Smart Fortwo Back Up Light Bulb Size

The Smart Fortwo back up light requires bulb sizes 921 and P21W depending on the specific version of the vehicle. Data for the components covers the range from 2008 to 2018.

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
2016-2018	921
2008-2015	P21W

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Wedge bulb (T15). 12V, 16W.	921
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

Other Locations

Additional Notes

The Smart Fortwo backup light specifications cover model years 2008-2018 and reference two distinct bulb types across that span: the 921 and the P21W.
The 921 is an ANSI-standard wedge bulb of the T15 type, rated at 12V and 16W, and is the designated backup light bulb for the Smart Fortwo in 2016-2018.
The P21W follows the ECE standard used in Europe and worldwide, features a BA15s bayonet base, and is rated at 12V and 21W. It is specified for the Smart Fortwo across 2008-2015.
The 921 and P21W are not interchangeable. The 921 uses a wedge base, while the P21W uses a BA15s bayonet base, making the two physically incompatible with each other's sockets.
The wattage difference between the two bulb types is notable. The P21W draws 21W compared to the 921's 16W, meaning a substitution would also alter the electrical load on the circuit.
The shift from P21W to 921 in the Smart Fortwo (2016) reflects a transition from ECE-standard to ANSI-standard componentry, which commonly corresponds to market-specific vehicle variants.
Both bulb types operate on a 12V system, which is standard for automotive lighting applications of this kind.
The P21W designation indicates a single-filament bulb, meaning it serves one lighting function only, consistent with its use as a dedicated backup light rather than a dual-function stop or tail lamp.