What Is an H4 Bulb? | Dual-Filament Headlight Standard

An H4 bulb is a dual-filament halogen headlight that combines high beam and low beam in one unit, using a P43t base and standard 60/55W power.

If you drive an older European, Asian, or classic car, the bulb under your headlight lens is likely an H4. Unlike single-beam bulbs that handle only one function, the H4 integrates both low and high beams into a single halogen capsule. It’s the most common dual-filament standard worldwide, though in the US it’s legally sold as the 9003 or HB2 — the three are physically identical. Understanding what makes this bulb different from the dozens of other types on the shelf is the difference between a proper beam pattern and dangerous glare.

How an H4 Bulb Is Different From Other Headlight Bulbs

The defining feature of an H4 is its dual-filament design inside one glass envelope. One filament produces the low beam; a second, brighter filament kicks in for the high beam. That design requires a specific base — the P43t-38 — that locks the bulb into the headlight housing at a precise angle. H1, H7, and H11 bulbs are single-filament units with completely different base shapes, so none can fit an H4 socket, and mixing types is the most common installation mistake.

Standard H4 bulbs run on 12V for passenger vehicles, with most rated at 60 watts for the high beam and 55 watts for the low beam. Halogen H4s produce a warm 2800K–3200K light color, while LED upgrades typically offer a cooler, whiter beam in the 4300K–6500K range.

H4 Versus 9003 Versus HB2

In Europe and Asia, the bulb is called H4. In the United States, the Department of Transportation classifies the same physical bulb as the 9003, and the Society of Automotive Engineers calls it HB2. The dimensions are identical: overall length of approximately 72.5 mm, filament position tolerance within ±0.2 mm, and the same P43t locking base. A bulb sold as 9003 or HB2 is an H4; the labels simply reflect different regulatory frameworks. When shopping for a replacement in the US, any of the three designations will fit your H4 socket.

If you’re ready to upgrade your H4 bulb, check our tested roundup of the best options for your vehicle: top-performing H4 LED bulb recommendations to see which models deliver the right beam pattern and certification.

Halogen vs. LED H4 Bulbs — What Changes

Halogen H4 bulbs use a traditional tungsten filament and typically last 300–500 hours. They’re affordable, widely available, and produce the orange-yellow light that reflector housings were designed for. The trade-off is short lifespan and lower brightness compared to modern alternatives.

LED H4 replacements swap the filament for semiconductor chips. They are significantly brighter, consume less power, and last much longer — often tens of thousands of hours. Most are plug-and-play, fitting the existing H4/9003 socket without housing modifications. However, vehicles with CANbus or PWM dimming systems may require LED bulbs with built-in load resistors to prevent flickering or dashboard error codes.

The catch: many LED H4 bulbs shift the light source position slightly, which can create glare or a scattered beam in housings originally designed for the precise ±0.2 mm tolerance of a halogen filament. Under UNECE Regulation 112, halogen bulbs are restricted to 2800K–3200K color temperature, and LED bulbs exceeding that may be non-compliant in jurisdictions that test to that standard.

H4 Bulb Facts at a Glance

Certification marks matter. A legitimate H4 bulb will have the ECE R37 mark (Europe) or DOT-SAE J1383 (USA) laser-etched into the glass. These marks are your assurance that the bulb meets the filament-position tolerance that keeps your beam pattern legal and safe.