BMW 733i Back Up Light Bulb Size

The BMW 733i back up light utilizes a P21W bulb size for illumination purposes. Available data for the specified component covers the range from 1978 through 1984.

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
1978-1984	P21W

Bulb Specifications

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

Other Locations

Additional Notes

The BMW 733i uses the P21W bulb for the back up light across all listed model years, spanning 1978-1984.
The P21W is an ECE-standard bulb, meaning it conforms to the United Nations Economic Commission for Europe regulations, which govern lighting equipment used in Europe and many other markets worldwide.
P21W bulbs operate on a 12-volt electrical system and draw 21 watts, which is typical for single-filament signalling applications such as reverse lights.
The P21W uses a BA15s bayonet base, where "BA" denotes a bayonet cap fitting, "15" refers to the 15mm base width, and the lowercase "s" indicates a single contact at the base.
Because the P21W is a single-filament bulb, it serves one lighting function only, distinguishing it from dual-filament variants such as the P21/5W, which would not be a direct substitute.
The BA15s bayonet base requires a quarter-turn insertion method to lock the bulb into its socket, and substituting a bulb with a different base type, such as a wedge or pin base, would be physically incompatible with the 733i's reverse light housing.