O-Ring Sizer with AS568 Chart

A free tool for sizing a sealing ring from its two defining numbers: the inner diameter (ID) of the hole and the cross-section (CS), the thickness of the cord. The inch-based AS568 standard pins every ring to one of a handful of fixed cross-sections — 1.78, 2.62, 3.53, 5.33 and 6.99 mm — and gives it a dash number, while metric rings quote ID × CS directly. Lay the ring flat over the calibrated circles, read the bore and the cord, and the tool names the dash or metric size.

Sizing an O-ring step by step

1
Lay the ring flat

Rest the relaxed, unstretched ring on the circles; never measure it stretched over a groove or shaft, which inflates the diameter.

2
Read the inner diameter

Find the printed circle whose edge sits just inside the ring's hole — that is the ID.

3
Measure the cord

Lay a section of the ring across the cross-section scale and read the cord thickness at its widest.

4
Match the standard

The tool snaps the cord to the nearest AS568 cross-section, pairs it with the ID, and offers the dash number or the metric ID × CS call-out.

Pin an ID × CS to its AS568 dash or metric size

What this gauge does

ID and CS together
Reads the inner diameter from the circle it sits over and the cross-section from the cord scale, because a seal is defined by both, not by outer diameter alone.
AS568 dash lookup
Snaps the cross-section to the nearest standard cord and pairs it with the ID to suggest the AS568 dash number printed in seal catalogues.
Metric and inch families
Tells an inch AS568 ring (fixed CS series) from a metric ring quoted as ID × CS, so you order from the right table.
Relaxed-ring rule
Measures the ring lying flat and unstretched, the only state that gives its true free dimensions, and warns against sizing a ring still on a shaft.
Cord wear awareness
Flags that a swollen or flattened cord reads oversize or undersize, so a perished seal is matched to a fresh standard size, not its worn shape.

Why two numbers beat one for a seal

Order a replacement O-ring off a single diameter and there is a good chance it will not seal — the cord could be too thin to fill the groove or too fat to seat. A seal lives or dies on the ID-and-cross-section pair, and the standards encode exactly that. This tool reads both from a relaxed ring without a caliper and resolves them to an AS568 dash number or a metric ID × CS, so you reorder the seal that actually fits the groove rather than one that merely looks the same size.

O-ring sizing questions

Why is outer diameter not enough to size an O-ring?

Because outer diameter is just inner diameter plus twice the cord, so it hides the two figures that matter. Two rings can share an outer diameter yet have a different ID and cord, and only the ID-and-cross-section pair tells you which groove a ring will seal.

What is an AS568 dash number?

AS568 is the standard inch O-ring sizing system. Each size has a dash number (for example -012 or -210) that encodes a fixed inner diameter and one of the standard cross-sections. Naming the dash number lets you reorder the exact ring from any supplier.

What are the standard cross-sections?

AS568 uses a short list of cord thicknesses — about 1.78, 2.62, 3.53, 5.33 and 6.99 mm. Your ring's cord will be close to one of these; the tool snaps to the nearest, which is why a slightly worn cord still resolves to a real size.

How do metric O-rings differ?

Metric rings are quoted directly as inner diameter by cross-section, such as 20 × 2.5 mm, rather than a dash number. They use their own cross-section steps, so the tool tells you whether your ring fits the inch AS568 series or a metric table.

Why must I measure the ring unstretched?

O-rings are elastic and are fitted with a slight stretch. Measuring one still on a shaft, or pulling it onto the gauge, reads a larger inner diameter than its free size. Let it relax flat first so the ID and cord match the catalogue dimensions.

My old ring is swollen and flat — can I still match it?

Often yes. A perished seal swells or takes a set, so read both numbers, then trust the nearest standard rather than the distorted shape. The cross-section in particular snaps cleanly to a standard cord, which usually fixes the match.

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