There are various car light bulb sizes in the automotive lighting system. H4 and H7 are common bulb fittings used in headlights and fog lights. If you don’t know how to choose between these two bulbs or are curious about the differences between H4 and H7, this guide is for you.

Here we will discuss what the H4 and H4 bulbs are respectively, and compare H4 vs. H7 bulbs so you can make an informed decision before getting your new headlight bulbs.

What is an H4 Bulb and What is it Used For?

H4 halogen bulb

An H4 bulb is a bilux or dual-filament halogen bulb with two filaments inside. It can serve as both high and low-beam lights, with each filament dedicated to one beam function. Consequently, an H4 bulb features only one lamp base plate.

Features of H4 Bulbs and How They Work

H4 lamps contain two filaments - one for low beams, and one for high. When the low beam is on, its filament lights up. Flick on the high beam, and its filament glows. Headlamp reflectors are aligned to the filaments, distributing light in front of the vehicle on low beam, and farther ahead on high. Today's H4 bulbs are typically halogen.

Common Uses and Applications of H4 Bulbs

Initially, H4 bulbs were primarily used in race cars. Their application in everyday vehicles was limited mostly to Europe for several years. In countries like the United States and India, H4 bulbs were popular in motorcycles, but their beam pattern did not meet the auto safety standards for cars.

To address this issue, an alternative bulb, the 9003/HB2, was developed. This bulb type could be used in road cars without dazzling other drivers. So today, we now have interchangeable H4, HB2, and 9003 bulbs. You may find all these labels on the packaging of bulbs that fit this specification. (Related guide: 9003 vs. H4 Headlight Bulbs: Are They The Same?)

For H4 LED headlight bulbs upgrade, click here

Also read: What is a 9003/H4 Bulb & Best 9003 LED Headlight Bulbs Choices

What is an H7 Bulb and What is it Used For?

H7 halogen bulb

H7 bulbs are single-filament headlight bulbs. In vehicles compatible with H7 bulbs, there are two base plates for each headlight.

Features of H7 Bulbs and How Does it Work

Since H7 bulbs are single-filament, they only serve one beam function at a time. This means you need two separate bulbs per headlight: one for the low beam and another for the high beam.

The reflectors in the headlight assembly are positioned in accordance with the filament in each bulb.

Common Uses and Applications of H7 Bulbs

The H7 light bulb has been a preferred choice of German and European automakers since hitting the market in 1993.

They were once xenon-based when used in luxury BMW, Audi, Mercedes, and Volkswagen vehicles. But the steep price and glare of xenon eventually gave way to more affordable, focused LED options.

For H7 LED headlight bulbs upgrade, click here

People also read: H7 vs. H11 Bulb: What’s the Difference Between These Headlights?

H4 vs H7 Bulbs: What’s the Difference?

Check the following comparison table of H4 vs. H7 for a quick reference:

*Note: The nominal power and Luminous Flux are for H4 and H7 halogen bulb types.

As we can see, H4 and H7 bulbs have quite different characteristics and applications. The H4 is designed as a dual-filament halogen bulb to provide both high and low headlight beams from a single unit. In contrast, the H7 is a single-filament bulb mainly used for high beams. Additionally, the H4 has a two-prong base while the H7 uses a three-prong base.

So not only do they serve different functions, but they have physically distinct attachments and are not interchangeable.

Now that you've had a quick glance at the differences, let's dive deeper into each aspect of these two bulb types.

Different Filament

H4 bulbs contain two filaments within a single bulb. These twin filaments allow the H4 to serve dual purposes: one filament for the low beam (dipped beam) and the other for the high beam (main beam).

On the other hand, H7 bulbs have a single filament, which means they can only perform one function at a time, either as a high beam or a low beam.

Different Base Type

When it comes to cross-compatibility, it's important to note that H4 and H7 bulbs are not interchangeable due to their different base designs and functionality.

The H4 has a three-pronged base, while the H7 has a two-pronged base. Additionally, their differing filament structures mean they cannot be used in place of each other.

So what bulb is the same as H4 or H7? The H4 bulb is similar to the HB2/9003 bulb used in the US. However, the H7 bulb does not have a direct equivalent in this system.

Power Draw and Luminous Flux

Power draw refers to the amount of power a bulb uses, while luminous flux indicates the perceived brightness produced by the bulb.

The stock power draw for an H4 bulb varies depending on the voltage — 60W at 12V for high beam and 55W for low beam. The H7 bulb, being a single-function bulb, has a power draw of 55W at 12V.

When it comes to luminous flux, H4 bulbs can produce 1,650 lm for high beam and 1,000 lm for low beam. H7 bulbs emit a consistent 1,500 lm.

Cost

With H7 lights, you need two bulbs for low beam and another two for high beam. It's always best to replace both bulbs in a pair at the same time to maintain consistent brightness. Therefore this doubles the cost of each replacement.

On the contrary, H4 systems, requiring only two bulbs, may have a lower cost factor over time, making them a bit more economical.

Why H4 and H7 Bulbs Aren't Interchangeable

Base Plate Differences

The primary reason lies in the design specifics of these bulbs. The base plate type in H4 and H7 bulbs is not identical, which means you cannot simply insert an H7 bulb into an H4 socket. This physical incompatibility is the first roadblock you'll encounter if you attempt to interchange these bulbs.

Reflector Compatibility

Moreover, the reflectors in the headlamp are matched to the position of the filaments in the bulbs. Since the filament in the H7 lamp is in a different position from the one in an H4 lamp, neither the light beam for the low beam nor that for the high beam would be cast correctly onto the road.

H4 vs. H7, Which Bulb Do I Need?

It depends on your vehicle. You must buy the exact fitting that is compatible with your vehicle. To find out what bulb size you need, you can:

• Check your vehicle owner's manual. The handbook should provide information on the type of bulb fittings your vehicle uses. You must buy the exact fitting as stated by the vehicle handbook.

• Use online vehicle bulb finder. Automotive lighting store like SEALIGHT allow you to confirm the bulb size your vehicle need quickly. Simply enter your vehicle’s information and you can find it out.

• Check your stock bulb. The most accurate way is to remove the old bulb and look at the bulb base. The printed number is the exact bulb size you need.

Wondering if your car has an H4 or H7 bulb? You can check the headlight beams. If your high and low beams come from separate bulb, it's H7. If they come from the same bulb then it's H4.

Final Words

Making the right bulb size choice is important when it comes to car bulb replacement. We hope that this guide has been helpful to you if you are looking for information on H4 vs. H7. You can click here for more car bulb size comparison guides. SEALIGHT is your choice for LED automotive lighting upgrades, whether you need headlight bulbs, fog lights bulbs, or backup lights bulbs. Visit the SEALIGHT automotive lighting store now.