Headlight Size Guide: Bulb Code vs. Housing Dimensions

“What size headlight do I need?” is three different questions wearing the same words — and answering the wrong one is how people end up with a bulb that won’t seat, a lamp that won’t fill the opening, or an assembly that won’t plug in.

Before you measure anything, work out which of these you’re actually replacing:

Get that classification right and the rest is straightforward. If it turns out you need the first row, the headlight bulb size reference chart is the lookup table and headlight bulb cross-reference covers which numbers are genuinely the same bulb. The other two rows are what this page covers.

Standardized lamp sizes — the dimension reference

Sealed beams were built to a small set of DOT-standardized shapes, which is exactly why a modern LED unit can drop into a decades-old bucket. The round sizes below are named by their true diameter, so the nominal name and the millimetre conversion match. The rectangular sizes are trade names, not literal dimensions — per FMVSS 108, the actual lens is larger than “5x7” or “4x6” suggests. Actual glass usually measures a fraction over nominal, so treat these as the family you’re matching to, not a tolerance to hold.

Three things about that table are worth spelling out, because each one causes real wrong-part orders:

5x7 and 7x6 are the same lamp — and both names round down. The rectangular H6054 actually measures about 7.9 inches wide by 5.6 inches tall; “5x7” and “7x6” are trade shorthand, not the literal size, which is why sellers round and reorder those numbers inconsistently — you’ll see 5x7, 7x6 and occasionally 6x7 on listings for identical parts. If a listing shows two of those numbers together, that’s a good sign it’s the standard rectangular lamp. Match the H6054 code where you can; it’s less ambiguous than the dimensions.

Quad systems use two different lamps per side. In both the 5-3/4” round and 4x6” rectangular families, the outer position takes a low/dual-beam lamp and the inner position takes a high-beam-only lamp. They look nearly identical in a photo and are not interchangeable. Buying four of the outer lamp is a common and frustrating mistake — the inners simply won’t light on low beam, which is normal behaviour for the wrong part.

The Wrangler changed size mid-life. CJ, YJ, TJ and JK all use the 7-inch round lamp, which is why so many 7-inch LED units are marketed straight at Wranglers. The JL and the Gladiator use a 9-inch round lamp — a 7-inch unit won’t mount or fill that opening. Confirm the generation, not just the model name.

For the full upgrade path once you know your size — connectors, DOT legality, wiring, and what a good LED sealed-beam replacement looks like — see the sealed beam headlight upgrade guide and the sealed beam category.

How to measure a headlight correctly

Four rules, and the first one accounts for most measuring errors:

1. Measure the lens, not the bezel. The chrome or painted trim ring around the lamp is a separate part. Measuring across its outer edge typically adds enough that a 7-inch lamp reads closer to eight, sending you hunting for a size that doesn’t exist.
2. Round lamps: measure the diameter straight across the widest point of the lens face, through the centre.
3. Rectangular lamps: measure width across and height top-to-bottom as the lamp sits installed. Orientation matters — a lamp described as 5x7 is wider than it is tall, so record the two numbers in the order you took them and note which is which.
4. If the lamp is already out, measure the bucket opening rather than the old lamp. That’s the dimension that actually has to be filled, and it removes any doubt about whether a previous owner fitted something non-standard.

Then check the lens itself before you order. Sealed beams carry their code — the H6024, H6054, H4651 and so on — molded directly into the glass, usually near the edge. That code is more reliable than your tape measure, and it’s the thing to search on.

Depth is the dimension nobody checks. Face dimensions get all the attention, but LED replacements often carry a heat sink or a driver box behind the lamp that a filament sealed beam never had. On tight installs — some grille-mounted quad systems and a few motorcycle buckets — that’s the clearance that bites. If the space behind the bucket is shallow, check the listing’s rear projection before ordering.

Modern composite assemblies have no size

Once manufacturers moved from standardized sealed beams to composite headlights, the housing became a styled body component shaped to that specific vehicle. There’s no shared dimension left to specify — so measuring a modern headlight tells you almost nothing useful.

What replaces the dimension is a five-part interchange identity:

• Year, make and model — including the generation, because mid-cycle facelifts routinely change the headlight even when the body looks similar.
• Trim level — a base trim and a top trim on the same car frequently carry different lamps.
• Side of the vehicle — headlights are mirror-image parts. Listings mark them LH/RH or driver/passenger, and “left” conventionally means the driver’s side on a left-hand-drive vehicle. Read which convention a listing uses.
• Factory lamp type — halogen, HID/xenon or full LED. This is the one that catches people. Different lamp types on the same body use different connectors and different internal hardware, and swapping between them isn’t a plug-and-play job. A car built with HID needs a ballast and a D-code bulb; the halogen version of the same headlight has neither.
• Extra hardware — auto-levelling motors, adaptive/AFS swivel actuators and integrated DRL modules add connector pins. An assembly that’s mechanically correct but missing a feature your car expects can throw a fault.

Finding the part number

The number that identifies the assembly is normally molded or stamped into the plastic — on a mounting tab, on the back shell, or on a flat area near the connector. You’ll usually need the assembly off the car to read it.

The lens carries separate markings, and they answer a different question. A DOT symbol indicates the lamp is certified to the US federal standard; an SAE code describes which functions the lamp performs (low beam, high beam, turn, position); manufacturer marks and a left/right indication are often alongside. Those markings tell you what the lamp does — the molded part number tells you exactly which lamp it is. For searching, you want the part number.

That number is also the single best search term on eBay Motors, where a used OEM assembly from a salvaged vehicle is by definition the correct size, connector and lens shape. For the rest of what’s inside the housing, see parts of a car headlight; for every other lamp position on the vehicle, see car light parts explained.

Sizing the rest of the lighting system

Fog lights follow the composite pattern more than the standardized one — most factory fog lamps are shaped to a specific bumper opening, so they’re matched by vehicle rather than by dimension. Universal round and rectangular pods are the exception and are sold by their face dimension plus a bracket. The bulb inside is a separate size question entirely, usually an H11, 880/881 or H10 — see best fog lights and the fog lights category.

Light bars are sold by overall length in inches — 20, 32, 42, 52 and so on — and that figure normally includes the end caps. Measure the span you have available, then account for the mounting feet, which sit inboard of the ends and determine where the bar can actually bolt down. A bar that fits the gap dimensionally but whose feet land on nothing isn’t a fit. The light bars category and best LED light bars cover the rest.

Bulbs, finally, have no relationship at all to housing size. An H11 is an H11 whether it lives in a compact car’s projector or a truck’s reflector. What does matter inside a reflector housing is the bulb’s chip geometry relative to the original filament — covered in best LED bulbs for reflector housings and projector vs. reflector headlights.

Where to buy which

Amazon — standardized sizes and universal parts. Because sealed beams are dimensionally standardized, LED replacements in 7-inch round, 5x7/7x6, 4x6, 5-3/4-inch and 9-inch round are genuine off-the-shelf products that fit thousands of vehicles. Same for bulbs, wiring harnesses and anti-flicker hardware. Browse sealed beam, LED headlights and bulbs.

eBay Motors — anything identified by part number. Composite assemblies, OEM housings, buckets, retaining rings and trim bezels, adjuster hardware, and the discontinued lamps that no longer exist as new stock. Searching the molded part number rather than a description is the difference between one right result and forty near-misses.

The short version

Classify first, measure second. If you’re replacing a bulb, you need a base code and the size chart answers it. If you’re replacing a sealed beam, measure the lens face — not the bezel — match it to one of the standardized families above, and confirm against the code molded in the glass. If you’re replacing a modern assembly, stop measuring entirely and find the part number, the side, and which lamp type the car was built with.

And whichever route you take: aim the headlights afterwards. A correctly-sized lamp pointed wrong is still a lamp pointed wrong.