Base types: E26, GU10, G9 and how to identify yours
The base code is the one specification where being close is the same as being wrong.
What this covers
- What the code means
- E26 and E12
- GU10
- G9
- MR16 and the voltage question
- A19, ST58 and shape codes
- The reliable identification method
- Physical size beyond the base
- Enclosed fixture ratings
What the code means
Base codes describe the fitting. The letter indicates the type – E for Edison screw, G for pin-based – and the number is usually a dimension in millimetres. E26 is an Edison screw twenty-six millimetres across, which is the standard American household socket.
E26 and E12
E26 is the standard screw base for most household lamps and fixtures. E12 is the candelabra base, much smaller, used in chandeliers, decorative fittings and some appliances. They are not interchangeable and adapters exist but change the geometry of the fitting.
GU10
A twist-lock base with two stubby pins that push in and rotate to lock. Common in track lighting and recessed downlights. The pins are set at a fixed spacing, and GU10 should not be confused with the similar-looking GU5.3 or MR16 pin bases used in low-voltage systems.
G9
A small bi-pin base with two looped wire pins that push directly into the socket. Widely used in modern wall lights and chandeliers where the bulb is small and partly visible. G9 bulbs are compact and the sockets are unforgiving of the wrong pin spacing.
MR16 and the voltage question
MR16 describes the reflector shape rather than the base, and MR16 lamps come in both mains-voltage GU10 and low-voltage GU5.3 versions. The low-voltage type needs a transformer. Getting this wrong means a bulb that either does not fit or does not light.
A19, ST58 and shape codes
These describe the bulb envelope rather than the base. A19 is the classic pear shape on an E26 base. ST58 is a tubular Edison style, also usually E26. Shape affects appearance and how the light spreads; the base determines whether it fits.
The reliable identification method
Bring the old bulb. Photographs and descriptions both go wrong, and the base is unmistakable in the hand. Where that is not possible, photograph the base against a ruler and count the pins or measure the screw diameter.
Physical size beyond the base
A bulb can have the right base and still not fit inside a shade or a recessed housing. Check the overall length and diameter against the fitting, particularly with larger decorative bulbs and with enclosed fixtures.
Enclosed fixture ratings
Some LED bulbs are not rated for fully enclosed fittings, because trapped heat shortens their life considerably. If the fixture is sealed, look for a bulb rated for enclosed use. This is a common cause of LED bulbs failing far earlier than expected.
How this applies when you are buying
Three numbers decide whether a bulb is right. The base code must match the socket. Lumens, not watts, tell you how bright it is. Kelvin tells you the colour: around 2700K is warm, 4000K neutral, 5000K and above is daylight. Mixing colour temperatures in one room is the thing people most often regret.
LED bulbs listed by base type, wattage equivalent, lumen output and colour temperature, in single and multi-packs.
On the shelf at the moment
These come straight from the A & E Bowery Lighting shelf as you read this, not from a list typed when the article was written, so the prices and the availability are current.
- Energetic 24-Pack A19 LED Light Bulb, 60 Watt Equivalent, Daylight 5000K, E26 Medium Base – $20.49
- Aipsun Black Modern LED Wall Sconce Hardwired Indoor Wall Lights Set of 2 Up and Down Wall – $6.64
- 10 Pack GU10 Led Bulb 6W Equivalent 50W, MR16 Led Bulb 5000K Daylight White Led Bulbs – $13.59
Where to go from here
All of the above is the reasoning we use behind the counter at A & E Bowery Lighting when someone asks. If you are weighing up two options and the listing does not settle it, ask: +1 212-219-3888 during opening hours, or service@bowerylights.shop, answered within one business day.
If you order from us
Standard shipping is a flat $11.95 anywhere in the United States, and it is free once the order passes $99. Weekday orders normally leave A & E Bowery Lighting the next working day, with three to seven business days in transit after that. Unopened items can go back within 30 days of delivery. Shipping Policy · Refund and Returns
Related sections of the shop
Related reading
- Colour temperature: choosing between 2700K and 5000K – The number that decides whether a room feels comfortable or clinical.
- Lumens, watts and what actually makes a room bright – Wattage equivalence is a rough translation. Lumens is the measurement.
- Dimming LEDs: why some flicker and how to avoid it – Two components have to agree, and the bulb is only one of them.
Who wrote this
Written by the team at A & E Bowery Lighting, a lighting, bulbs and fixtures shop at 131 Bowery, New York, NY 10002. The guidance here is what we tell customers across the counter, and it is based on the products we actually stock and the questions we are actually asked. Where we are not certain of something, we say so rather than fill the gap. More about the shop · Contact us
