DIN is a car audio standard developed by the German standards body Deutsches Institut für Normung (DIN). It specifies a height and width for car head units. When a unit is referred to as a single DIN car stereo or a single DIN car radio, this means that it is the height and width described in the DIN standard.

Car manufacturers and car stereo manufacturers around the world use this standard, which is why most head units are interchangeable. Although the wiring is not standardized, the DIN standard is the reason why you can replace OEM car stereos with aftermarket units.

Although the DIN standard specifies only one height and width, manufacturers of head units also produce devices that are twice as high. These double-height units are called double DIN, because they are twice as high as the single DIN standard. A small number of head units are 1.5 times as high as the DIN standard, which technically makes them 1.5 DIN.

The easiest way to determine if a car radio is single DIN is to measure it. If the radio is about two inches high, it is probably a single DIN. If it is about four inches high, it is a double DIN. A 1.5 DIN radio, which is quite rare, falls between those measurements. There are no other standardized DIN measurements.