Find a brightness target for replacement bulbs, whether you are matching an old bulb or lighting a room with existing fixtures.
How to use our Light Bulb Buying Calculator
- Choose whether to match an old bulb or estimate room lighting.
- For a brightness match, choose the old incandescent label or enter its lumen rating. For a room, enter dimensions and your target lux level.
- Enter the number of fixtures and bulbs in each. Use Advanced options for package preferences or room assumptions.
- Calculate, then compare the per-bulb lumen target with bulb packages. Check base, shape, voltage, actual wattage, dimmer and enclosure compatibility before buying.
Definitions
Lumens: A bulb's rated light output. Lux: Light reaching a surface, in lumens per square meter. Equivalent watts: A brightness comparison with an older bulb, not the new bulb's electrical power.
Common mistakes and quick fixes
Do not buy a 60 W LED merely because the old bulb was 60 W. Match lumens for brightness, and compare the new bulb's actual watts with the fixture limit. Do not assume that equal lumens guarantee equal beam pattern or room lighting.
Limitations & Key Assumptions / Boundary Conditions
No brand or product compatibility is certified. Room estimates assume evenly shared general illumination and do not account for a particular shade, beam angle, high ceiling or fixture layout. Task lighting and unusual rooms may need a separate assessment. Check manufacturer labels even if the brightness target looks suitable.
Methodology
Matching mode uses approximate incandescent replacement anchors: 40 W → 450 lm, 60 W → 800 lm, 75 W → 1,100 lm and 100 W → 1,600 lm. Enter package lumens directly when known [1, 2].
Room mode uses: lumens per bulb = floor area in m² × target lux / (utilization factor × light loss factor × number of bulbs). Feet are converted using 1 ft = 0.3048 m. The starting assumptions are 150 lux, 0.6 utilization and 0.8 light loss; they are editable estimates, not a lighting standard.
Example: replacing six 60 W incandescent bulbs gives a shopping target of about 800 lumens each, or 4,800 lumens in total. A 12 m² room at 150 lux with six bulbs and the starting factors instead gives 625 lumens per bulb.