BMW 325i front and rear side marker bulbs utilize 12961 and P21/5W sizes for the front position, with P21/4W bulb size specified for the rear location. The replacement chart covers 1988 to 2006 model years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The BMW 325i front side marker position uses two distinct bulb types across its model years: the 12961 and the P21/5W.
The 12961 is a Philips miniature wedge bulb of the T10 form factor, rated at 12V and 5W, and fitted with a W2.1x9.5d base. It is the front side marker bulb for the 325i across model years 1988-1991, 1995, and 2002-2006.
The P21/5W is an ECE-standard dual-filament signalling bulb rated at 12V with a 21W main filament and a 5W secondary filament. It uses a BAY15d bayonet base and serves as the front side marker bulb for the 325i in 1993-1994 and 2001.
The P21/4W is an ECE-standard dual-filament signalling bulb rated at 12V with a 21W main filament and a 4W secondary filament. It is the rear side marker bulb for the BMW 325i across model years 1993-1994 and 2001-2005.
The P21/5W and P21/4W are not interchangeable despite sharing a similar designation. The P21/5W uses a BAY15d base, while the P21/4W uses a BAZ15d base. These two bayonet bases have different pin offsets, which physically prevents cross-installation.
The 12961 wedge base and the P21/5W bayonet base are mechanically incompatible. A socket designed for one will not accept the other, making substitution between these two front side marker types impossible without a socket adapter or housing modification.
The ECE designations P21/5W and P21/4W are internationally standardized, meaning bulbs from multiple manufacturers conforming to those standards are considered direct replacements for the original fitment in the applicable 325i model years.
The 12961 designation is a Philips-specific part number rather than an industry-wide standard. Equivalent T10 wedge bulbs from other manufacturers are generally cross-referenced by their base type (W2.1x9.5d) and electrical rating (12V, 5W) rather than by this number alone.