05 Feb The Complete O-Ring Size Chart Guide
Selecting the right O-ring might seem straightforward until you’re staring at three different sizing charts. At I.B. Moore Manufacturing, we’ve watched engineers and purchasing managers wrestle with this problem more times than we can count. The confusion isn’t your fault: The industry uses three major sizing systems, and each one speaks a different language. Let’s break down the metric, AS568, and British standards, and help you avoid measurement mistakes that cause seal failures.
O-Ring Size Charts Matter for Your Design
O-rings create pressure-tight seals in everything from hydraulic systems to medical devices. When a seal measures even slightly off-spec, you’re looking at leaks, equipment damage, or complete system failure. Some industries work exclusively in metric measurements, while others swear by AS568 standards or British sizing conventions.
AS568 O-Ring Sizes
The AS568 standard was developed by the Aerospace Size Standard committee and remains North America’s go-to sizing system. Each O-ring is assigned a dash number that corresponds to specific inner-diameter and cross-section measurements. For example, the AS568-010 O-ring has an inner diameter of 0.239 inches and a cross-section of 0.070 inches.
The system runs from -001 through -475, though plenty of numbers in that range don’t actually exist. Smaller dash numbers generally mean smaller O-rings, so the logic makes sense once you’ve used it a few times. Most suppliers keep common sizes like -010, -011, -012, and -015 in stock, since these are used frequently in standard applications.
The Metric O-Ring Size Chart
European and Asian manufacturers typically work in metric units, measuring both the inner diameter and the cross-section in millimeters. Unlike AS568, there are no dash numbers here. Instead, you get the actual measurements listed directly; “10 x 2” means a 10mm inner diameter with a 2mm cross-section.
ISO 3601-1 sets the international standard for metric O-ring dimensions. This standard O-ring size chart in mm divides O-rings into four series based on cross-sectional diameter, running from 1mm to 7mm. Because metric measurements fit with how most of the world does engineering, they’re the preferred choice for multinational projects where everyone needs to be on the same page.
British O-Ring Sizes
British Standard BS1806 and BS4518 define the O-ring dimensions in the United Kingdom and Commonwealth countries. These standards mix metric and imperial measurements, which can get confusing when you’re dealing with these suppliers. BS1806 handles smaller sizes, while BS4518 tackles larger industrial applications.
British standards use size codes instead of continuous numbering systems. Each code points to specific dimensions, and manufacturers might list them alongside metric or imperial equivalents. You won’t run into British sizes as often as AS568 or metric standards, but they’re still important for maintaining older equipment or meeting UK-based specifications.
Converting Between O-Ring Size Chart Systems
Converting between sizing systems takes more work than just switching units because each standard defines dimensions differently. AS568 uses inner diameter, while some engineers prefer working with outer diameter or groove dimensions. Cross-reference charts help you find equivalent sizes across systems, but perfect matches don’t always exist.
When specifications from one system don’t line up neatly with another, we pick the closest available size while keeping compression ratios and groove dimensions in mind. Software tools and online calculators make this faster, though double-checking critical dimensions by hand prevents expensive mistakes. Manufacturing tolerances differ across standards, too, so reviewing detailed specifications is important for precision work.
Beyond O-Ring Size Charts
Correct O-ring selection involves more than just matching numbers on a chart. Your application’s pressure, temperature, and chemical exposure all play a role in which material and size will actually work. Start by calculating the required groove dimensions based on your O-ring’s size and the amount of compression you need.
Standard compression ranges from 10% to 30%, depending on the application. Static applications usually need higher compression than dynamic seals, which require some room for movement without creating excessive friction. Don’t forget about thermal expansion, since temperature swings can significantly affect seal performance and the amount of groove clearance you need.
Partner With Our O-Ring Experts on Your Next Project
Being unable to find the right O-ring size shouldn’t hold up your entire project. I.B. Moore Manufacturing provides technical support to companies using any sizing standard, whether converting between systems or selecting seals for custom applications. Our team will help you make sense of size charts, recommend the right materials, and track down hard-to-find dimensions. Contact our Lexington, KY, team for nationwide service.