Scan the code on the bulb or socket, measure the diameter or pin spacing in millimeters, then match the shape: screw (E), pins (G/GU), or bayonet (B).
Quick Steps To Identify Your Bulb Base
- Look for print on the bulb neck, metal cap, or the socket ring. Codes like E26, E12, GU10, G9, B22d are typical. Some fixtures print the base near “MAX WATT” text.
- Note the base shape. Threads mean Edison screw (E). Two pins mean a G family base. Two stubby posts with twist tabs mean GU10. Two side lugs with a push-and-turn action mean a bayonet (B).
- Measure: screw bases use the outer thread diameter in millimeters; G and GU use pin spacing; bayonet bases use shell diameter.
- Match your findings to the charts below. If you are between sizes, remeasure and check for regional variants like E26 vs E27 or E12 vs E14.
Base Families At A Glance
| Base Family | Common Codes | Where You See It |
|---|---|---|
| Edison screw (E) | E26/E27 medium, E12 candelabra, E17 intermediate, E39 mogul, E14 small EU | General lamps, ceiling fixtures, pendants, decorative chandeliers |
| Bi-pin straight (G) | G4, G9, G13, G5, GY6.35, GU24 | Under-cabinet pucks, T-series tubes, desk lamps, CFL twist-lock sockets |
| Twist-lock pins (GU) | GU10, GU5.3, GU4 | Track heads, recessed spots, small projectors |
| Bayonet (B) | B22d, BA15d, BA15s | Many UK, AU, and some retrofit lamps; indicator bulbs |
| Linear ends | R7s | Double-ended flood bars and security lights |
| Strip sockets | S14s, S14d | Bathroom strip bars and sign lights |
Need a reference with visuals while you compare? A clear brand chart from GE Lighting lists base groups and typical codes. For a quick sanity check on LED choices, the U.S. Department of Energy’s Energy Saver page gives care tips and selection basics. Bayonet users can see Philips guidance on the B22 cap on the official Hue site: B22 details.
Identifying A Light Bulb Base Type At Home
You do not need a caliper. A small ruler or tape in millimeters works fine. Set the lamp on a table, grab a notepad, and work through these checks.
Step 1: Read The Markings
Many bulbs print the base on the neck or metal shell. Fixtures often show it near the socket, on a sticker, or stamped on the shell. You might see “E26 120V,” “E12,” “GU10,” “G9,” or “B22d.” Write that down before it rubs off.
What A Socket Label Might Say
Common prints look like “E26 120V 60W MAX,” “E12 120V,” “GU10 230V,” “G4 12V,” or “B22d 240V.” The first part is the base. The next part is the supply. The last part on many labels is the watt cap. Match all three and you avoid misfits.
Step 2: Confirm The Shape
- Threads present? That is an E base. The number is the thread diameter in millimeters.
- Two pins straight out? That is a G base. The number is the pin spacing center-to-center in millimeters. A “Y” in the code adds thicker pins.
- Two pins with little block heads that twist in? That is a GU style. The number is the pin spacing.
- Two opposite lugs and a push-turn action? That is a B bayonet. Suffix letters like “d” or “s” show the contact style.
- Long tube with end caps? Look for G13 or G5 on T8, T12, or T5 tubes.
- Flat strip bar? Look for S14s or S14d along the back of the bar.
Step 3: Measure The Critical Dimension
Hold your ruler right at the feature that defines the base:
- E bases: measure the outside thread diameter. E26 measures about 26 mm; E12 about 12 mm.
- G and GU bases: measure from the center of one pin to the center of the other pin. GU10 is 10 mm; GU5.3 is 5.3 mm; G9 is 9 mm.
- B bases: measure the shell across. B22d is about 22 mm.
- Tubes: measure the pin spacing: G13 is 13 mm; G5 is 5 mm.
Step 4: Cross-Check Watts, Volts, And Shape
Match the base and the bulb shape your fixture expects. A pendant with “E26 60W MAX” wants a medium screw base. A puck light marked “G4 12V” needs a small bi-pin on a low-voltage driver. Track heads with “GU10 120V” take twist-lock spots.
Regional Base Codes And Common Mix-Ups
Edison screw sizes look the same at a glance, yet there are regional norms. E26 is common in North America and parts of Japan. E27 is common in many other regions. The thread diameter is close, yet fixture ratings and voltage differ by market. Use lamps that match your local supply and fixture label. Candelabra E12 and small EU E14 are a similar case: both small, not the same thread.
Pin bases also create mix-ups. A GU10 twists and locks with a quarter turn. A GU5.3 simply pushes in. Both drive spot lamps, yet sockets are not the same. G9 has looped pins that push in, while G4 uses two thin straight pins. Bayonet B22d lamps push in and turn; many screw sockets cannot take them at all. When in doubt, rely on the measurement and the photo cues in the sections above.
Decoding The Letters And Numbers
What The Letter Means
- E = Edison screw thread
- G = two pins
- GU = two pins with a twist-lock head
- B = bayonet with side lugs
- R7s = recessed double contact at each end of a linear lamp
- S14 = strip bar with back contacts
What The Number Means
- E family: the thread diameter in millimeters: E26 ≈ 26 mm, E27 ≈ 27 mm, E12 ≈ 12 mm, E39 ≈ 39 mm, E17 ≈ 17 mm.
- G/GU family: the pin spacing in millimeters: GU10 = 10 mm, GU5.3 = 5.3 mm, G9 = 9 mm, G4 = 4 mm, GU24 = 24 mm.
- B family: the shell diameter in millimeters: B22d = 22 mm, BA15d = 15 mm, BA15s = 15 mm with single contact.
Close Variations That Trip People Up
E26 Versus E27
Both are medium screw sizes that look alike. Many lamps with one size will seat in the other, yet the safe pick is to buy for the region and supply printed on the fixture. If you moved or imported a lamp, check the label and match it.
E12 Versus E14
Both are small screw sizes on decorative lamps. E12 is common in the U.S. and Canada. E14 is common across large parts of Europe. They are not cross-fit sizes. Look at the socket stamp and measure the thread before you order a bunch of chandelier bulbs.
GU10 Versus GU5.3
Both are used on spots and tracks. GU10 pins are short, blocky, and twist to lock in place. GU5.3 pins are thin and push straight in. The socket shape gives it away before you measure.
G9 Versus G4
G9 uses looped wire pins on grid adapters and mini pendants. G4 uses two thin straight pins on tiny pucks and accent heads. Both are compact. The pin style and spacing settle the answer in a second.
Find The Match By Measurement
Use this guide when the print has faded. Grab your ruler and compare your number to the ranges here.
Measurement Cheat Sheet
| Measure You See | Likely Base Code | Notes |
|---|---|---|
| ~26 mm thread | E26 | North America medium screw on A19, BR30, ST19 |
| ~27 mm thread | E27 | Medium screw common outside North America |
| ~12 mm thread | E12 | Chandeliers, sconces, night-lights |
| ~17 mm thread | E17 | Intermediate base on ceiling fans and signs |
| ~39 mm thread | E39 | Mogul base on high output lamps |
| 10 mm pin spacing | GU10 | Twist-lock spots, 120–240 V mains |
| 5.3 mm pin spacing | GU5.3 | MR16 low-voltage push-in |
| 9 mm pin spacing | G9 | Loop pins on grid adapters and mini pendants |
| 4 mm pin spacing | G4 | Micro bi-pin on pucks and accents |
| 22 mm shell | B22d | Bayonet push-turn common in UK and AU |
| 15 mm shell | BA15d/BA15s | Signals, vans, and some marine fittings |
| 13 mm tube pins | G13 | T8/T12 fluorescent and LED tube retrofits |
| 5 mm tube pins | G5 | T5 fluorescent and LED tube retrofits |
How The Base Ties To Bulb Shape
Base and shape often travel together. Medium screw E26 or E27 shows up on A-shape bulbs, vintage ST, BR floods, and many globes. E12 rides with candle shapes and small globes. GU10 pairs with short PAR or GU10 spotlight forms. GU5.3 rides with MR16. G9 rides with small grid adapters and minimalist pendants. Tubes use G13 or G5. Bayonet B22d appears on A-shape and candle styles sold in bayonet markets.
Why Pins And Bayonets Exist
Some fixtures need a tight lamp focus or a shallow neck. Pins give a low profile and set the beam position. Bayonet sockets hold firm against vibration. The base style follows the job the fixture must do.
Safety, Ratings, And Fit
Match the base, match the voltage, and stay within the fixture watt cap. If the label shows “120V E26 60W MAX,” pick an E26 LED that draws less than 60 watts. If the label shows “GU5.3 12V,” that socket runs from a driver or transformer, so use the right low-voltage lamp.
Taking Measurements: Handy Tips
- Use millimeters. Base codes are tied to metric sizes.
- Write the code and size in your notes so you can shop once.
- If the thread is nicked, measure across two high points and average.
- For pin spacing, mark the pin centers with a pencil dot, then read center-to-center.
Two Quick Paths For Shoppers
Path A: You Know The Base Code
Search by that code at a trusted brand or retailer. Filters match the base to many shapes.
Path B: You Only Have A Ruler
Use the cheat sheet above, then cross-check with the first chart. After that, pick lumens for brightness, CCT for color, and CRI for color quality.
Taking Care Of Sockets And Bases
- Power off and let lamps cool before you swap.
- Do not force a fit. If threads feel gritty or pins fight you, back off and confirm the code.
- Wipe dust from the shell and socket rim with a dry cloth. Clean metal makes better contact.
- On threaded shells, a tiny dab of dielectric grease helps in damp areas. Keep it off the center contact.
- On bayonets, align the lugs with the slots, press, then turn until the stop.
Common Base Codes With Typical Uses
This list helps when you need quick recall during a project.
Screw Bases
- E26: standard medium in North America for A19, BR30, PAR38, ST19.
- E27: medium used across many regions for the same shapes.
- E12: small screw on candles and night-lights.
- E17: intermediate on fans, signs, and some retro lamps.
- E39: large mogul on high output lamps and yard lights.
Pin And Twist Bases
- GU10: twist-lock on mains-voltage spots and tracks.
- GU5.3: low-voltage MR16 push-in on cans and displays.
- G9: loop-pin grid adapters and compact pendants.
- G4: tiny pucks and accents.
- GU24: twist-lock CFL sockets and matching LED retrofits.
Bayonet And Specialty
- B22d: push-turn A-shape and candles in bayonet markets.
- BA15d/BA15s: signals, marine, vans.
- R7s: double-ended bars for floods and security lights.
- S14s/S14d: strip bars for mirrors and signs.
Identify Your Light Bulb Base With Confidence
Read the code, learn the shapes, and measure once. Start with the letter, confirm the number, and match it to the tables. The right lamp then seats cleanly and lights on the first try.