What is an O-Ring?

An O-Ring is a torus, or donut shaped seal that is typically used to prevent the leaking of air or fluid in or out of a closed hydraulic system. O-Rings are commonly used as a seal but have been used as drive belt, bumpers, vibration dampeners, decoration, and even as jewelry.

How Does an O-Ring Seal?

The O-Ring seals by squeezing it between two surfaces that fluid or air may escape through. When squeezed, the O- Rings want to return to its original shape which applies a force between the sealing surfaces thus blocking the pathway which the fluid would escape. This is how the O-Rings seal under low to no pressure. If pressure is applied, the O-Ring is pushed against the side of the groove causing it to expands and increasing its ability to seal.

O-Ring Size Standards

The most common standard is Aerospace Standard 568 (AS568) which assigns a dash number and tolerances to a size of an O-Ring. These are commonly referred to as standard size O-Rings and are the most common used and stocked in the US.

There are also other international O-Ring size standards such as DIN3771 tolerances for metric O-Rings, British Standard (BS), Japanese Industrial Sizes (JIS-B-2401) and many others.


Since O-Rings can’t be made exactly to their dimensions every time, manufactures are allowed to make them within range of their actual dimensions. Variations in the rubber’s compound, ingredients, and in the manufacturing process cause slight variations in the shrink of the material and affect the finished size of the O-Rings.

O-Rings are dimensioned and toleranced on the Inside Diameter (I.D.) and the Width (W). The Outside Diameter is given as a reference only and does not usually have a tolerance. Tolerances can be expressed as a “±” figure like 1.239 ± 0.011, or stated as a range like 1.228-1.250 inches. Therefore 1.239± 0.011 is the same as 1.228-1.250.

Type of Elastomers

O-Ring seals are commonly made of rubber, but they can also come in Teflon®, Thermoplastic elastomer and Teflon® Encapsulated. There are 36 different types of rubber compounds on the market today, because of the different temperatures, chemical exposures and environments that O-Rings are subjected too. There is no one type of rubber that is compatible with all the environmental conditions. Therefore the need for different kind of rubber and different grades of each rubber are necessary.