How to Size O Rings

O rings are used to create a secure seal between two surfaces in either static or dynamic applications. Because the purpose of an O ring is to create an impenetrable seal, getting the size exactly right is essential. If it is too large, fluids, dirt, grit and gasses can flow around it. If it is too small, overstretching it to get it into place will compromise the seal’s integrity and it will fail prematurely. Knowing how to size O rings is important to avoid seal failures and the waste of time, money and productivity. Fortunately, learning how to size O rings is not overly complicated. It does, however, demand absolute precision. The old adage to measure twice and cut once definitely applies to O rings. Callipers and micrometres are the best tools to measure a small O ring. An O sizer or Pi-Tape will work better for larger ones. Sizing cones are designed so you can slip an O ring onto the cone and see at a glance what standard size it is, but this is not always the most precise measure because of the potential to stretch the O ring as you place it on the cone. You can size an O ring in metric or imperial and, if ever in doubt, you can always count on us and our O-ring size chart that includes both measurements.