G4 and G9 bulbs with different pin bases

G4 vs G9 Bulbs: Pins, Voltage and Which One Your Fixture Needs

G4 and G9 are two small bi-pin bulb bases, and they do not swap. A G4 has two straight pins 4 mm apart and usually runs on low voltage, often 12V through a transformer. A G9 has two looped pins 9 mm apart and usually runs on standard U.S. household current, 120V. Check the pins first, then match the voltage printed on your fixture or old bulb.

That is the short version. The rest of this guide shows how to tell the two apart in seconds, why voltage matters more than size, and how to choose a replacement that looks right in your chandelier, sconce or lamp.

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TL;DR

  • Pins: G4 has straight pins 4 mm apart. G9 has U-shaped loops 9 mm apart.
  • Voltage: In the U.S., G4 is typically 12V and G9 is typically 120V. Always confirm against the fixture, because the base code alone does not guarantee voltage.
  • Interchangeable? No. Different contacts, spacing and electrical supply.
  • Transformer: A 12V G4 fixture uses one. A 120V G9 fixture does not.
  • Upgrade path: Both bases come in LED versions, but check size, dimming and transformer compatibility first.

Table of Contents

What the G and the number actually mean

The “G” refers to glass, the material of the original bulbs, and the number is the distance between the pins in millimeters. Family Handyman explains that G4 and G9 prongs sit 4 mm and 9 mm apart. The bi-pin lamp base reference adds that the suffix after the G indicates the pin spread, and that a GU prefix usually means the lamp has a mechanism for physical support by the fixture.

G4 and G9 pin spacing compared

So G4 is a 4 mm pin spread and G9 is a 9 mm pin spread. Nothing in the name tells you the voltage, and that is where most wrong purchases start.

G4 vs G9 at a glance

G4 G9
Pin shape Two thin, straight pins Two looped (U-shaped) wire contacts
Pin spacing 4 mm center to center 9 mm center to center
Typical U.S. voltage 12V (some 6V or 24V) 120V
Needs a transformer? Usually yes, in the fixture No
Typical size Very small capsule Slightly larger capsule
Common homes for it Under-cabinet, accent, small decorative fixtures Chandeliers, sconces, pendants, vanity and ceiling fixtures

The pin and voltage details come from the bi-pin base reference: G4 is used for small halogen capsules at 6 or 12 volts, and G9 at 120V in the U.S. (230V in Europe). The same page notes G9 is also used for LED lamps.

How to tell them apart in 30 seconds

The loop is the giveaway. If the pins are straight wires, it is a G4. If each pin curls into a small U or loop, it is a G9. You do not need a ruler, though a 4 mm versus 9 mm gap is easy to see once you hold them side by side: the G9 pins sit more than twice as far apart.

Here is a quick routine:

  1. Switch off the fixture and let it cool. Small halogen capsules get very hot.
  2. Remove the bulb carefully. Use a clean cloth or glove rather than bare fingers on a halogen capsule.
  3. Look at the pins. Straight is G4. Looped is G9.
  4. Read the markings. Wattage and voltage are often printed on the glass or the package. If the print has rubbed off, rely on the pin shape.
  5. Photograph the old bulb and the socket. It helps when you shop.

Family Handyman suggests bringing the old bulb with you to the store, since packages usually carry a true-to-size diagram you can hold up against it. If you are also sorting out other bases around your home, our guide to identifying bulb base types in four steps walks through the full set.

Watch for look-alikes

G4 is sometimes loosely called GU4, and G9 is not the same as G9.5 (GY9.5), which has 9.53 mm spacing, according to SizeSheet’s comparison. If your fixture says G9.5, shop for exactly that. The near match in name is not a near match in fit.

Can you use a G4 in a G9 socket, or the reverse?

No. The pins are different shapes and spaced differently, so one will not seat properly in the other’s socket. Even if you could force it, the electrical supply differs. Pushing a 12V G4 capsule onto a 120V circuit is a fire and shock risk, and a 120V G9 bulb in a low-voltage socket will not do what you expect. Use the exact base and electrical rating that the fixture specifies.

If you are unsure what your fixture is rated for, look for the label near the socket or on the fixture’s body. For anything involving rewiring, swapping a transformer or hardwired changes, hire a licensed electrician.

Voltage: the difference that matters most

In the U.S., G4 bulbs are often 12V, though 6V and 24V versions exist, and G9 bulbs are often 120V. Both base types are recognized in the American market: the ENERGY STAR Lamps specification (Version 2.0, effective January 2, 2017) lists G4 and G9 among the standard bases for lamps rated at 120, 240 or 277 VAC, or 12 or 24 VAC or VDC.

That range is a good reminder: the base code does not guarantee voltage. Treat “G4 means 12V, G9 means 120V” as a strong clue, then confirm on the fixture label.

What a transformer does

A low-voltage G4 fixture has a transformer or electronic power supply that steps household current down to 12V. It may be built into the fixture, tucked in a canopy, or sitting in a wall or cabinet. A G9 fixture runs straight off line voltage, so it has no transformer.

This changes how you upgrade:

  • G4: the bulb you buy must match the transformer’s output, including AC or DC. If the fixture supplies 12V, buy a 12V bulb.
  • G9: the bulb must be rated for 120V, and for the fixture type, such as an enclosed or damp-location one if that applies.

Choosing between G4 and G9 when you are buying a fixture

Often you do not choose the base, the fixture does. But when you are picking a new lamp or chandelier, the base shapes what you will live with.

G4 fixtures suit delicate, small-scale pieces where a tiny light source is part of the design: compact accent lamps, small pendants, cabinet and display lighting. The capsules are very small, so they sit neatly inside crystal, glass or fine metalwork.

G9 fixtures show up in chandeliers, wall sconces, pendants, ceiling fixtures and vanity lights. Because they run on household voltage, the system is simpler: no transformer to hide, replace or match. Norman Lamps notes its 120V G9 LED is suited to chandeliers, pendants, sconces, ceiling fixtures, vanity lights and under-cabinet installations, which reflects how widely the base is used.

LEDVANCE adds that G-series bulbs are commonly used at home for under-cabinet kitchen lighting, decorative bathroom accents and small task lighting in display cases, in its U.S. bulb size guide.

Choosing between G4 and G9 when you are buying a fixture

Lighting a display or collection

Small capsules are popular for showcasing objects because they are compact and put out a crisp, focused light. If you collect sculpture, glass or stones, this look at mineral display lighting shows how light quality changes what a piece looks like, which is worth keeping in mind when you pick your color temperature.

Halogen or LED: what to check before you buy

Both bases come in halogen and LED. LED is the usual upgrade, since it runs cooler and uses far less power. TECO’s product FAQ states that G4 LEDs can save 80% to 90% of the energy of traditional G4 halogens. For a wider look at the trade-off, see our piece on LED vs incandescent bulbs for the home.

Before you buy, run through this list.

1. Base and voltage

Match pin shape, then voltage. This is the step that prevents returns.

2. Physical size

These capsules are small, but fit still varies. A G4 LED’s dimensions have to fit inside the fixture, as TECO’s FAQ points out, and some LED versions are longer or wider than the halogen capsule they replace. Measure the gap in tight glass shades or enclosed globes.

3. Brightness in lumens, not watts

Wattage describes power draw, not brightness. Compare lumens instead. As a rough reference, a G9 halogen of 35 to 60 watts is typically replaced by an LED using only a few watts. Shop by the lumen figure on the package, and aim for the brightness your room needs. For the table-lamp end of things, our bedside lamp wattage guide explains how to think in lumens.

4. Color temperature

Warm white (around 2700K to 3000K) gives the glow most people want in living rooms and bedrooms. Cooler white and daylight suit kitchens, bathrooms and workspaces. Our guide to soft white vs. daylight bulbs helps you choose by room.

5. Dimming

Not every LED dims, and not every dimmer works with every LED. Look for “dimmable” on the package and use a dimmer rated for LEDs. Our explainer on whether LED bulbs are dimmable covers the details.

6. The transformer (G4 only)

Old halogen transformers can need a minimum load to run properly. TECO’s FAQ says traditional transformers often require a minimum electrical load, for example 20W, and that flicker in G4 LEDs usually comes from voltage instability or transformer incompatibility. Swapping in a dedicated LED driver usually solves it. If a new LED flickers or buzzes, our 10-minute LED flicker diagnosis can help you narrow it down by symptom. Replacing a transformer is a job for a licensed electrician if it is hardwired.

7. Enclosed or damp locations

If the fixture is enclosed, or sits in a bathroom or over a tub, check that the bulb’s packaging says it is suited to that use.

Common mistakes

  • Buying by size alone. Both are tiny capsules, so they look similar on a shelf. Check the pins.
  • Assuming G9 means 230V. That is the European rating. In North America, G9 is 120V per the bi-pin reference and SizeSheet.
  • Ignoring the transformer on G4 fixtures. A bulb that fits the pins can still flicker if the transformer is wrong for it.
  • Touching halogen glass. Skin oils can shorten a halogen capsule’s life. Handle with a cloth.
  • Replacing with a higher wattage. Stay at or below the fixture’s maximum rating.

Perspective

We see the same pattern again and again: someone falls in love with a fixture, then hits a wall at the bulb. The bulb base is a small detail, but it decides how the whole piece performs.

Our view is simple. Choose the fixture for its proportion and style first, because that is what you will see every day. Then read the base and voltage on the product page before you add it to your cart, so the bulb is never a surprise.

A G9 fixture is a straightforward choice if you want a chandelier, sconce or pendant on household current with no transformer to think about. A G4 fixture rewards you with a delicate, compact light source in small-scale pieces, as long as you match the transformer. Layer either one with ambient light from a ceiling fixture and a floor or table lamp, and the room feels curated rather than merely lit.

That is why we pair decorative lighting with complementary accents in one place. When the light, the lamp and the sculpture on the shelf are chosen together, the result looks intentional.

Norm Blain

Matching the bulb to the room

Once the technical match is right, the bulb is part of the design.

  • Dining room chandelier: a warm, dimmable bulb keeps the table flattering. If your fixture takes G9, a warm-white LED on a compatible dimmer makes it easy to move from dinner to dishes.
  • Bathroom vanity: brighter, slightly cooler light helps with grooming, so a neutral or cool white works well, provided the bulb is rated for the location.
  • Kitchen under-cabinet: small capsules put light right on the counter. Confirm voltage and fit with the existing strip or puck before you buy.
  • Living room accent lamp: go warm and dimmable, and keep the lumens modest so it supports the room’s main light.
  • Smart upgrades: if you want app or voice control in a chandelier, see our guide to smart bulbs in chandeliers, and for lamps, smart bulbs for table lamps. Smart options differ by base, so confirm yours is offered.

If you want more on how bulb shape and finish affect a room, read what a decorative bulb is.

Find a fixture that fits your style

If you are replacing a fixture rather than just a bulb, it is a good moment to rethink the whole layer of light in the room. At Lights & Things you can browse ceiling lighting, floor and table lamps, and curated home decor in one place, with free shipping thresholds, easy returns, secure checkout, price matching and financing to make larger purchases comfortable.

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Start with our floor and table lamp collection, or see everything on the Lights & Things homepage. Before you order, check the bulb type listed on the product page so you know what to stock.

FAQ

Is G9 brighter than G4?

Not by base alone. Brightness depends on the bulb’s lumens, and both bases come in a range of outputs. G9 capsules are slightly larger and are often used where a bit more light is wanted, but compare lumens on the package rather than the base name.

Do G4 bulbs need a transformer?

Most U.S. G4 bulbs are rated 12V, so the fixture supplies power through a transformer or electronic power supply. Some exist at 6V or 24V. Match the voltage the fixture provides, and check whether it needs AC, DC or both.

Can I replace a halogen G9 with an LED?

Usually yes, if the LED is rated for 120V, fits the fixture’s space and is dimmable if you use a dimmer. LED versions use a fraction of the power of a halogen of the same base, per the bi-pin base reference, which notes G9 is also used for LED lamps. Confirm size and enclosure suitability first.

How do I know if my bulb is G4 or G9 if the label is worn?

Look at the pins. Straight pins 4 mm apart mean G4, and looped pins 9 mm apart mean G9. Bring the old bulb with you when you shop, or compare it against the true-to-size diagram on the package.

Why does my new G4 LED flicker?

The most common cause is an old transformer that does not suit low-wattage LEDs. A dedicated LED driver usually fixes it. Have a licensed electrician handle any hardwired transformer change.

Sources

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