You pull a dead bulb from a recessed light, ready to order a new one. You squint at the base. There is no label, no model number, no clue. The fine print has faded into a smooth, glassy nothing. This is a common moment of household frustration. You could go to the hardware store and guess, buying three different bulbs hoping one fits. Or you can solve the puzzle with some basic deduction.
Finding a replacement when the original specs are lost is about observation and knowing a few key terms. The goal is to match three things: the bulb’s shape and size (its form factor), its electrical connection (the base), and its light output (the brightness and color). People often get hung up on wattage, but that’s just power consumption. Modern LED replacements use far fewer watts for the same light. The real targets are lumens and Kelvin. A tool like the greenic finder simplifies this by letting you input physical features to get a match, but understanding the process yourself saves time and ensures you always get it right.
Step one: identify the base type
The base is non-negotiable. It has to screw or pin into the socket perfectly. Take the old bulb and look straight at its bottom metal connector. For screw bases, measure the diameter in millimeters across the threads. A standard household bulb is an E26 base (26mm). A smaller candelabra bulb is typically E12. For pin bases common in under-cabinet or vanity lights, count the pins and note the distance between them.
Write this down. It is your first filter. No amount of lumens will help if the bulb won’t physically connect.
Step two: decode the bulb shape
Bulb shapes are coded with a letter and a number. The letter indicates the form. ‘A’ stands for Arbitrary, the classic light bulb shape. ‘BR’ is Bulged Reflector, often used in recessed cans. ‘PAR’ is Parabolic Aluminized Reflector, a spotlight with a flat face. ‘G’ is Globe.
The number that follows is the diameter at the bulb’s widest point, measured in eighths of an inch. So an A19 bulb is the standard Arbitrary shape, 19 eighths of an inch in diameter (about 2.375 inches). A BR30 is a bulged reflector that is 30 eighths across (3.75 inches). Hold your dead bulb up, recognize its silhouette, and estimate its width with a measuring tape.
Forget watts, remember lumens and kelvin
This is where most guesswork goes wrong. We used to buy bulbs based on wattage: 60W for a lamp, 100W for a porch light. That measured energy use, not light output. With LEDs, wattage tells you almost nothing about brightness.
You need to find lumens. Lumens measure total visible light. To replace a traditional 60-watt incandescent bulb, look for an LED offering about 800 lumens. For a 100-watt equivalent, target 1600 lumens.
Then consider Kelvin, which describes light color. 2700K is warm white, cozy like old incandescents. 3000K is a slightly crisper soft white. 4000K is cool white, more neutral and alerting. 5000K+ is daylight blue-white.
Where tricky fixtures need extra attention
Some fixtures have constraints beyond shape and base. Enclosed fixtures like ceiling domes trap heat; you must use LEDs rated for enclosed spaces or risk a shortened lifespan. Dimmable fixtures require dimmable LED bulbs, but note that not all LEDs work well with all dimmer switches.
Outdoor lights need wet or damp location ratings depending on whether they’re exposed to rain or just covered porch humidity. For recessed downlights (‘can lights’), check if it’s a remodel or new-construction housing; some bulbs are too long for shallow cans.
Building a reliable matching system
The best outcome is to never face this puzzle again for that particular fixture. When you successfully identify a replacement, do one more thing: write it down on tape inside the fixture’s housing or on the back of the faceplate.
A better system is to keep a simple home maintenance log—digital or paper—with a section for lighting. Note the room, fixture type, and final bulb specs you bought: Base (e.g., E26), Shape (e.g., BR30), Lumens (e.g., 850), Kelvin (e.g., 2700K), Dimmable (Yes/No). This record turns a future chore into a quick shopping trip.
A practical list of what to document looks like this:
- Fixture location (Kitchen over sink)
- Bulb base code (GU10)
- Bulb shape code (PAR20)
- Required lumens (650)
- Preferred color temperature (3000K)
- Any special ratings (Dimmable)