If you’ve ever pulled an under-cabinet puck light down, looked at the tiny capsule bulb inside, and thought “these all look the same”—you’re not alone.
The good news: G4, G8, and G9 bases are easy to identify once you know what to look for. In this guide, we’ll show you how to tell them apart in under-cabinet puck lights, what measurements matter, and how to avoid ordering the wrong replacement.
Quick tip: Don’t guess by “shape” alone—measure the pin spacing in millimeters and check whether the pins are straight or looped.
Why puck light bulbs are confusing?
Under-cabinet puck lights often use small halogen capsules (and now LED replacements) that:
- look very similar from a distance
- may have no readable printing after years of heat
- may be installed in fixtures that are 12V low-voltage or full line-voltage (120V/230V)
Because of that, two different bases can seem interchangeable until you try to install them.
Safety first (30 seconds)
Before you remove anything:
- Turn off power at the switch—and ideally at the breaker.
- Let halogen bulbs cool. They can stay hot for several minutes.
- If you’re replacing a halogen with LED, confirm whether your puck system is 12V or 120V (more on that below).
The fastest way to identify G4 vs G8 vs G9
Step 1: Look at the “pins”
- G4 and G8 are typically two straight pins (bi-pin) that push into two round holes.
- G9 uses two looped pins (they look like small wire loops) that push into a special socket.
If you see loops, you’re almost certainly dealing with G9.
Step 2: Measure the pin spacing (center-to-center)
If you have straight pins (G4/G8), the base name is literally the spacing:
- G4 = 4 mm pin spacing
- G8 = 8 mm pin spacing
This is the standard identification method used by lighting suppliers: G4 bulbs have 4mm spacing between pins, and G8 bulbs have 8mm spacing. (Sources: 1000Bulbs identification notes on G4 and mini bi-pins.)
For G9, the “9” refers to 9 mm spacing between the loop pins. (Source: LiquidLEDs description of G9.)
Comparison table: G4 vs G8 vs G9 for under-cabinet puck lights
G4: 2 straight pins • 4 mm spacing • Common in low-voltage puck systems • Often 12V (sometimes 6V, 24V)
G8: 2 straight pins (wider than G4) • 8 mm spacing • Often in halogen puck lights • Mostly used in the US - 120V
G9: 2 loop pins • 9 mm spacing • More common in decorative fixtures (sometimes in pucks) • Often 120V/230V/240V
Detailed identification: what each base looks like in real life
G4 (bi-pin, 4mm)
What you’ll see: Two thin, straight pins very close together.
How to confirm: Measure center-to-center spacing—about 4mm.
Where it shows up: Many under-cabinet puck systems that use a small transformer (low-voltage). G4 LEDs are a popular retrofit when replacing hot-running halogens.
Common mistake: Mixing up G4 vs G6.35 (another straight-pin base). They look similar, but G6.35 has wider pin spacing.
Shop G4 Halogen replacement bulbs
G8 (bi-pin, 8mm)
What you’ll see: Two straight pins, but noticeably farther apart than G4.
How to confirm: Measure center-to-center—about 8mm.
A quick way to think of it: if it looks like G4 but “wider”, it’s often G8. Still—measure to be sure.
Where it shows up: Under-cabinet puck lights that originally shipped with small halogen capsules. Some systems are line-voltage, some are low-voltage.
Common mistake: Assuming G8 always means 120V. The base type is about mechanical fit, not voltage—your fixture determines voltage.
Shop G8 Halogen replacement bulbs
G9 (loop-pin, 9mm)
What you’ll see: Two looped pins (not straight posts). They push into a socket that looks different from a bi-pin holder.
How to confirm: If you can measure, spacing is about 9mm, but in practice the loop shape is the giveaway.
Where it shows up: Some puck lights, but also many decorative fixtures (pendants, wall sconces, etc.).
Common mistake: Trying to insert a G9 into a bi-pin holder (or vice versa). They are not compatible.
Shop G9 Halogen replacement bulbs
How to measure pin spacing (without fancy tools)
You don’t need calipers, although they help. Here are three easy options:
- Ruler method (quick): Hold a millimeter ruler behind the pins and estimate center-to-center spacing.
- Phone photo + zoom: Take a sharp close-up photo, zoom in, and compare to a ruler.
- Printable mm reference card: If you have one, place the bulb directly over it.
Measuring tip: If the pins are slightly bent, straighten them gently before measuring.
Don’t forget voltage: 12V vs 120V matters more than you think
Base type tells you what fits. Voltage tells you what survives.
Before ordering, check one of these:
- the old bulb printing (sometimes shows 12V or 120V)
- a label inside the puck light
- the power supply/transformer (often in a cabinet, basement, or above cabinets)
Rule of thumb
- If you find a transformer and thin low-voltage wiring, you’re likely on 12V.
- If the puck lights are daisy-chained with standard house wiring and no transformer, it may be line voltage.
If you’re unsure, don’t guess—send a photo of your bulb and fixture and we’ll help you identify it before you buy.
LED retrofit notes (so your new bulbs don’t flicker)
When replacing halogen capsules with LED:
- Check whether your system uses an electronic transformer or driver.
- Some LED capsules need a compatible driver to avoid flicker or dropouts.
- Match dimmable vs non-dimmable to your setup.
(We can add a short internal link here to a “LED flicker troubleshooting” guide if you have one.)
FAQ
Can I replace a G4 with a G8 if it “kind of fits”?
No. If the spacing doesn’t match, it’s the wrong base. Forcing it can damage the socket.
Are G4 and G8 always 12V?
Not always. Many G4 systems are low-voltage, but voltage is fixture-dependent. Confirm before ordering.
My puck light bulb has no markings—what do I do?
Measure pin spacing and take a clear photo of the base and socket. That usually solves it fast.
Get the right replacement (and avoid returns)
Best next step: Identify your base and shop the matching replacement:
- Shop G4 Halogen bulbs → https://12vmonster.com/collections/all/g4
- Shop G8 Halogen bulbs → https://12vmonster.com/collections/all/g8
- Shop G9 Halogen bulbs → https://12vmonster.com/collections/all/g9
Still not sure? Send us a photo of:
- the old bulb (pins visible)
- the socket inside the puck
- any labels showing voltage
We’ll help you confirm the base type so you can order confidently.