O-ring groove design to ISO 3601-2 — squeeze and gland fill

An O-ring is only half of the seal — the other half is the groove you seat it in. The most expensive FKM ring will leak if the groove was designed by eye, while a plain NBR ring holds reliably for years when it sits in a groove cut to the standard. ISO 3601-2 defines groove dimensions as ratios of the ring cross-section (CS), so the same rules apply to a small ring with a 1 mm cross-section and to a large hydraulic seal alike. In this article you will find the recommended table values, how to calculate squeeze and gland fill, and the most common design mistakes we see in warranty claims.

The two numbers that decide everything: squeeze and gland fill

Rubber is practically incompressible — it changes shape, not volume. Two basic groove design rules follow from that. First, the ring must be compressed in the groove so that contact pressure builds up on the sealing faces: this is the squeeze. Second, the groove must be large enough for the rubber to have somewhere to go: this is the gland fill. Most seal failures that are not material-related can be traced back to one of these two numbers.

Squeeze: how many percent for static and how many for dynamic seals

Squeeze is calculated from the ring cross-section CS and the groove depth: squeeze in % = ((CS − groove depth) / CS) × 100. For a static seal (no relative motion between the faces — a cover, a flange, a fixed joint) ISO 3601-2 recommends a groove depth of roughly 0.78 × CS, which gives a squeeze of about 22 %. For a dynamic seal (reciprocating piston, piston rod, slow rotation) the recommended depth is roughly 0.90 × CS, i.e. a squeeze of about 10 %.

Why does dynamic get less? Every percent of squeeze increases contact pressure and with it friction. In a static application friction does not matter, so you can afford a firmer grip and a bigger sealing margin. In motion, however, friction means heat and wear — so the squeeze is dialled down to a level that still seals safely but does not brake the ring or overheat it unnecessarily.

Table ratios per ISO 3601-2

The standard expresses groove dimensions as multiples of the cross-section CS. Recommended values for a radial seal:

• Static groove: width ≈ 1.30 × CS, depth ≈ 0.78 × CS (squeeze ≈ 22 %), bottom corner radius ≈ 0.10 × CS, edge chamfer 15–20°.
• Dynamic groove: width ≈ 1.50 × CS, depth ≈ 0.90 × CS (squeeze ≈ 10 %), bottom corner radius ≈ 0.10 × CS, edge chamfer 15–20°.

An example for the common cross-section CS = 3.53 mm: the static groove works out 4.59 mm wide and 2.75 mm deep, the dynamic one 5.30 mm wide and 3.18 mm deep, with a 0.35 mm corner radius in both cases. The ratios are valid for cross-sections from 1.0 to 10.0 mm — that covers practically the entire O-ring catalogue. Larger cross-sections (big hydraulic seals run up to 12 mm) are no longer covered by the tables of the standard and have to be designed individually.

Gland fill: why never 100 %

Gland fill is the ratio of the ring cross-section area to the groove cross-section area. Because rubber does not change volume, it needs free space in the groove for three things: thermal expansion (elastomers expand considerably more than steel when heated), swell in the medium, and the deformation from the squeeze itself — a compressed ring spreads sideways.

The recommended ratios of the standard keep the fill in the safe band automatically. The ring cross-section area is π/4 × CS², roughly 0.785 × CS². The static groove has a cross-section of 1.30 × 0.78 ≈ 1.014 × CS², so the fill comes out at about 77 %. The dynamic groove, 1.50 × 0.90 = 1.35 × CS², gives a fill of about 58 %. In other words: the standard leaves 30 to 40 % of the groove volume free — and that is not spare margin to give away, it is the working space of the rubber.

What happens at 100 % fill? The ring warms up or swells slightly, has nowhere to expand, and the pressure in the groove spikes. The rubber finds the only escape route — the gap between the parts — and gets forced into it (extrusion). The result looks like a nibbled ring edge and the seal is destroyed. If you change the material or the medium in an existing application, check the swell in the chemical resistance chart — a groove designed for the original material may not be enough for the new one.

Radial or axial groove?

A radial groove compresses the ring across its cross-section between two cylindrical surfaces — typically a piston in a bore or a shaft in a housing. Static radial seals and all dynamic applications belong here. An axial (face) groove compresses the ring perpendicularly — between the groove bottom and a bolted cover or flange. A face seal uses very similar ratios to a static radial one, so the table above serves here too.

With a face groove, also watch the pressure direction: position the groove so that the medium pressure pushes the ring against the groove wall, not away from it. And under continuous fast shaft rotation an O-ring wears out quickly — there the right answer is a radial shaft seal (Simmerring), not a deeper groove.

Groove inner diameter and ring stretch

Besides width and depth you also have to choose the seat diameter. The safe starting point is a free fit: groove inner diameter equal to the ring inner diameter, i.e. 0 % stretch. A static seal tolerates 1 to 5 % stretch — the ring seats nicely and does not fall out during assembly. Demanding dynamic seals, on the contrary, are deliberately kept at exactly 0 %, because stretch reduces the cross-section and with it the actual squeeze.

The most common groove design mistakes

• Groove too deep (too little squeeze): the ring loses contact pressure and the seal leaks — typically at low pressure or in the cold, when the rubber stiffens.
• Groove too shallow (too much squeeze): in dynamic service high friction, heat and rapid wear; in static service needlessly high compression set and difficult assembly.
• Overfilled gland: fill close to 100 % leaves no room for temperature and swell — it ends in rubber extrusion into the gap.
• Sharp groove bottom corners: a missing radius (recommendation ≈ 0.10 × CS) creates a notch that gradually cuts into the ring over pressure cycles.
• Missing 15–20° chamfer on the lead-in edges: the ring gets cut or nicked on a sharp edge during assembly — a failure that looks like a material defect but is a drawing error.
• Swapped tables: a static groove with dynamic ratios seals worse, a dynamic groove with the static 22 % squeeze overheats and wears out fast.

Frequently asked questions

How much squeeze should a static O-ring have?

About 22 % of the cross-section. You get it with a groove depth of 0.78 × CS per ISO 3601-2. Less means a leak risk; considerably more means needless compression set and difficult assembly.

Can the gland be filled to 100 %?

No. Rubber is incompressible and needs free volume when it heats up or swells. The recommended ratios of the standard leave roughly 30 to 40 % of the groove free; fill close to 100 % leads to extrusion and a destroyed seal.

Do these ratios apply to every ring size?

The tables cover cross-sections from 1.0 to 10.0 mm, which corresponds to the vast majority of catalogue O-rings. Outside that band (extremely thin cords, hydraulic cross-sections above 10 mm) treat the ratios only as guidance and have the design verified.

Calculate your groove in a minute

No need to multiply by hand: the groove calculator on oringy.sk computes the width, depth, radius and resulting squeeze for both static and dynamic applications — just enter the inner diameter (2 to 500 mm) and the ring cross-section. Identify an unknown ring with the size converter, verify the right material for your medium in the chemical chart, and if the seal has already failed, walk through the failure analysis — a wrong groove is one of the most frequent causes. The configurator will then find an in-stock ring straight from your dimensions. And if you are still not sure even with the result, send Filip the groove drawing — we will review it.