Size identification

What size is my O-ring? Two measurements are enough

Enter any two of inside diameter, cross-section and outside diameter. We work out the third, check that they agree, find the closest standard size in four systems and show the grades we stock.

Your measurements

Loading size references…

Where to measure an O-ring: ID across the hole, CS across the cord, the outer edge as an optional check Front view of an O-ring. The ID arrow runs from inner edge to inner edge across the hole. The cross-section view shows the two cut cords; CS is the thickness of one cord. A dotted outer-edge line is an optional check: inside diameter + 2 × cross section. FRONT VIEW ID inside diameter Outer check · optionalCROSS-SECTION CS CS = thickness of the cord ID Outer check = ID + 2 × CS
ID is the hole, CS is the cord thickness. The outer edge is only used to check the other two.

What you get

  • The third dimension, worked out from any two, plus a consistency check.
  • The closest standard size in AS568, metric, JIS B 2401 and BS 1806.
  • The grades we list at that size, with packs and prices.

No ring to hand? See an example: 24.99 × 3.53 mm

How to measure

Four steps to a reliable size

Two vernier calipers and several black and red O-rings laid flat on a technical drawing, ready to be measured
Lay the ring flat and relaxed. Close the jaws gently: rubber compresses under pressure.
  1. Clean and lay it flatWipe off oil and grease. Do not stretch the ring while you handle it.
  2. Cross-section firstClose a caliper gently on the wall at several points and record each reading. Do not squeeze the rubber. If readings differ materially, inspect wear or deformation before choosing an original size.
  3. Then the inside diameterUse the inside jaws of a caliper on a relaxed ring. A sizing cone gives a nearby standard-size candidate, not an actual diameter measurement. For an intact circular ring, outside circumference gives an estimate: ID ≈ circumference ÷ π − 2 × CS.
  4. Check with the ODIf you can measure the outside too, enter it: ID + 2 × CS should equal the OD. If it does not, measure again.

Size references are compiled from public standards, not an official validation. Nominal dimensions only. A nearby size is not an equivalent.

Used rings, units, custom sizes

Measuring questions

My old ring is flattened. Which size do I order?

Flattening changes a used ring's cross-section. Keep its readings as reference and compare them with the groove, mating hardware, a confirmed drawing or a known-good sample. A nearby or larger standard size is a candidate, not proof of the original size.

Why do I only need two numbers?

An O-ring is fully described by its inside diameter and cross-section. The outside diameter is always ID + 2 × CS, so any two values give the third.

What if no standard size matches?

We make custom sizes to order, molded or spliced. Send the measurements and a photo of the ring next to a ruler, and we reply with the options.

Inches or millimetres?

Either. Switch the unit before you type. Results are shown in both, and AS568 sizes are inch-based.

Used or damaged rings

What can this sample tell you?

A ring taken out of service is evidence, not a specification. Find its condition below to see which readings you can use and what to check before you order.

  • Measureuse the reading
  • Estimatecalculate, then confirm
  • Reference onlycompare, do not order from it alone
  • Not usableuse another source
Which readings a used O-ring can give, by condition
Ring conditionInside diameter (ID)Cross-section (CS)Check next
Intact and relaxedReadings repeat when you measure again Measure Measure Find the size in the chart and check its tolerance. If readings disagree, or the ring will not lie relaxed, treat them as reference.
FlattenedFlat faces where it sat in the groove (compression set) Reference only Reference only Groove dimensions, mating hardware or the drawing. A flattened cross-section is not the original one.
Worn on one side Reference onlyseveral points Reference onlyunworn section only Measure an unworn section, then confirm with the groove or drawing. If wear reaches every section, or the ring is oval, use another source.
Cut or broken, all pieces kept Estimatefrom the total length Measureintact section, several points Match the estimate to the size chart, then confirm with the groove, drawing or a good ring. Not valid if the pieces are stretched or flattened.
Broken, pieces missing Not usable Reference onlyif a section is intact Drawing, groove or a good ring. The original length is unknown, so the diameter cannot be rebuilt.
Swollen or softened Not usable Not usable Note the fluid it ran in, then size from the groove, drawing or a good ring. The fluid has changed the ring's volume.
Stretched or out of round Reference only Reference onlyseveral points Groove or mating hardware, then the size chart. If the diameter changes with direction, the ring cannot give its original size.
Hardened, brittle or cracked Not usable Reference only Drawing, groove or a good ring. There is no reliable way to correct a hardened ring back to its original size.
Twisted or spiral damage Not usable Reference onlyundamaged section only Groove or drawing. The ring cannot relax into a stable round shape.
Standard-size identification cone/tape kit result only Not measured Not measured Treat a standard-size identification kit result as a nearest standard candidate, then verify ID and CS or check the drawing. This limitation does not apply to every precision circumference tape: use its stated measurement capability and accuracy.

Stop and use another source when…

  • A reading is marked “Not usable” aboveMissing pieces, no intact section, flattening on two sides, swelling, severe stretch, hardening or twisting: size from the drawing, groove or a good ring instead.
  • Your readings disagreeDo not average them into one size. Keep every reading and ask for a review: the ring may be oval, worn or deformed.
  • The ring has shrunk or dried outIt may now be smaller than new. Use another source and note what it was exposed to.
  • You only have a photoA photo shows the condition, not the size. Send the ring, the drawing or the groove dimensions.
  • The nearest size is only “close”There is no general close-enough rule. Check the tolerance for that size, or ask for a review.

What each source can prove

Drawing or confirmed equipment spec
Good for: nominal size, profile and compound, when the revision matches your equipment.
Not enough for: proving what is installed now.
Groove and mating hardware
Good for: cross-checking a deformed ring and sizing the replacement.
Not enough for: ordering directly. Groove dimensions are not ring dimensions.
Intact, relaxed ring
Good for: direct ID and CS readings when repeat readings agree.
Not enough for: material, hardness or suitability.
Damaged used ring
Good for: showing the condition and narrowing the candidates.
Not enough for: the original size after set, swell, stretch, wear, twist or hardening.
Standard-size identification cone/tape kit
Good for: a nearest standard size candidate within the kit scope.
Not enough for: actual dimensions or a pass/fail check. Precision circumference tapes are a different tool category; their capability and accuracy must be checked separately.
Photo
Good for: recording the damage.
Not enough for: an exact size or a single root cause.

Not sure what your sample supports? Send the readings with a photo, the drawing or the groove dimensions, and we reply with the candidate sizes.

Ask for a size check Size from the groove