BMW X4 Back Up Light Bulb Size

The BMW X4 back up light uses a P21W bulb size for replacement purposes. The replacement chart covers vehicle production spanning 2015 to 2018.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
2015-2018	P21W

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

The BMW X4 uses the P21W bulb for the back up light across the 2015-2018 model years.
The P21W is an ECE-standardized bulb, meaning it conforms to regulations recognized in Europe and internationally rather than SAE standards used in North America.
P21W bulbs operate on a 12-volt electrical system and draw 21 watts, which is typical for single-filament signaling applications.
The P21W uses a BA15s bayonet base, where "BA" denotes a bayonet-type fitting, "15" refers to the 15mm base width, and the lowercase "s" indicates a single contact at the base.
Because the P21W is single-filament, it serves one lighting function only, distinguishing it from dual-filament variants such as the P21/5W, which would not be a compatible substitute.
The BA15s bayonet base requires a twist-and-lock installation rather than a screw-in method, and substituting a bulb with a different base type would result in an incompatible fit.