What Bulb For

Light bulb bases and shapes

Each bulb base and shape on its own page: what it looks like, where it's used and what it gets mixed up with.

GU10 bulbsThe push-and-twist spotlight base on household 120 volts, the bulb shapes it comes on, and how it differs from 12 volt MR16s. MR16 bulbsThe small reflector spotlight on a 12 volt GU5.3 base with a transformer, or a 120 volt GU10, and which LEDs work on the transformer. G9 bulbsThe small capsule bulb with two looped pins, on 120 volts, as halogen or LED: lengths, dimming and the fixture's watts. GU24 bulbsThe twist-lock pin base on ordinary bulb shapes such as the A19, in LED and CFL, and why it doesn't swap with E26. E12 candelabra bulbsThe small candelabra screw base: the candle, globe, fan and night light bulbs it comes on, and E12 vs E11, E14 and E17. E17 intermediate bulbsThe intermediate screw base between E12 and E26: microwave, refrigerator, sign and some ceiling fan bulbs. E26 vs E27The US and European medium screw bases: close in size, used on different voltages, so match the one made for your country. A19 vs A21The standard bulb in four sizes: how wide A15, A19, A21 and A23 are, how long, and whether the bigger one fits. A15 bulbsThe standard bulb shape at 1-7/8 inches across, on E26, E12 or E17, for ceiling fans and appliances. BR30 vs PAR30Two 3-3/4 inch reflector bulbs with different beams, plus R30 and short or long neck PAR30s. PAR38 bulbsThe 4-3/4 inch reflector flood on the E26 base: lengths, wet or damp ratings bulb by bulb, dusk-to-dawn PAR38s and dimming. Edison bulbs: ST19The ST19 and the other filament shapes (ST15, ST20, T10, G25): widths, lengths, bases and what to check on the box. Chandelier bulbsCandle bulb shapes for chandeliers and sconces (B10, B11, BA10, CA10, CA11), their tips and sizes, on E12 or E26. E39 mogul bulbsThe large mogul screw base: 3-way torchiere lamps, corn-cob LEDs for garages and yards, and what to leave to an electrician. T8 vs T12How wide T8 and T12 tubes are, the watts and light of some 4 ft. tubes, and what an LED tube needs to fit either. T8 LED tubesWhich LED tube fits a fluorescent fixture: Type A on the ballast, Type B wired by an electrician, or Type AB. Measure a bulbMeasure the glass across its widest part in eighths of an inch to get the shape code's number, with a chart.

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General guidance from manufacturer and standards data. Products vary, so check the label and the maker's instructions.