How this instrument works
Lumens measure how much visible light a bulb actually produces; watts measure how much electrical power it draws to do it. The two aren't the same thing, and how many watts it takes to produce a given number of lumens — a bulb's luminous efficacy — depends entirely on the bulb technology. Some technologies convert electricity to light far more efficiently than others, which is exactly why an LED and an old incandescent bulb can look equally bright while drawing very different power.
This calculator divides a target lumens figure by a chosen bulb type's typical efficacy (lumens per watt) to find the wattage that bulb type needs to hit that brightness — incandescent at roughly 14 lm/W, halogen around 20, CFL around 60, and LED around 90, representative midpoint figures for each technology rather than exact specs for any single bulb model.
This is the calculation behind every 'this LED is a 60-watt equivalent' claim printed on LED packaging: the box isn't saying the LED draws 60 watts, it's saying the LED produces roughly as many lumens as a 60-watt incandescent bulb used to, while itself drawing far fewer watts to get there.
- Enter Brightness (lumens) — the light output you're targeting, often taken from an old bulb's packaging or a room's recommended total.
- Select Bulb type — incandescent, halogen, CFL, or LED, each using a different typical lumens-per-watt efficacy.
- Read Equivalent wattage (W) — the wattage that bulb type needs to draw to produce your target lumens.
- Compare the same lumens figure across different bulb types to see how dramatically wattage — and running cost — differs by technology.
- Use a bulb's own printed lumen rating, not its wattage, when comparing brightness across different bulb technologies.
Worked example — 800 lumens across two bulb technologies
800 lumens is roughly the output of a classic 60-watt incandescent bulb. Selecting LED at its typical ~90 lm/W efficacy, Equivalent wattage (W) comes to 800 / 90 = 8.888..., about 8.9 watts — matching the '60W equivalent, ~9W actual' rating printed on many real LED bulb packages.
Selecting Incandescent instead, at its much lower ~14 lm/W efficacy, the same 800 lumens needs about 800 / 14 = 57.1 watts — close to the bulb's actual old wattage, since incandescent is the baseline technology the '60W equivalent' marketing language is built around. The roughly 48-watt gap between 8.9W and 57.1W for identical brightness is the entire reason LED replacement bulbs save money to run.
Questions
What does 'LED equivalent wattage' actually mean on a light bulb box?
It means the LED produces roughly the same number of lumens as a bulb of that wattage using the older, less efficient incandescent technology — not that the LED itself draws that many watts. A '60W equivalent' LED is typically only drawing around 8-10 actual watts, which is exactly the gap this calculator's lumens-to-watts conversion makes explicit for any bulb type and target brightness.
Why do different bulb types need such different wattage for the same lumens?
Because each bulb technology converts electricity into visible light at a very different efficiency, called luminous efficacy, measured in lumens per watt. Incandescent bulbs waste most of their energy as heat rather than light, giving low efficacy (~14 lm/W); LEDs convert far more of their input power directly into visible light, giving much higher efficacy (~90 lm/W) — hence the large wattage gap for identical brightness.
Are the lumens-per-watt figures used here exact for every bulb?
No — they're representative midpoints for each general bulb category, cross-checked against commonly published efficacy ranges. A specific bulb model can sit meaningfully above or below its category's typical figure depending on its particular design and manufacturer, so treat the wattage this calculator returns as a close estimate rather than an exact spec for any one product.
How does this relate to the site's LED savings calculator?
This tool answers 'how many watts does a bulb type need for a given brightness'; the LED savings calculator takes an old and new bulb's wattage directly and works out the annual energy and cost saved by switching between them. Use this one first if you only know your target brightness and need a wattage figure to start from.
Should I buy a bulb based on its wattage or its lumens?
Lumens, if you're comparing across different bulb technologies — wattage alone only tells you power draw, not brightness, and a low-wattage LED can easily outshine a higher-wattage incandescent. Compare the lumens rating printed on the packaging directly, and use wattage mainly to estimate the bulb's running cost once you've matched the brightness you actually want.