Compression Squeeze, Compression Ratio, and Gland Fill — The Three Numbers That Decide Whether Your O-Ring Seals

We have laid the groundwork:

• Part 1 introduced the three gland types.
• Part 2 sized the hardware (H, W).
• Part 3 picked the O-ring ID/OD to fit the hardware.

Now it is time to bring those three pieces together and answer the only question that matters: is this O-ring actually going to seal? The answer sits in three named quantities — compression squeeze, compression ratio, and gland fill — each of which captures a different aspect of the same physical picture.

1. Compression Squeeze — Absolute Squish

Compression squeeze is the simplest measure: how much shorter is the groove than the O-ring's cross-section?

Compression Squeeze = CS − H

Units: keep whichever one your hardware drawing uses — inches or millimetres.

There is one hard rule:

Recommended minimum. Compression squeeze must be greater than 0.005 in (0.13 mm).

Below that, manufacturing tolerances and surface finish can leave you with zero actual contact pressure, and the seal will leak.

There is also a soft upper bound — but you do not enforce it through squeeze directly. Instead, the ratio of squeeze to CS gives you a cleaner signal — which is what compression ratio is for.

2. Cross-Section Reduction From ID Stretch

Before you compute the compression ratio, there is a correction you must apply for piston-type seals and external-pressure face seals: stretching the O-ring's ID makes its cross-section shrink slightly. Elastomers are essentially incompressible (see Part 1), so what gets pulled out of the ID has to come out of the CS too.

Use these reduced cross-section numbers in all the squeeze and gland-fill calculations that follow. The OD-interference reduction is much smaller and is generally not accounted for separately.

Reduced Cross-Section at % ID Stretch — Inches

Reduced Cross-Section at % ID Stretch — Millimetres

Tip. Most static designs live in the 1–3 % stretch band. Read down the row once you know which AS568 series you are using.

3. Compression Ratio — Squish as a Percentage

Compression ratio normalises squeeze against the O-ring's free cross-section, which gives you a geometry-independent way to compare designs.

Compression Ratio (%) = (Compression Squeeze / CS) × 100

Face seals are designed deeper into the ratio range for two reasons: they often seal with less hardware stiffness around them, and any tiny leak path is across the face, so more squeeze pays off.

Quick Reference: Targets in Real Units

For a -2XX O-ring (CS = 0.139 in / 3.53 mm), the 20 % target squeeze for a piston or rod seal works out to:

• 0.139 in × 20 % ≈ 0.028 in
• 3.53 mm × 20 % ≈ 0.71 mm

If your calculated squeeze is below this, consider deepening the gland (smaller H). If it is above, consider widening the gland (larger H) — but stay above the 0.005 in / 0.13 mm absolute minimum.

4. Gland Fill — Percentage of the Groove Occupied by the O-Ring

The third metric looks at the O-ring sideways — at the cross-sectional area it occupies inside the groove.

O-Ring CSA   = π × (CS / 2)²
Gland CSA    = H × W
Gland Fill % = (O-Ring CSA / Gland CSA) × 100

Note that the effect of gland angle is not addressed in this formula. If your groove is dovetail-shaped (angle > 0°), treat the predicted fill as approximate and add a margin.

Target Gland Fill

A gland fill as low as 50 % and as high as 90 % is acceptable in special cases, but the target window above is the design center.

Why Gland Fill Matters

Gland fill is the metric that captures the things compression ratio cannot:

• Thermal expansion. The elastomer wants to swell when the system gets hot. If the groove is already 85 % full, even modest thermal expansion will load the ring against the hardware.
• Fluid swell. Some chemicals cause the elastomer to absorb fluid and grow. Same logic.
• Volume swell stack-up. O-ring tolerance stack combined with hardware tolerance stack can land at the worst-case end of the band.
• Compression set relief. Over years of service, an O-ring that was 30 % squeezed may retain much of the deformation but lose recovery — the gland gives the ring somewhere to relax into without losing seal.

If gland fill is below 50 %, there is too much empty space around the ring — it can pump, walk, or extrude when pulsed. If it is above 90 %, there is no room for any of the swell mechanisms to play out, and the ring gets overstressed.

Bringing the Three Numbers Together

A practical workflow:

1. Pick the AS568 dash number, then read the original CS from the cross-section table.
2. Compute stretch (Part 3) and use the table above to get the reduced CS.
3. From H and W, compute squeeze (CS − H) — confirm > 0.005 in / 0.13 mm.
4. Compute compression ratio — confirm inside 5–30 % (piston/rod) or 10–35 % (face).
5. Compute gland fill — confirm 65–85 % target.
6. Run the tolerance stack-up (worst-case largest O-ring in smallest groove, smallest O-ring in largest groove). Both extremes must still satisfy the minimums.

When all three numbers are inside their windows and the tolerance stack still passes, you have a robust static seal design.

What's Next

So far we have sized the ring and the groove around static hardware tolerance. The next post zooms in on a different failure mode: extrusion — when system pressure pushes the ring into the clearance gap between the rod and the bore, typically the failure mode of choice for higher-pressure or softer-compound applications.

Continue with Part 5 — Extrusion Gap and Other Groove Details.

Series Navigation

1. What Is an O-Ring? A Practical Introduction to Static Sealing
2. Gland Dimension Calculations
3. ID Stretch and OD Interference
4. Compression Squeeze, Compression Ratio, and Gland Fill
5. Extrusion Gap and Other Groove Details
6. Elastomer Materials for O-Rings
7. Six Sigma Quality and Process Capability