# G4 vs G9 bulbs: straight pins or wire loops > Learn how G4 and G9 bulb bases differ in pin spacing, voltage, shape, and fit, plus the checks to make before installing a replacement. Canonical page: [https://getbulbmatch.com/articles/g4-vs-g9-light-bulbs/](https://getbulbmatch.com/articles/g4-vs-g9-light-bulbs/) Published: 2026-08-17 Last reviewed: 2026-08-21 Category: Bulb bases and compatibility ## Short answer G4 and G9 are not interchangeable: G4 has two straight pins on 4 mm centers, while G9 uses looped contacts spaced 9 mm apart. In U.S. products, G4 is often 12 V and G9 often 120 V, but the base does not guarantee voltage; match the fixture label, voltage, and full lamp dimensions. ## G4 vs G9 bulb comparison The pins provide the quickest visual distinction, but voltage and capsule dimensions must still come from the exact lamp and fixture. | Check | G4 | G9 | | --- | --- | --- | | Contact style | Two thin straight pins | Two U-shaped wire loops | | Nominal spacing | 4 mm center to center | 9 mm center to center | | Installation | Push straight in | Push straight in | | Common U.S. voltage pattern | Often 12 V; verify | Often 120 V; verify | | Typical shape | T3 / JC capsule or MR11 | T4 / JCD capsule | | Interchangeable? | No | No | ## The physical difference between G4 and G9 A G4 base has two thin, straight pins with a nominal center-to-center spacing of 4 millimeters. A G9 base has two looped wire contacts with a nominal spacing of 9 millimeters. The loop shape is usually the quickest visual clue when the printing on an old capsule is unreadable. These codes identify the base interface, not the entire bulb. Terms such as capsule, T3, and T4 describe the lamp body or shape separately. The supplied manufacturer sample includes G4 LEDs described as capsules, T3, and T4, while several G9 examples use a T4 body. Matching a familiar-looking glass or plastic capsule is therefore not enough. Do not bend straight pins, squeeze loops, or force a lamp into a socket. A correct base should align with the contacts without modification. G4, GU4, GY4, and other similar codes can share a nominal pitch while differing in pin dimensions or other standardized details, so copy the complete base code. - G4: two straight pins, 4 mm nominal center spacing. - G9: two looped contacts, 9 mm nominal spacing. - Base code and bulb shape are separate specifications. ## Voltage is a separate compatibility check The number in a G-base designation describes the contact arrangement; it is not a dependable voltage label. In the limited U.S. catalog sample supplied for this article, the listed G4 LEDs are 12-volt products and the listed G9 LEDs are 120-volt products. That is a useful recognition pattern, not a universal rule. For example, the focused Feit G4 model is marked for 12-volt AC operation only, while the cited Feit and Westinghouse G9 examples are rated 120 volts. Those are model facts, not definitions of the bases, which is why the base alone cannot settle the electrical question. Read the voltage on the fixture label, old lamp, transformer, or fixture documentation. A low-voltage fixture commonly supplies the lamp through a transformer or electronic power supply. The replacement must match the specified voltage and any AC, DC, or AC/DC marking. Never connect a lamp marked 12 V to a 120 V supply, even if someone has altered the socket. - Match the stated voltage exactly. - Check whether the lamp requires AC, DC, or supports both. - Treat common U.S. voltage patterns as clues, not guarantees. ## Check the fixture rating and light output separately The fixture label may name the required base and set a maximum lamp wattage. Stay at or below that limit using the replacement lamp’s actual power consumption. An LED package’s “20 W replacement” or “40 W equivalent” statement compares its intended light output with an older lamp; it is not the LED’s electrical wattage. Brightness is measured in lumens. If the old lamp’s lumen value is known, compare that directly. Otherwise, its wattage-equivalent description can be a rough starting point, but light distribution also affects the result. In the limited sample, G4 and G9 LEDs with the same base are offered at multiple lumen levels, so base code alone says nothing about brightness. Color appearance is another independent choice. Match the old lamp’s Kelvin value when you want the room to look unchanged. A 3000 K lamp and a 5000 K lamp can share the same base, voltage, and wattage while producing noticeably different-looking light. - Use actual lamp watts for the fixture’s wattage limit. - Use lumens to compare brightness. - Use Kelvin to match light appearance. ## Confirm dimming and low-voltage equipment compatibility A dimmable fixture does not make every replacement lamp dimmable. The lamp itself must be marked dimmable, and its manufacturer should identify compatible dimmers or control types. Even among the G4 examples in the supplied sample, some models are dimmable and others are not. With a low-voltage G4 installation, the dimmer, transformer or power supply, and LED lamp operate as a system. Check the lamp and fixture documentation for compatible equipment. Flicker, a limited dimming range, failure to start, or light that remains on at the lowest setting can indicate an unsuitable combination rather than the wrong pin spacing. A non-dimmable lamp is appropriate only when the circuit is not controlled by a dimmer. Do not assume that leaving an incompatible dimmer at full output resolves the issue unless the lamp or control manufacturer specifically permits it. - Look for an explicit dimmable marking on the lamp. - Check the recommended dimmer list when available. - For G4, include the transformer or power supply in the compatibility check. ## Measure clearance and read environmental ratings Correct pins do not guarantee a physical fit. LED capsules can be wider or longer than the halogen lamps they replace because they contain electronic components. Measure maximum overall length and maximum diameter, then account for the direction in which the lamp enters the socket, nearby glass, retaining clips, and the fixture cover. Enclosure suitability is separate from base, shape, and voltage. Fully enclosed fixtures retain heat, which can affect LED performance and life. Use a replacement specifically approved for an enclosed fixture when the lamp compartment is sealed. The Feit and Westinghouse examples cited here are both listed for enclosed use, but that property must be checked model by model. Location ratings are separate again. A damp-location marking addresses moisture exposure; it does not by itself establish suitability for direct rain, water contact, or a sealed enclosure. For outdoor use, the fixture and lamp must have ratings appropriate to the actual location, and the fixture instructions remain controlling. - Measure overall length and diameter before ordering. - Verify enclosed-fixture suitability independently. - Do not treat a damp rating as a wet-location rating. ## What to watch after installation Turn off the fixture and allow the old lamp to cool completely. Unplug a portable fixture when practical. Remove a straight-pin G4 lamp by pulling it evenly outward; remove a G9 according to the fixture instructions, without twisting unless the holder specifically calls for it. Inspect the holder before fitting the new lamp. Stop if contacts are burned, cracked, loose, or badly corroded. Handle the replacement as directed on its package and keep the contact pins clean; use a clean, dry cloth if the manufacturer calls for one. Insert the correctly matched lamp without forcing it, restore power, and observe startup and dimming behavior. If it flickers, buzzes, cycles off, or fails to start, switch it off and recheck voltage, AC/DC marking, dimmer compatibility, transformer compatibility, and seating. The exact fixture label and model documentation take priority over general conventions. - Switch off power and let the lamp cool. - Pull evenly and avoid stressing the socket. - Stop if the socket is damaged or the lamp requires force. ## Replacement checklist - Copy the complete base code from the old lamp or fixture label. - Confirm 4 mm straight pins for G4 or 9 mm looped contacts for G9. - Match voltage and AC, DC, or AC/DC requirements. - Keep actual lamp wattage within the fixture limit. - Compare lumens and Kelvin separately. - Verify lamp, dimmer, and transformer compatibility. - Measure overall length, diameter, and cover clearance. - Check enclosure and damp or wet-location ratings separately. ## Frequently asked questions ### Can a G4 bulb replace a G9 bulb? No. Their contact shapes and spacing differ, and their typical U.S. operating voltages also differ. Use the exact base and electrical rating specified by the fixture. ### Does G4 always mean 12 volts? No. Many household and landscape G4 products are rated 12 V, but G4 lamps also exist at other voltages. Read the fixture and lamp markings instead of inferring voltage from the base. ### Does G9 always mean 120 volts in the United States? No. Many U.S.-market G9 lamps are 120 V, including the manufacturer examples used here, but G9 identifies the base rather than guaranteeing the supply voltage. ### Can any G4 or G9 LED be used in an enclosed fixture? No. Enclosed-fixture suitability is a model-specific rating. Check the lamp package or technical sheet because retained heat can affect an LED that is not approved for that application. ### Why does a correctly sized G4 LED flicker? Possible causes include an incompatible dimmer, transformer, or power supply, an AC/DC mismatch, poor socket contact, or a faulty lamp. Recheck the complete electrical system and manufacturer compatibility guidance. ## Primary sources - [LEDVANCE — Guide to Light Bulb Shapes, Types, and Sizes](https://www.ledvance.com/en-us/professional-lighting/insights/blog/lighting-basics/guide-light-bulb-shapes-types-and-sizes) (accessed 2026-08-21) - [Feit Electric — 2W 20W Replacement Warm White G4 Base T3 Dimmable Specialty LED](https://www.feit.com/products/20-watt-equivalent-dimmable-warm-white-g4-base-capsule-specialty-led-bp20g4-830-led) (accessed 2026-08-21) - [Feit Electric — 3.5W 35W Replacement Warm White G9 Base T4 Dimmable Specialty LED Bulb](https://www.feit.com/products/35-watt-equivalent-dimmable-warm-white-g9-base-t4-specialty-led-bp35g9-830-led) (accessed 2026-08-21) - [Westinghouse Lighting — 5 Watt 40 Watt Equivalent G9 Dimmable LED Light Bulb](https://www.westinghouselighting.com/light-bulbs/led-bulbs/specialty/5-watt-%2840-watt-equivalent%29-g9-dimmable-led-light-bulb-5164100.aspx) (accessed 2026-08-21) - [Bulbrite — LED T4 G9 120V 4.5W 500LM 3000K Dimmable](https://bulbrite.com/products/770597) (accessed 2026-08-21) - [ANSI / NEMA — ANSI NEMA C81.61-2025 — Specifications for Bases for Electric Lamps](https://www.nema.org/standards/view/american-national-standard-for-electrical-lamp-bases-specifications-for-bases-for-electric-lamps) (accessed 2026-08-21) ## Editorial note BulbMatch is an independent reference and does not sell products. A matching bulb code is only one compatibility check; verify voltage, dimensions, actual wattage, fixture ratings, dimming, location, and the fixture manufacturer's instructions where applicable.