The BMW 328i tail light requires bulb sizes P21/4W, P21/5W, and 12821 depending on the specific version of the lighting component. Data for the bulbs covers the range from 1996 through 2000.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The P21/4W is an ECE-standard dual-filament signalling bulb operating at 12V with a 21W primary filament and a 4W secondary filament. It uses a BAZ15d bayonet base, which is an offset pin configuration that prevents incorrect installation.
The P21/5W shares the same ECE-standard dual-filament design as the P21/4W but carries a higher 5W secondary filament rating. Its base type is BAY15d, a different offset bayonet configuration that is not interchangeable with the BAZ15d base of the P21/4W.
Because the BAZ15d and BAY15d bases differ in pin offset geometry, P21/4W and P21/5W bulbs are not physically interchangeable despite their similar appearance and shared primary wattage.
The 12821 is a Philips part number designation for a miniature wedge-style T10 bulb with a W2.1x9.5d base, rated at 12V and 5W. As a single-filament wedge bulb, it serves a fundamentally different electrical and mechanical role than the dual-filament bayonet types listed alongside it.
The BMW 328i (1996-1998) Inner Tail Light position uses the P21/4W across all three model years, indicating a consistent fitment for that sub-part within that generation.
A separate Inner Tail Light dataset for the BMW 328i covers 1996-2000 and shows variation across model years: the 1999-2000 entries specify P21/4W, the 1997-1998 entries specify P21/5W, and the 1996 entry specifies the 12821 wedge bulb.
The 1996 model year in the second dataset is the only year across both datasets to specify the 12821 wedge bulb for the Inner Tail Light position, which uses a W2.1x9.5d base entirely distinct from the bayonet bases used in all other listed years.
The shift from the 12821 wedge type in 1996 to the P21/5W dual-filament bayonet type in 1997-1998 represents a change in both base type and filament configuration for the same Inner Tail Light position.