersed in does not fail silently. It leaks and when someone realizes, it's not the few rupees worth of the ring, it's downtime and an assembly stripped apart. This guide takes you through the AS568 O-ring size chart system correctly: what a dash number really means, how the series are organized, how to measure a ring that you already have in your hand, and how to select a compound without having to get a materials degree. Written from the sizing questions we get from customers, not a book, we manufacture rubber O-rings in all of these standards. If you already have your dash number and only need to get a quote, go to the bottom. When you're looking at a groove, a drawing or a used O-ring and wondering what to order, read this entire blog to choose the best O-ring for your business.
What Is AS568?
AS568 is a SAE Aerospace Standard covering standard sized O-rings measured in inches and suitable for use in Aerospace, Hydraulic, Pneumatic, Automotive and general industrial applications. The standard was first published in 1971 and provides a universal numbering system, the dash number being used to identify the O-rings (e.g., -214). The tolerances, materials and standard format have been revised in most of the changes in AS568, the revised editions of which are AS568A, AS568B, AS568C and AS568D. The basic dash-size dimensions of these o-rings (with the designation -214) will remain the same for all these revisions. In choosing an AS568 O-ring, it is important to consider its size, tolerances, materials, and applications, and not revision letters.
How the Dash Number System Works
The single most useful thing to understand about AS568 is that the dash number is not arbitrary. The first digit tells you the cross section, and the last two tell you where the ring sits in the size progression for that cross section. Each series has one fixed cross section. Every ring in that series is exactly that thick, regardless of how large the diameter gets:
| Series | Dash Range | Cross Section (inch) | Cross Section (mm) | Typical Use |
|---|---|---|---|---|
| 000 | -004 to -050 | 0.070 | 1.78 | Small bores, instrumentation, miniature fittings |
| 100 | -102 to -178 | 0.103 | 2.62 | General purpose, the most widely stocked series |
| 200 | -201 to -284 | 0.139 | 3.53 | Hydraulic and pneumatic cylinders, medium bore |
| 300 | -309 to -395 | 0.210 | 5.33 | Larger bore hydraulics, heavier static seals |
| 400 | -425 to -475 | 0.275 | 6.99 | Large diameter flanges and housings |
| 900 | -901 to -932 | Varies | Varies | Straight-thread boss / ORB gaskets (different logic — see below) |
The ID, CS and OD Relationship
An O-ring is fully described by two dimensions, and the third falls out of them. AS568 charts list the inner diameter and the cross section; the outer diameter is calculated:
OD = ID + (2 × CS)
The cross section is counted twice because the ring's wall sits on both sides of the bore. This sounds obvious written down, and it is still the single most common source of ordering errors we see, usually because someone measured an outer diameter with a caliper and ordered it as an inner diameter.
A Worked Example
Take an AS568-214. The leading 2 puts it in the 200 series, so the cross section is 0.139 inch (3.53 mm). The 14 places it in the size progression, giving an inner diameter of 0.984 inch (24.99 mm). Applying the formula:
OD = 0.984 + (2 × 0.139) = 1.262 inch, or 32.05 mm.
Watch the nominal-versus-actual trap. The 200 series is often described as the "one eighth inch" cross section and a -214 as a "one inch ID" ring. Both are round-number shorthand. The real figures are 0.139 inch and 0.984 inch. If you design a groove to the nominal numbers rather than the actual ones, you will end up with the wrong squeeze.
AS568 Quick Reference Size Chart
A sample spanning all five main series, in both inch and metric, to show how the progression works. The full standard runs to several hundred sizes.
| Dash No. | Series | ID (in) | ID (mm) | CS (in) | CS (mm) | OD (in) | OD (mm) |
|---|---|---|---|---|---|---|---|
| -004 | 000 | 0.070 | 1.78 | 0.070 | 1.78 | 0.210 | 5.33 |
| -010 | 000 | 0.239 | 6.07 | 0.070 | 1.78 | 0.379 | 9.63 |
| -020 | 000 | 0.864 | 21.95 | 0.070 | 1.78 | 1.004 | 25.50 |
| -110 | 100 | 0.362 | 9.19 | 0.103 | 2.62 | 0.568 | 14.43 |
| -120 | 100 | 0.987 | 25.07 | 0.103 | 2.62 | 1.193 | 30.30 |
| -132 | 100 | 1.737 | 44.12 | 0.103 | 2.62 | 1.943 | 49.35 |
| -210 | 200 | 0.734 | 18.64 | 0.139 | 3.53 | 1.012 | 25.70 |
| -214 | 200 | 0.984 | 24.99 | 0.139 | 3.53 | 1.262 | 32.05 |
| -222 | 200 | 1.484 | 37.69 | 0.139 | 3.53 | 1.762 | 44.75 |
| -309 | 300 | 0.412 | 10.46 | 0.210 | 5.33 | 0.832 | 21.13 |
| -320 | 300 | 1.100 | 27.94 | 0.210 | 5.33 | 1.520 | 38.61 |
| -425 | 400 | 4.475 | 113.66 | 0.275 | 6.99 | 5.025 | 127.63 |
| -430 | 400 | 5.100 | 129.54 | 0.275 | 6.99 | 5.650 | 143.51 |
How to Measure an O-Ring
If you have a ring in hand and no part number, you can identify it with a vernier or digital caliper in about a minute. Two measurements are all you need.
Measuring the Inner Diameter
Lay the ring flat on a clean surface without stretching it. Open the caliper's inside jaws across the hole and take the reading at the widest point, rotating the ring a quarter turn and re-measuring to confirm. An O-ring is flexible enough that a light pull adds several hundredths of an inch, which is more than enough to land you on the wrong dash number.
Measuring the Cross Section
Close the outside jaws gently on the ring's wall until they just make contact, without compressing the rubber. Since every AS568 series has one fixed cross section, this measurement alone tells you which series you are in, which usually narrows the candidates to a handful.
Then Match It
With the cross section giving you the series and the inner diameter giving you the position within it, look up the dash number in a full AS568 chart. Calculate the outer diameter from the formula above as a cross-check; if it does not match what you measured across the outside, one of your two readings is off.
Never trust a used O-ring as a dimensional reference. A ring that has been in service has been compressed in a groove, often for years, and may have taken a permanent set. If it has been sitting in oil, fuel or solvent, it may also have swollen well beyond its original size. Measuring a failed seal and ordering to those numbers reproduces the deformation, not the specification. Where the equipment drawing or the original part number is available, use that. Where it is not, measure the groove instead of the ring.
When the Size Falls Between Two Dash Numbers
Sometimes a measured ring sits neatly between two standard sizes. That usually means one of three things: it is a metric ring rather than an AS568 one, it is a non-standard size specific to that OEM, or it is a used ring that has deformed. Check the metric standards first, since that is the most common answer. If it is genuinely non-standard, it can be tooled as a custom size.
Choosing the Right O-Ring Material
Getting the size right only solves half the problem. The compound determines whether the seal survives the fluid, the temperature and the environment it is sitting in. Rather than working through a properties table, it is usually faster to start from the application.
Hydraulic Oil, Fuel and General Mineral Oil Systems
NBR, also called nitrile or Buna-N, is the default and covers the large majority of industrial sealing. It handles mineral oils, hydraulic fluids and greases well, holds up mechanically, and is the most cost-effective compound in the range. Its limits are heat, weathering and ozone. It is the right first choice for a hydraulic cylinder, a gearbox cover or a fuel line, and the wrong choice for anything mounted outdoors.
Outdoor Exposure, Water, Steam and Brake Fluid
EPDM is the answer wherever NBR fails on weathering. It resists ozone, UV, ageing and hot water or steam, which makes it standard for outdoor equipment, water handling, and brake systems. The critical warning: EPDM has very poor resistance to mineral oils and petroleum fluids. Swapping an NBR ring for an EPDM one because it was the compound on the shelf is a common and fast failure.
High Temperature and Aggressive Chemicals
FKM, widely known by the Viton trade name, is the compound for sustained high heat and chemical exposure. It handles temperatures well above NBR's ceiling and resists fuels, solvents and aggressive media that would degrade a nitrile ring in service. It costs considerably more, so it is specified where the duty genuinely requires it rather than as a blanket upgrade.
Food Contact, Pharmaceutical and Medical
Silicone is used where the seal contacts food, beverages or pharmaceutical product, and where a very wide temperature range matters more than mechanical strength. It is physiologically inert and holds its flexibility at both temperature extremes. The trade-off is poor tear strength and abrasion resistance, which is why it is a static-sealing material and rarely the right answer for a dynamic application.
Those four cover most requirements, but we also compound in NR, SBR, CR (Neoprene), HNBR, ACM, CSM and others depending on the duty. The full material comparison table on our O-Ring page sets out hardness ranges, high and low temperature limits and steam resistance for all ten compounds side by side, which is the fastest way to confirm a choice once you have narrowed it down.
Static vs Dynamic O-Ring Applications
How the seal moves in service changes what you should specify, sometimes more than the fluid does.
A static seal sits between two parts with no relative motion. A flange face, a cover plate, an end cap, a threaded port. The ring is compressed once on assembly and stays there. Static sealing is forgiving: surface finish matters less, and softer or lower-strength compounds like silicone work fine.
A dynamic seal has surfaces moving against it, either reciprocating like a hydraulic cylinder rod or rotating like a shaft. The ring is being worn every cycle. That demands better abrasion and tear resistance, tighter control of surface finish on the mating part, and closer attention to squeeze, since too much accelerates wear and too little leaks. Compounds that are perfectly adequate statically fail quickly in dynamic duty.
We supply both standard and custom O-rings for static and dynamic service. For rotating shaft applications specifically, an O-ring is often not the right component at all, and a lipped rubber oil seal will outlast it considerably. If you are sealing a flat joint rather than a groove, a rubber gasket is the better fit, and our guide to rubber gaskets vs rubber seals covers where each one belongs.
O-rings, rubber seals and gaskets are frequently specified together on the same assembly, particularly in automotive and hydraulic applications, where a single machine may use all three across different subsystems.