Flange Standards

There are several different flange standards to be found worldwide with standardised dimensions.
The most common standards are ASA/ANSI (USA), PN/DIN (European), BS10 (British/Australian), and JIS/KS (Japanese/Korean).

ASA150, 300, 400, 600, 900, 1500, 2500

Metric PN6, 10, 16, 25, 40

BS10 Table D, E, F, H

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.
The flange faces are also made to standardised dimensions and are typically "flat face", or "raised face" but other types are available.