How do I measure an O-ring size?

Measure the inside diameter and the cross-section thickness with calipers. The outside diameter equals the ID plus twice the cross-section. Never size from a stretched, worn ring.

How much should an O-ring compress in its groove?

For static seals, target roughly 20 to 30 percent squeeze: groove depth of about 75 percent of the cross-section, and groove width of about 135 percent so the rubber has room to displace.

Should an O-ring stretch over its groove?

Only slightly. The groove inside diameter should equal the O-ring ID with at most 1 to 2 percent stretch. Over-stretching thins the cross-section and weakens the seal.

O-Ring Size Calculator — ID, Cross-Section, OD & Groove Dimensions

O-Ring Size Calculator

Enter your O-ring's inside diameter and cross-section — get the outside diameter and recommended static groove dimensions (25% squeeze).

Inside diameter (ID), inches:

Cross-section (thickness):

Measuring a worn ring? Use calipers on ID and thickness — not the stretched old ring. Compound questions: rubber compatibility chart.

Use the calculator above to size an O-ring from its two defining measurements. The math: OD = ID + (2 x cross-section), and for static seals the groove should compress the ring roughly 20-30% — groove depth about 75% of the cross-section, groove width about 135% so the displaced rubber has room.

Standard Cross-Sections

Cross-section	Decimal	Typical use
1/16"	0.0625"	Small fittings, faucets, instruments
3/32"	0.103"	Plumbing, small pumps
1/8"	0.139"	General industrial, filters
3/16"	0.210"	Larger housings, machinery
1/4"	0.275"	Heavy equipment, large flanges

Measuring a Worn O-Ring

Old rings stretch and flatten — never size by eye from a worn ring. Measure the ID and thickness with calipers, or measure the groove it came from: groove ID = ring ID, and cross-section ≈ groove depth ÷ 0.75. Between sizes? Round the ID down (slight stretch is fine); never round the cross-section down (lost squeeze = leak).

Shop rubber O-rings — and if the seal lives in oil, sun, or heat, check compound choice on the rubber compatibility chart. Flat sealing under a bolt head instead? That's a rubber washer.