Light Bulb Base & Socket Chart

How to read a base code

The letter or letters name the family: E is Edison screw, GU and G are pin bases, B is bayonet, MR is a multifaceted reflector, and R7s is a linear double-ended lamp. The number that follows is the key dimension, almost always in millimetres: E26 is a 26 mm screw, GU10 has 10 mm between its pins, G9 has 9 mm between its pins, and B22 is a 22 mm bayonet. MR is the exception that proves the rule: MR16 means the reflector is 16 eighths of an inch across (2 inches), while its pins are separately specified as GU5.3 — 5.3 mm apart. Voltage is not in the code at all, which is why the chart states it separately: GU10 is line voltage (120 V in the US), while MR16/GU5.3 is low voltage (12 V) and needs a transformer. A bulb that fits mechanically but runs at the wrong voltage is a bulb that fails fast or dangerously.

Light bulb base chart

Print this table before a hardware-store trip: the code on the old bulb is the whole answer, and this chart translates it. If the old bulb's code is worn away, measure the screw diameter or the pin spacing in millimetres and match it to the diameter column.

Two confusions to avoid

E26 vs E27: nearly the same, not quite. E26 is 26 mm and E27 is 27 mm — one millimetre apart. In practice an E27 bulb will usually screw into an E26 socket and light up, and an E26 bulb will usually fit an E27 socket, because the thread pitch is the same and the difference is within the tolerance of most sockets. But "usually" is not "always": a loose fit can mean poor contact, flicker, or a bulb that works its way out with vibration, and a bulb rated for 230 V European mains must never be assumed correct for a 120 V US fixture (or the reverse) just because it screws in. Buy the code your fixture specifies; treat cross-fitting as a stopgap, not a plan.

GU10 vs MR16: do not interchange — line voltage vs low voltage. GU10 and MR16 spotlights look alike in a ceiling, and both are sold as small reflector lamps, but they are different systems. GU10 is a twist-lock base running directly on 120 V line voltage, with no transformer. MR16 (GU5.3) is a bi-pin base running on 12 V low voltage through a transformer. The pins are different shapes and spacings (10 mm twist-lock vs 5.3 mm straight pins), so they should not physically interchange — but adapters exist, and an adapter that puts a 12 V lamp on 120 V, or a 120 V lamp on a 12 V transformer, is a fast way to destroy the lamp and risk the fixture. If the old lamp twists to lock, it is GU10; if it pushes straight in with two thin pins, it is MR16/GU5.3. Match the system, not just the shape.

Output · formula · meaning

Who uses this

• The homeowner standing in the lighting aisle: reads E26 off the dead bulb, confirms 26 mm Edison screw here, and buys the right replacement instead of guessing between the lookalike boxes.
• The renter replacing a chandelier bulb: looks up E12, learns it is a 12 mm candelabra base, and stops trying to force a standard E26 bulb into a socket half its size.
• The DIYer rewiring a spotlight: checks GU10 vs MR16 before buying, discovers the existing track is 12 V MR16, and buys the low-voltage lamp plus the right transformer instead of a line-voltage lamp that would fail.
• The landlord stocking spares for several properties: prints the chart, notes which properties use E26, which use GU10, and which use B22, and stops sending the wrong spares to the wrong buildings.

Accuracy & limitations

Diameters and pin spacings in the chart are the nominal dimensions the codes are defined by, and they are exact as definitions; real bulbs vary by fractions of a millimetre with manufacturing tolerance, which is why E26 and E27 cross-fit in practice. Wattages are typical ranges for the fixtures listed, not ratings for any specific bulb — always check the wattage printed on the bulb and the maximum wattage printed on the fixture, and never exceed the fixture rating, especially with halogen lamps that run hot. This chart covers the common household and commercial bases only; specialty bases (automotive, medical, stage, and miniature electronics) are outside its scope, and a code not in the chart should be looked up by its full manufacturer specification rather than guessed from a similar code. Error policy: an empty or unknown base code is reported as an error rather than guessed, so no lookup ever returns a plausible-looking wrong base.