The BMW 318i uses multiple bulb sizes for turn signal applications, including P21/5W for front inner and front turn signals, 12821 for front outer turn signals, and P21W for rear turn signals. The replacement chart covers 1984 to 1998 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 P21/5W is the most frequently specified bulb across the BMW 318i turn signal positions, appearing in the front turn signal position across 1984-1985 and 1991-1998, as well as in the front inner turn signal position for 1997-1998.
The P21/5W is a dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base. Its dual-filament design allows it to serve both signalling and parking light functions within a single housing.
The P21W, specified for the rear turn signal across all listed years, is a single-filament bulb rated at 12V, 21W, and uses a BA15s bayonet base. The BA15s base is not interchangeable with the BAY15d base of the P21/5W, as the two offset pins on the BAY15d are positioned to prevent incorrect installation.
Both P21/5W and P21W follow ECE standards, indicating they are designed for European and worldwide regulatory compliance rather than SAE standards used in North American markets.
The front outer turn signal for the BMW 318i (1997-1998) uses the Philips 12821, a miniature wedge-type bulb in the T10 form factor with a W2.1x9.5d base, rated at 12V and 5W.
The side position of the front turn signal for 1997-1998 uses the Philips 12961, which shares the same T10 form factor, W2.1x9.5d base, and 12V, 5W rating as the 12821. Despite identical electrical and physical specifications, these are listed under separate Philips part numbers and are assigned to distinct positions.
The wedge base of the T10 bulbs (12821 and 12961) is mechanically distinct from the bayonet bases used by the P21/5W and P21W, making cross-position substitution between these bulb families physically incompatible.
The front turn signal position for 1997-1998 specifies two separate bulbs: a P21/5W for the front-facing element and a 12961 for the side element, indicating a split-function housing arrangement for those model years.