Light Bulb Base Sizes Chart: A Guide to the Right Fit

A light bulb base sizes chart makes it much easier to replace a bulb without guessing. The base is the part that connects the bulb to the socket, and it must match in shape, diameter, pin spacing, and voltage requirements. Start by reading the code printed on the old bulb or fixture. A code such as E26, E12, GU10, or G13 tells you the base family and size.

Key takeaways - The bulb base must match the socket. A bulb that looks similar may not fit or work safely. - On Edison screw bases, the number is the base diameter in millimeters. E26 is 26 mm wide; E12 is 12 mm wide. - Pin and twist-lock bases use letters and numbers to describe the pin style and spacing. - Measure the base, not the glass bulb, when you need a replacement. - Match the fixture's voltage and maximum wattage, not just the base type.

Light bulb base sizes chart for common household bulbs

Use this chart as a quick reference. The most common base in US table lamps, floor lamps, and standard ceiling fixtures is E26. Smaller decorative fixtures frequently use E12. Recessed lights and under-cabinet fixtures may use GU10 or bi-pin bases instead.

Base code	Common name	How it connects	Base measurement	Common uses
E10	Miniature Edison screw	Screw	10 mm diameter	Small flashlights, novelty lights
E11	Mini-candelabra screw	Screw	11 mm diameter	Some specialty and halogen fixtures
E12	Candelabra screw	Screw	12 mm diameter	Chandeliers, sconces, ceiling fans
E14	Small Edison screw	Screw	14 mm diameter	Some imported decorative fixtures
E17	Intermediate screw	Screw	17 mm diameter	Appliance lights, specialty fixtures
E26	Medium Edison screw	Screw	26 mm diameter	Most US lamps and household fixtures
E27	Medium Edison screw	Screw	27 mm diameter	Many international fixtures
E39	Mogul screw	Screw	39 mm diameter	Large outdoor and commercial fixtures
GU10	Twist-lock two-pin	Push and twist	10 mm pin spacing	Line-voltage recessed spotlights
GU24	Twist-lock two-pin	Push and twist	24 mm pin spacing	Some dedicated energy-saving fixtures
G4	Bi-pin	Push in	4 mm pin spacing	Low-voltage cabinet and landscape lights
G5.3	Bi-pin	Push in	5.3 mm pin spacing	MR16 low-voltage spotlights
G8	Loop-pin	Push in	8 mm pin spacing	Some under-cabinet and puck lights
G9	Loop-pin	Push in	9 mm pin spacing	Compact decorative and enclosed fixtures
G13	Medium bi-pin	Push in and turn	13 mm pin spacing	Linear fluorescent and LED tube fixtures

The letter before the number matters. An E base has Edison screw threads. A G base has pins, loops, or other contact points. The U in GU10 or GU24 refers to the pin shape and locking design.

How to read a light bulb base code

Most bulb codes give you useful physical information in only a few characters. They are not random product labels.

Edison screw codes: E12, E17, E26, and E39

Edison screw bases are the familiar metal threaded bases found on many household bulbs. The letter E means Edison screw. The number that follows is the outer diameter of the threaded base in millimeters.

For example:

E12 has a 12 mm threaded base and is called candelabra size.
E17 has a 17 mm threaded base and is called intermediate size.
E26 has a 26 mm threaded base and is called medium size in the US.
E39 has a 39 mm threaded base and is called mogul size.

An E26 bulb and an E27 bulb are very close in diameter, but they are not always interchangeable. Thread pitch, fixture design, voltage, and regional electrical standards can differ. Use the exact base called for by the fixture whenever possible.

Pin base codes: G4, G5.3, G9, and G13

G bases use metal pins or wire loops instead of threads. In many common G-base codes, the number describes the distance between the centers of the pins in millimeters.

A G4 bulb has two straight pins spaced 4 mm apart. A G5.3 bulb has pins spaced 5.3 mm apart. A G9 bulb uses two looped wire contacts 9 mm apart. These differences are small enough to confuse at a glance, so measuring is important.

G13 bases appear on many four-foot tube lights. Each end of the tube has two pins. The fixture's tombstone-shaped socket holds the pins, and you turn the tube a quarter turn to lock it in place.

Twist-lock codes: GU10 and GU24

GU bases have pins that are shaped to twist into matching slots. A GU10 bulb has short, thick pins with widened ends. You push the bulb into the socket and turn it to secure it.

GU24 bases also twist into place, but their pins and spacing differ from GU10. GU24 sockets were used in some fixtures designed to prevent use of standard screw-in bulbs. Never force one twist-lock bulb into a different twist-lock socket.

How to measure for a light bulb

The safest way to identify a base is to remove the old bulb after switching off power and letting it cool. Bring the bulb with you when shopping if you can. If the marking has worn off, a ruler or caliper will narrow it down.

Measure a screw base diameter

For a screw base, measure straight across the widest outer edge of the metal threads. Do not measure the glass, the brass contact at the bottom, or the socket opening.

A small ruler can identify clear differences:

About 12 mm is E12 candelabra.
About 17 mm is E17 intermediate.
About 26 mm is E26 medium.
About 39 mm is E39 mogul.

A digital caliper gives the clearest reading, but it is not required. Comparing the bulb to a known E26 household bulb also helps. An E12 base looks much slimmer, about half the width of an E26 base.

Measure pin spacing and pin shape

For a two-pin bulb, measure from the center of one pin to the center of the other. That is the pin spacing used in the base code.

Also look closely at the contacts. Straight, thin pins point to a bi-pin base such as G4 or G5.3. Bent wire loops suggest G8 or G9. Thick pins with mushroom-like ends indicate GU10.

Do not use pin length as your only clue. Two bulb bases can have similar pin lengths but different spacing or locking shapes.

Check the full light bulb dimensions

Base size tells you how the bulb connects. It does not tell you if the glass portion will fit inside a shade, globe, recessed can, or appliance compartment. Check the full light bulb dimensions before buying.

Measure these areas:

Maximum bulb width: Measure the widest part of the glass or plastic bulb.
Overall length: Measure from the bottom contact or pin ends to the top of the bulb.
Neck clearance: Check for a narrow opening in a globe or shade.
Socket depth: A deep recessed socket may need a longer neck, while a shallow cover may need a shorter bulb.

A bulb can have the correct E12 or E26 base and still be too wide for a small chandelier cup or too long for an enclosed fixture cover.

Which base size is most common in US homes?

E26 is the standard medium screw base in most US homes. You will see it in many table lamps, floor lamps, porch lights, ceiling fixtures, and garage fixtures. Its popularity does not make it universal.

E12 is the next common household size. It is used for flame-tip and globe bulbs in chandeliers, wall sconces, some ceiling fans, and decorative fixtures. E17 appears less often, mainly in appliance lights and specialty fixtures.

GU10 is common in recessed directional lighting. G4, G5.3, G8, and G9 are more likely in compact accent lights, under-cabinet fixtures, and low-voltage systems. G13 is associated with tube fixtures rather than a standard screw-in bulb.

This light bulb size chart identifies base size, not the bulb's outer shape. A19, BR30, PAR38, MR16, and T8 are shape or tube design labels. They answer a different question: what will the light-producing part look like and where will it fit?

Match the base to the fixture's power source

A correct physical fit is only one part of a safe replacement. You also need to match the fixture's power source and electrical requirements.

Many E26 and E12 household fixtures run directly on 120-volt household current in the US. A GU10 bulb also commonly runs on line voltage, though you should still read the fixture label. Small bi-pin bulbs such as G4 and G5.3 frequently run on 12 volts through a transformer or driver.

Using a 12-volt bulb in a 120-volt socket can destroy the bulb and create a hazard. Using a 120-volt bulb in a low-voltage fixture will not give you a safe workaround. The base style alone does not prove the voltage.

Check three places before installing a replacement:

The label inside the fixture or near the socket
The label on the old bulb
The fixture manual, especially for integrated drivers or specialty systems

Look for voltage, maximum wattage, bulb type, and any warning about enclosed fixtures. Stop and consult a qualified electrician when fixture markings are missing and the wiring or socket seems damaged.

Base size and energy efficiency are separate choices

Base size does not determine energy efficiency. An E26 socket can hold an efficient LED bulb or a less efficient older bulb design. The same is true for E12, GU10, and many pin bases.

After you match the base and voltage, choose the bulb based on light output, heat limits, dimmer compatibility, and fixture requirements. Light output is measured in lumens. Wattage shows how much electricity the bulb uses, not how bright every bulb will be.

LED replacements are available with many common base types. They use less electricity than older incandescent bulbs for similar light output and produce less heat at the bulb surface. Still, LED bulbs need enough room for heat to escape from their electronic parts.

For a closed globe or recessed fixture, look for a bulb labeled for enclosed fixtures. A standard LED in a tight, sealed cover can overheat and fail early. The fixture's maximum wattage label remains important, even with LED bulbs.

Light bulb sizing tips that prevent wrong purchases

A few checks take less than a minute and prevent most base-size mistakes. Use these light bulb sizing tips before putting a replacement in your cart.

Read the socket label before removing the old bulb

Many fixtures print the maximum wattage and bulb type near the socket. The label may say something like “Use E12, 40W max” or “GU10, 120V.” Take a clear phone photo before removing the bulb.

The fixture label is more reliable than guessing from the bulb shape. Previous owners may have installed the wrong bulb, especially in lamps with replaceable sockets.

Bring the old bulb or take clear photos

A photo of only the glass portion is rarely enough. Photograph the side of the base, the bottom contact, the printed code, and the socket. Place a ruler next to an unmarked base for scale.

A removed bulb is even better. Keep it in a small box or wrap it in a towel so the glass does not break during transport.

Account for shades, globes, and trim

Decorative fixtures have more clearance limits than open lamps. A wide globe bulb can hit the sides of a fabric shade. A long flame-tip bulb can show below a chandelier's cup. A reflector bulb may sit too far out of recessed trim.

Check your fixture from the side as well as the front. The visible part of the bulb affects appearance, while the hidden section affects heat and fit.

Do not rely on “standard” as a size description

“Standard base” is commonly used for E26 in the US, but the phrase is vague. “Standard bulb” can also refer to an A19 bulb shape. Look for the actual code, such as E26, rather than a casual label.

Check dimmer and smart fixture requirements

A bulb with the right base still needs to work with the controls in your fixture. A dimmer needs a dimmable bulb. A smart bulb needs the right type of wall control and enough space around its wider electronic housing.

For ceiling fans, choose bulbs rated for fan use when the fixture maker requires them. Vibration, enclosed shades, and frequent switching can shorten the life of an unsuitable bulb.

How to install a bulb without damaging the base or socket

Turn off the wall switch first. For ceiling fixtures, lamps with loose sockets, or any fixture that feels warm, unplug it or switch off power at the breaker as an added precaution. Let the old bulb cool completely.

Installing a screw-base bulb

Hold the bulb by its base or lower body. Align the threads and turn clockwise with gentle pressure. Stop when the bulb is snug and sits straight.

Do not overtighten it. Excess force can crack a glass bulb, damage a plastic base, or make later removal difficult. With a candelabra bulb, use a light grip because the narrow glass neck can be fragile.

Installing pin and twist-lock bulbs

Line up straight pins with the socket holes and push them in evenly. For GU10 and GU24 bulbs, insert the pins into the wide part of the socket slots, then twist until the bulb locks into place.

Never twist a straight bi-pin bulb unless the fixture instructions say to do so. Do not bend pins to make them fit. Bent contacts can create poor electrical connection and damage the socket.

After installation, restore power and test the light. Flickering, buzzing, sparking, a loose bulb, or a burning smell calls for immediate shutoff. Remove the bulb after it cools and inspect the socket. An electrician should assess damaged or discolored wiring.

Care and maintenance for bulb sockets and bases

Most bulbs need little care, but a clean, undamaged socket helps them make reliable contact. Turn off power before inspecting any socket.

Dust the outside of a bulb with a dry microfiber cloth after it is cool. In kitchens, remove greasy film from the fixture cover according to the manufacturer's care instructions. Do not spray cleaner into a socket or onto a hot bulb.

Look for these warning signs during a bulb change:

Brown or black marks inside the socket
A loose metal contact tab at the bottom of a screw socket
Cracked ceramic or melted plastic around pin sockets
Corrosion on pins or screw threads
A bulb that repeatedly loosens, flickers, or burns out early

Do not try to repair a damaged socket while power is live. Socket replacement involves electrical connections, so it is best handled by a qualified person when you are unsure.

Common mistakes when choosing a replacement bulb

The wrong base is not the only issue that causes a poor fit. These mistakes can leave you with a bulb that technically screws in but does not work well.

Confusing bulb shape with base size

An A19 is a common pear-shaped household bulb. It can have an E26 base, but A19 does not mean E26. A small globe bulb may use E12, E17, or E26 depending on the fixture.

Read both the shape designation and base code. The first addresses physical bulb shape; the second addresses socket fit.

Replacing a specialty socket with an adapter

Adapters exist for some base combinations, but they can add length, block a shade, trap heat, or create a voltage mismatch. They can also defeat the fixture's intended bulb limit.

An adapter is not a default fix for an unavailable bulb. Use the fixture's specified base and voltage whenever you can. For a hard-to-find specialty lamp, a lighting professional can help identify a suitable replacement.

Ignoring an enclosed-fixture rating

A bulb tucked inside a sealed globe runs hotter than one in open air. This matters most with LED bulbs, which have electronic drivers in their bases. Choose a bulb rated for enclosed use when the fixture has a closed cover.

Forcing a bulb that almost fits

An E12 bulb will not properly fit an E17 socket, even though both are small screw bases. A G4 bulb will not safely substitute for a G5.3 bulb. Resistance is a signal to stop, not to apply more pressure.

A simple checklist before you buy

Use this final checklist with the light bulb base sizes chart whenever you need a replacement:

Find the base code on the old bulb or fixture.
Confirm the fixture voltage and maximum wattage.
Measure the base if the code is missing.
Measure bulb width and length for shades, covers, and recessed trim.
Check for dimmer, enclosed-fixture, appliance, fan, or outdoor requirements.
Match the new bulb's base exactly, then choose the light output and color you prefer.

Taking those measurements once can save a return trip and protect your fixture. Keep a note of the base codes used in each room, especially for chandeliers, appliance bulbs, and recessed lights that use different sockets.

Frequently Asked Questions

What is the most common light bulb base size?

E26 is the most common light bulb base size in US homes. It is the medium Edison screw base used in many lamps and standard ceiling fixtures. Decorative fixtures commonly use the smaller E12 candelabra base, so check the socket or old bulb instead of assuming every screw base is E26.

Are E26 and E27 light bulb bases the same?

E26 and E27 bases are very similar but are not officially the same specification. E26 is common in North America, while E27 appears in many other regions. Their small diameter difference can seem unimportant, but voltage and thread standards can differ, so use the exact base listed on the fixture.

How do I know if I need an E12 or E26 bulb?

Measure the threaded metal base or read the code printed on the old bulb. An E12 base is about 12 mm across and looks narrow, while an E26 is about 26 mm across and is much wider. Chandeliers and sconces frequently use E12; standard lamps usually use E26.

Can I use a bulb with the same base but a higher wattage?

No, do not exceed the maximum wattage marked on the fixture. The base may fit, but a higher-watt bulb can create too much heat for the socket, wiring, shade, or cover. An LED with similar brightness and lower wattage can be a suitable option when it meets the fixture's other requirements.

What does GU10 mean on a light bulb?

GU10 describes a two-pin twist-lock base with pins spaced 10 mm apart. You push the bulb into a compatible socket and twist it to lock it. GU10 bulbs are common in recessed spotlights and directional fixtures, but you should also confirm the fixture voltage before replacing one.

Can I replace a G4 bulb with a G5.3 bulb?

No, a G4 bulb and a G5.3 bulb do not share the same pin spacing. G4 pins are 4 mm apart, while G5.3 pins are 5.3 mm apart. Forcing either bulb can bend the pins or damage the socket. Match the code exactly and confirm the low-voltage requirement as well.