O-Ring Groove Calculator
Free O-Ring groove calculator. Pick a gland type and an AS568 dash number and get groove depth, width, corner radius, lead-in chamfer, squeeze, and gland fill.
Pick the kind of gland you are cutting and the O-Ring going in it, and this gives you the groove depth, width, corner radius, and lead-in chamfer, plus the squeeze and gland fill those numbers produce. Enter your bore or rod diameter and it works out the groove diameter and how much the ring stretches to get there. The Copy link to this gland button puts everything in the address so you can send it straight to whoever runs the machine.
Seal and size
A face seal squeezes along the bolt axis. A dovetail is a face groove cut narrower at the mouth so it keeps hold of the ring when the joint opens. A piston groove rides on the outside of a piston and seals to the bore. A rod groove is cut in the housing and seals against a rod.
Give it the hardware diameter and it works out the groove diameter, plus the stretch or compression on the ring.
Groove
Guideline values from the Freudenberg FST 2025 Technical Manual, ch. 6 (Static Seals), §8.2. They are a sound starting point, not a substitute for a tolerance study on your own parts. Confirm against the controlling drawing before you cut metal, and ask us if the duty is unusual.
Which gland type do I pick?
| Gland | Where the groove is | Typical job |
|---|---|---|
| Face seal (axial) | Cut into a flange face, squeezed as the bolts pull down | Covers, housings, port plates |
| Piston groove (radial) | Around the outside of a piston, sealing to the bore | Hydraulic pistons, plugs |
| Rod groove (radial) | In the housing bore, sealing against a rod | Rod glands, shafts, spools |
| Pneumatic variants | Same as above, air service | Air cylinders, valves |
| Dovetail (axial) | A face groove cut narrower at the mouth than at the base | Lids, tooling, upside-down joints |
Freudenberg publishes the same guideline dimensions for static and dynamic hydraulic duty, so one entry covers both. Pneumatic runs slightly less squeeze to keep friction and stick-slip down, so it gets its own entry.
About the dovetail
A dovetail is a face groove with the walls cut at an angle so the opening is narrower than the bottom, about 24 degrees off vertical. That undercut grips the ring and holds it in the groove when the joint comes apart, which is the whole point. Use it on a lid that gets opened often, on tooling, or anywhere the groove faces down and a ring in a square groove would simply fall out.
What you give up is machining time. A dovetail is harder to cut than a square groove and the width has to be measured at the mouth, before deburring, or the number means nothing. The mouth radius matters too: too sharp and it nicks the ring going in.
Because the section is a trapezoid rather than a rectangle, the calculator works the gland fill from the trapezoid area. Run the numbers on a square groove and you get 112 to 142 percent, which is meaningless. Dovetails are cut tight on purpose, so expect fill in the high 70s to high 80s, and on a 0.070 cord expect it to come out essentially full.
Dovetail dimensions are published for the five preferred AS568 cross-sections only (0.070, 0.103, 0.139, 0.210, 0.275). Pick anything else and the calculator says so.
The four numbers that matter
Squeeze is how much the ring is flattened, given as a percentage of the cross-section. It makes first contact and holds the seal before pressure ever arrives. Face grooves run the most squeeze, around 30 percent, and pneumatic the least.
Gland fill is how much of the groove the ring takes up. Aim for 60 to 85 percent and never go past about 90. Rubber is nearly incompressible, so a squeezed ring spreads sideways and grows when it heats or swells. Overfill the groove and it has nowhere to go but into the clearance gap, which is extrusion.
Stretch applies to piston grooves, where the ring is pulled over the piston to get in. Keep it under 15 percent. Past that the ring thins out, the cross-section drops, and you lose the squeeze you thought you designed in.
Clearance gap is the space at the low-pressure side where extrusion starts. It is set by your fits, not by the groove, so it is not calculated here. As a rule of thumb a standard 70 durometer ring starts to extrude around 1,500 psi with a typical gap. Tighten the gap, go to 90 durometer, or add a back-up ring to buy headroom.
Before you cut metal
These are guideline values, and good ones, but they assume nominal parts. Tolerances stack up. Freudenberg’s own advice is to run a tolerance study on the installation space and the O-Ring together so that neither leakage nor damage to the ring can happen at the extremes, and that is worth doing on anything that matters. Thermal expansion moves squeeze too: it falls as things get cold and rises as they get hot.
If the duty is unusual, high pressure, hard vacuum, wide temperature swing, or a fluid that swells the compound, give us a call. Getting the groove right the first time is cheaper than machining it twice.
For the background behind these numbers, see Groove & Gland Design and Sealing Principles. For the ring itself, the Master O-Ring Size Chart has every dimension and the materials guide covers compound choice.
Groove dimensions for the face, piston, and rod glands: Freudenberg FST 2025 Technical Manual (Imperial), ch. 6 (Static Seals), §8.2 Guideline values for the design, Tab. 37 through Tab. 43. Dovetail dimensions come from the handbook’s own gland dimension charts. Squeeze, gland fill, and stretch are calculated from those dimensions.
