Sizing & Spec5 min read
O-Ring Size Chart: AS568, ISO 3601-1 and BS 1806 Explained
An O-ring is described by two numbers — inside diameter and cross-section — and by at least four numbering systems that express them differently. This guide shows how to read each one, how to convert between them, and how to measure a ring when you have the part but not the paperwork.
Published 2 September 2026 · Updated
How do you identify an O-ring size?
Four numbering systems, two of them current
| Standard | Units | Status | Where you meet it |
|---|---|---|---|
| AS568 | Inch | Current | North American drawings and most global hydraulics. Dash numbers −001 to −475, 369 sizes |
| ISO 3601-1 | Metric | Current | The international standard. Inside diameters, cross-sections, tolerances and designation codes. European and Asian machinery |
| BS 1806 | Inch | Superseded by BS ISO 3601 | Legacy UK drawings and spares lists. Covers the AS568 cross-section groups plus sizes between them |
| BS 4518 | Metric | Legacy British metric | The metric counterpart to BS 1806 — sizes given as metric inside diameter and cross-section |
| JIS B 2401 | Metric | Current, Japanese | Japanese-built equipment. Its own series, not interchangeable by number with ISO 3601 |
The two inch systems and the metric systems describe the same rings from different directions, and an AS568 size and an ISO 3601-1 size will often be the same physical ring expressed in different units. They are not interchangeable by number. Convert by dimension, never by assuming the codes line up.
Reading an AS568 dash number
An AS568 dash number is not arbitrary. The first digit group sets the cross-section; the rest counts up through inside diameters at that cross-section. So every −2xx ring is the same thickness of rubber and they differ only in how big the hole is. Once you know the series, you know the thickness without a chart.
| Series | Cross-section (in) | Cross-section (mm) | Example |
|---|---|---|---|
| −0xx | 0.070 | 1.778 | −012 |
| −1xx | 0.103 | 2.616 | −115 |
| −2xx | 0.139 | 3.531 | −214 |
| −3xx | 0.210 | 5.334 | −325 |
| −4xx | 0.275 | 6.985 | −432 |
The full series runs from −001 to −475, 369 sizes in all. Take −214: the 2 series fixes the cross-section at 0.139 in (3.531 mm), and 14 is its position in the inside-diameter sequence at that thickness. Half the identification is done before you look anything up.
ISO 3601: the standard current drawings cite
ISO 3601 comes in parts, and a drawing may reference more than one of them.
Part 1 — inside diameters, cross-sections, tolerances and designation codes. This is the size standard. It defines two tolerance classes: Class A, the tighter one, and Class B, the standard one. Class A size codes correspond to the AS568 dash numbers, which is the bridge between the inch and metric worlds and the single most useful fact on this page.
Part 2 — housing dimensions for general applications. The groove, not the ring. Squeeze, groove width and depth, and how full the groove is come from here.
Part 3 — quality acceptance criteria. Which surface imperfections are allowed and how large they may be.
A complete O-ring specification usually references all three: the size from Part 1, the groove from Part 2, the acceptance grade from Part 3. A drawing that gives only a size has specified a third of the part.
BS 1806 and BS 4518: what to do with a legacy drawing
BS 1806 is the British imperial series, and it was widely used before ISO 3601 replaced it. It covers the same cross-section groups as AS568 plus a number of sizes that fall between AS568 sizes — which is why an old British drawing occasionally calls for a ring that has no AS568 equivalent at all.
BS 4518 is its metric counterpart, giving sizes as a metric inside diameter and cross-section rather than as a dash number.
Both have been superseded by BS ISO 3601, and neither should be specified for new work. Both are still worth knowing, because spares lists outlive the drawings they came from and a machine built in 1985 is often still running. Send us the old number and we will identify the ring; where the size has no modern equivalent we will make it to the original dimensions.
How to measure an O-ring you already have
Most people arrive at a size chart holding a part, not a drawing. Measure the cross-section first — the thickness of the rubber — with a vernier or a micrometer, gently. Then measure the inside diameter, laid flat on a surface, without stretching it.
Three cautions. A ring that has been in service is not the size it was made. It may have swollen in the fluid, in which case it measures large; it may have taken a compression set, in which case the cross-section measures flat on two sides. Measure a spare if you have one, and if you do not, say that the sample is used.
Second: an O-ring has a circular cross-section. If the part in your hand is square, rectangular or has a lip on one side, it is not an O-ring — it is a quad ring, a square-section seal or a rotary shaft seal, and it is specified a different way.
Third: measure the groove too. The ring tells you what was fitted; the groove tells you what should have been.
The groove is half the specification
An O-ring does not seal because of its size. It seals because the groove squeezes it — the ring is compressed between the groove floor and the opposing face, and that compression is what closes the joint. Get the groove wrong and a perfectly correct ring will leak.
Three things have to be right. Squeeze — too little and it does not seal, too much and it takes a set or extrudes. Groove fill — the ring swells and expands with heat, and the groove must have room for it; a groove filled to capacity has nowhere to put that growth. Surface finish — a scored groove or a rough opposing face abrades the ring, particularly where it moves.
ISO 3601-2 gives housing dimensions for general applications, and the figures belong there rather than in a rule of thumb. Take the groove from the standard, or send us the application and we will work it with you — the numbers vary with static or dynamic duty, pressure and compound, and a single quoted percentage would be wrong more often than right.
Quality grades: what ISO 3601-3 allows
ISO 3601-3 classifies the surface imperfections an O-ring may carry and sets limits on them — flash, parting-line projection, back rind, flow marks, non-fills, indentations, blisters and cracks. It sorts them into grades: Grade N, the general-purpose standard of acceptance; Grade S, for optimum-performance industrial and aerospace applications, with tighter tolerances and fewer allowable imperfections; and Grade CS for the most critical service.
Most industrial work is Grade N, and specifying above it costs money for no benefit. Ask for Grade S when the consequence of a leak justifies it, not by default.
One exclusion, stated plainly. ISO 3601-3 is written to cover aerospace O-rings as well as industrial ones. We do not supply aerospace — that requires AS9100, which we do not hold. We hold ISO 9001:2015, certificate 09111526A, Integral Certification Ltd, valid to 21 February 2027, scope: rubber moulded components, and we are upgrading to IATF 16949, targeted by the end of 2026.
Five mistakes when ordering O-rings to a size chart
Measuring a used ring
A ring that has swollen measures large; one that has taken a set measures flat. Copying it dimensionally reproduces the failure. Measure a spare, or say the sample is used.
Cross-referencing by number
AS568 dash numbers and ISO 3601-1 codes do not map onto each other arithmetically. Convert by dimension.
Specifying BS 1806 for new work
It has been superseded by BS ISO 3601. Quote it only when you are matching an existing drawing.
Giving the size and nothing else
A size is one of three references a complete O-ring drawing carries. The groove and the acceptance grade are the other two.
Ordering a size without naming the fluid
The dimension identifies the ring; the compound decides whether it survives. The same −214 in nitrile and in EPDM are different parts.
Frequently asked questions
What do the numbers in an AS568 O-ring size mean?
The series sets the cross-section and the remaining digits set the inside diameter. A −2xx ring has a 0.139 in (3.531 mm) cross-section; a −3xx ring has 0.210 in (5.334 mm). The full range runs from −001 to −475, 369 sizes.
Is BS 1806 still valid?
It has been superseded by BS ISO 3601, so it should not be specified for new designs. It is still widely quoted on legacy UK drawings and spares lists, and it includes some sizes that fall between AS568 sizes, so it remains useful for cross-reference. Send us the old number and we will identify the ring.
How do I measure an O-ring?
Measure the cross-section — the thickness of the rubber — then the inside diameter, laid flat and unstretched. Use a spare rather than a used ring if you can: a ring that has swollen in service measures large, and one that has taken a compression set measures flat on two sides.
Are AS568 and ISO 3601 sizes interchangeable?
Often the same physical ring, but not interchangeable by number — the two systems count differently. ISO 3601-1 Class A size codes correspond to the AS568 dash numbers, which is the usable bridge between them. Convert by dimension, and check the tolerance class as well as the size.
Can you make an O-ring to a non-standard size?
Yes — we mould to your dimensions in any compound we mix, including obsolete British sizes with no modern equivalent. The minimum run is 10,000 pieces per part number, per compound. Prototype and first-article lots sit outside the minimum. We respond within 48 hours.
Send the two measurements, or the old part number, and we will identify the ring — and tell you the compound the application needs, which the size chart cannot. We respond within 48 hours.