What is the standard wheel diameter in inches for a BMX bicycle?

The standard wheel diameter for a BMX bicycle is 20 inches.

The 20-inch format became closely associated with BMX as the sport developed in California during the late 1960s. Compact wheels helped young riders handle jumps, banked turns and tight dirt-track layouts, while also making the bikes strong and maneuverable. The common racing wheel uses an ISO 406 mm bead-seat diameter, although the inflated tyre makes the complete wheel larger than 20 inches.

BMX categories do include other sizes. A 24-inch BMX is generally called a cruiser and is used in racing and recreational riding, while smaller wheels appear on bikes designed for younger children. Freestyle and race bikes may also differ in frame geometry, gearing and brakes while retaining the familiar 20-inch wheel format.

The “20 inches” in the answer describes the nominal wheel size, not the exact outside measurement from tyre edge to tyre edge. It is also easy to confuse BMX wheels with 26-inch mountain-bike wheels or 700c road wheels, which belong to different bicycle categories.