BMW 850i Back Up Light Bulb Size

The BMW 850i back up light requires a P21W bulb size for the rear lighting assembly. Data for the specific component covers the range from 1991 to 1992.

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
1993-1997	P21W

Bulb Specifications

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

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Additional Notes

The BMW 850i uses the P21W bulb for the back up light across the 1993-1997 model years.
The P21W is an ECE-standardized bulb type, meaning it conforms to regulations used in Europe and internationally rather than SAE standards common in North American automotive applications.
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, distinguishing it from dual-contact variants.
Because the P21W is single-filament, it serves one lighting function only, which is consistent with its use as a dedicated back up light rather than a combined stop or tail light.
The BA15s bayonet base requires a twist-and-lock installation method, and substituting a bulb with a different base type, such as BA15d, would result in an incompatible fit despite a similar physical appearance.
Replacement bulbs for the BMW 850i back up light must match the P21W specification precisely, as using a bulb with a different wattage rating risks improper illumination output or electrical load issues within the vehicle's signalling circuit.