How to Choose the Right Light Bulb: Base, Shape & Brightness Explained

Standing in front of a wall of light bulbs—or scrolling through hundreds of them online—is enough to make anyone freeze. The good news: almost every bulb purchase comes down to just three decisions. Get the base, the shape, and the brightness right, and you'll never bring home the wrong bulb again.

1. Start With the Base (How It Connects)

The base is the part that physically connects the bulb to the fixture, and it's the single most important spec. If the base is wrong, nothing else matters—it simply won't fit.

The most common base in North America is the E26 medium screw base, the standard "Edison" thread you'll find in most table lamps and ceiling fixtures. Smaller fixtures and decorative fixtures often use the E12 candelabra base, while large commercial fixtures use the E39 mogul base. Pin-based bulbs (like GU10, GU24, and G-series) twist or push into place instead of screwing in.

The letter tells you the base type and the number is the diameter in millimeters—so E26 is a 26 mm screw base. If you're not sure what you have, take the old bulb with you or measure the base. Our Bulb Bases & Filaments guide shows every common base side by side.

2. Match the Shape and Size

Bulb shapes are described with a letter-and-number code: a letter for the shape and a number for the diameter in eighths of an inch. A few you'll see constantly:

• A19 – the classic "light bulb" pear shape, the default for most lamps and fixtures.
• BR30 / BR40 – bulged reflector floodlights used in recessed cans.
• PAR20 / PAR30 / PAR38 – parabolic spot and flood bulbs for track and outdoor lighting.
• B10 / CA10 – candle and bent-tip shapes for chandeliers and sconces.
• G25 / G16 – globe bulbs for bathroom vanities and pendants.

Shape matters for both fit and looks—a bulb that's too large may not seat in an enclosed fixture, and an exposed bulb in a clear fixture becomes part of the décor. Our Bulb Shapes & Sizes reference breaks down every code with diagrams.

3. Choose Brightness and Color

Here's the modern twist: brightness is no longer measured in watts. Watts measure energy used; lumens measure light produced. Because LED bulbs sip power, a 9-watt LED can easily out-shine an old 60-watt incandescent.

As a rough guide:

• ~450 lumens = old 40W
• ~800 lumens = old 60W
• ~1100 lumens = old 75W
• ~1600 lumens = old 100W

Then choose a color temperature, measured in Kelvin (K). Lower numbers are warmer and cozier; higher numbers are cooler and crisper. 2700K is warm white (great for living rooms and bedrooms), 3000K is soft white, 4000K is neutral, and 5000K is daylight (great for garages, workshops, and task areas). Our Characteristics of Light page explains Kelvin, CRI, and beam angle in plain English.

4. Confirm the Last Few Details

Before you check out, make sure the bulb is dimmable if it's going on a dimmer switch—not all LEDs are. Check whether the fixture is enclosed or damp/wet rated for bathrooms and outdoors, and confirm the voltage (most household bulbs are 120V, but low-voltage landscape and track systems are often 12V).

Why LED Is the Easy Answer

For nearly every replacement today, an LED bulb is the smart choice: it uses up to 85% less energy, lasts 10–25 times longer than incandescent, and runs cool. With incandescent and many fluorescent types being phased out under federal and state law, switching to LED also keeps you ahead of changing regulations.

Still unsure which bulb you need? Browse our LED bulb collection, check the Batteries & Bulbs Glossary for any term you don't recognize, or reach out—we've been helping people find the right bulb since 1992.