The BMW 325 brake light requires a P21W bulb size for proper installation and function within the lighting assembly. Data for the specific component covers the range from 1986 to 1988.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The P21W is an ECE-standard single-filament signalling bulb rated at 12 volts and 21 watts, fitted with a BA15s bayonet base. Its designation follows the international ECE naming convention, where "P" denotes a single-filament lamp and "21W" reflects its wattage.
The BA15s bayonet base uses a single contact pin offset to one side, which physically prevents incorrect installation of double-contact variants such as the BA15d. This mechanical distinction is relevant when sourcing replacement bulbs.
The BMW 325 uses the P21W across both the standard brake lights and the center high mount stop light for the 1986-1988 model years, meaning all three brake light positions share the same bulb specification throughout that production span.
Because the brake lights and the center high mount stop light share an identical bulb specification, a single bulb type services all stop-light functions on the vehicle, with no position requiring a different wattage or base type.
The P21W operates on a 12-volt electrical system, consistent with the standard automotive voltage architecture used in the BMW 325 during the 1986-1988 period.
At 21 watts, the P21W produces a relatively high light output for a single-filament signal bulb, which is why it is commonly assigned to brake and stop-light applications rather than lower-wattage positions such as parking or side marker lights.