Steel pipe flange is most commonly used to join lengths of pipe. Normally found at one end of a pipe, or occasionally both ends, a steel pipe flange is a lip or ring that sticks out from the pipe or tube, perpendicular to the length. By design, this lip provides a mating surface that can be bolted to another flange or connect a handrail pipe to a wall. The handrail pipe-to-wall connection also uses a type of steel pipe flange. Although there are alternate methods for these types of connections, such as welding or threading, a steel pipe flange can make this type of assembly much easier.

       And flange is an external or internal ridge, or rim (lip), for strength, as the flange of an iron beam such as an I-beam or a T-beam; or for attachment to another object, as the flange on the end of a pipe, steam cylinder, etc., or on the lens mount of a camera; or for a flange of a rail car or tram wheel. Thus flanged wheels are wheels with a flange on one side to keep the wheels from running off the rails. The term "flange" is also used for a kind of tool used to form flanges. Pipes with flanges can be assembled and disassembled easily.

There are many types of steel pipe flanges, the majority of which provide corresponding bolt patterns for a simple bolted assembly. Therefore, to allow easy functionality and inter-changeability, these are designed to have standardized dimensions. Common world standards include ASA/ANSI (USA), PN/DIN (European), BS10 (British/Australian), and JIS/KS (Japanese/Korean).

       In most cases these are not interchangeable (e.g. an ANSI flange will not mate against a JIS flange). Further, many of the flanges in each standard are divided into "pressure classes", allowing flanges to be capable of taking different pressure ratings. Again these are not generally interchangeable (e.g. an ANSI 150 will not mate with an ANSI 300). These pressure classes also have differing pressure and temperature ratings for different materials. Unique pressure classes for piping can also be developed for a process plant or power generating station; these may be specific to the corporation, engineering procurement and construction (EPC) contractor, or the process plant owner.