Car Bulb Size Guide

Why Bulb Size Matters

Car bulbs are not interchangeable. Each position in a vehicle uses a specific bulb type, determined by the size of the housing, the fitting type (how it locks into the socket), and the wattage required. Fitting a bulb with the wrong base type will prevent it from fitting correctly. Fitting one with the wrong wattage can damage the wiring or cause the light to burn out prematurely.

Types of Car Bulb Fittings

There are several common fitting standards used in modern vehicles:

• H4, H7, H11: Halogen bulbs for headlights, with different base shapes and pin configurations. H7 is the most common single-filament headlight bulb in European cars.
• T10 (W5W): Small wedge-based bulbs used for sidelights, number plate lights, and some interior lights.
• 501 / T10: Common sidelight and interior lamp bulb.
• 380 / P21/5W: Dual-filament bulbs used for combined stop/tail lights.
• 382 / P21W: Single-filament bulbs for indicators, reversing lights, and brake lights (depending on the circuit).
• PY21W: Amber-tinted indicator bulb used in some vehicles.
• D1S, D2S, D3S, D4S: HID (xenon) bulb types — do not replace these without specific knowledge, as they operate at very high voltage.
• H8, H9, H16: Halogen fog light bulbs.

How to Find the Correct Bulb for Your Vehicle

1. Check the owner's manual. The bulb specification chart is usually in the maintenance or technical data section.
2. Use the AutoWire app. The Bulb Size Guide feature lists the correct bulb type for each position on your specific make, model, and year.
3. Check the existing bulb. If you remove the blown bulb, the type is usually printed or stamped on the glass or base (e.g., "H7", "W5W").
4. Use an automotive parts lookup. Many parts retailers will look up the correct bulb by vehicle registration or make/model/year.

Common Bulb Positions and Their Types

While exact specifications vary by vehicle, these are typical bulb positions and common fitting types:

• Headlight (main beam / dipped beam): H4, H7, H11, or H1 depending on the vehicle and headlight design
• Sidelight / position light: W5W (T10) or similar small capless bulb
• Front indicator: PY21W (amber) or P21W (with amber lens)
• Stop / brake light: P21W or P21/5W (combined stop/tail)
• Tail / rear light: W5W, R5W, or P21/5W depending on position
• Reversing light: P21W
• Rear fog light: P21W or P21/5W
• Number plate light: W5W (T10) or C5W festoon
• Interior dome light: C5W or C10W festoon, or W5W
• Front fog light: H8, H11, or H16

Tips for Replacing Car Bulbs

• Always turn off the ignition and allow the vehicle to cool before replacing bulbs — halogen bulbs become extremely hot.
• Do not touch the glass of a halogen bulb with bare hands. Skin oils cause hot spots that can shorten bulb life or cause premature failure. Use a cloth or gloves.
• When replacing headlight bulbs, replace both sides at the same time — if one has failed, the other is likely close to failing as well.
• After fitting, test every function of the replaced bulb before reassembling the housing or trim.