For decades, we chose light bulbs by watts: a 60-watt bulb for reading, a 100-watt bulb for a brightly lit kitchen. But that worked only because all incandescent bulbs had roughly the same efficiency. LEDs changed everything — now a 9-watt LED produces the same light as a 60-watt incandescent, making watts a poor measure of brightness.
Watts vs. Lumens: The Key Difference
- Watts (W) = energy consumed per second. It tells you how much electricity the bulb uses, not how bright it is.
- Lumens (lm) = visible light output. It directly measures brightness as perceived by the human eye.
💡 Simple RuleWhen buying a bulb, shop by lumens for brightness, then choose the lowest-watt option that produces those lumens. That's your most efficient choice.
Incandescent to LED Conversion Chart
| Old Incandescent Watts | Lumens Output | Equivalent LED Watts | Energy Savings |
|---|---|---|---|
| 25W | ~250 lm | 3–4W | 85% |
| 40W | ~450 lm | 5–6W | 87% |
| 60W | ~800 lm | 8–10W | 85% |
| 75W | ~1,100 lm | 11–13W | 84% |
| 100W | ~1,600 lm | 14–17W | 85% |
| 150W | ~2,600 lm | 20–25W | 85% |
How Many Lumens Do You Need?
This depends on room function (task vs. ambient lighting) and room size. General guidelines:
| Room Type | Recommended Lumens | Foot-Candles |
|---|---|---|
| Bedroom | 10–20 lm/sq ft | 10–20 fc |
| Living room (ambient) | 10–20 lm/sq ft | 10–20 fc |
| Kitchen (general) | 30–40 lm/sq ft | 30–40 fc |
| Kitchen (task/counter) | 70–80 lm/sq ft | 70–80 fc |
| Bathroom | 70–80 lm/sq ft | 70–80 fc |
| Office/Study | 50–70 lm/sq ft | 50–70 fc |
To calculate total lumens needed: multiply the room area by the recommended lm/sq ft value.
Other Bulb Specs to Know
- Color Temperature (Kelvin): 2700K = warm white (cozy), 3000K = soft white, 4000K = cool white (offices), 5000–6500K = daylight (task lighting)
- CRI (Color Rendering Index): 80+ for general use, 90+ for makeup/art spaces where color accuracy matters
- Dimmable: Not all LEDs are dimmable — check the packaging if you have a dimmer switch
- Beam Angle: Narrow (less than 40°) for spotlights, wide (more than 100°) for general room lighting
Luminous Efficacy: The Efficiency Metric
Luminous efficacy is lumens per watt (lm/W) — the higher, the more efficient the bulb:
- Incandescent: 10–17 lm/W
- CFL: 45–75 lm/W
- LED: 80–150+ lm/W
Efficacy = Lumens ÷ Watts
Try the Lumens to Watts Calculator
Free, instant results — no sign-up needed.