Free Lumens to Watts Calculator

Understanding Light Bulb Efficiency

Brightness is expressed in lumens (lm), while power consumption is given in watts (W). The ratio between these two—lumens per watt—defines luminous efficacy, the key metric for judging how efficiently a bulb turns electricity into visible light. A lumens to watts calculator, also known as a brightness to power converter or light bulb efficiency calculator, makes it simple to find the power a bulb needs for a given brightness, or to determine its efficacy.

With this free online tool you can:

• Calculate LED wattage – find the power required for a specific luminous flux.
• Determine luminous flux – compute the light output from a known wattage and efficacy.
• Evaluate luminous efficacy – see how efficiently any source produces light.
• Compare bulb types – use the built‑in reference tables for an instant LED bulb comparison with older technologies, helping you choose the most energy‑saving option.

Using the Brightness‑to‑Power Converter

1. Enter the luminous efficacy of your light source (default is 100 lm/W, typical for modern LEDs). If you have a different bulb type, look up its typical value in the efficacy table below.
2. Input the brightness in lumens or nits (a unit often used for displays). For example, a bulb might emit 1200 lm or a screen might be 350 nits.
3. The calculator immediately shows the required wattage.
   Example: With an efficacy of 100 lm/W, producing 1200 lm needs $1200 \div 100 = 12$ W.

You can also reverse the process: if you know the wattage and brightness, the tool will compute the luminous efficacy.

Luminous Efficacy Reference Table

Typical efficacy ranges vary by bulb type. The table below lists common values (actual numbers may differ between manufacturers and models).

Light bulb type	Luminous efficacy (lm/W)
LED bulbs	80–120
Compact fluorescent (CFL)	45–75
Fluorescent tube lights	40–105
High‑pressure sodium	85–150
Low‑pressure sodium	100–190
Sulfur plasma lights	50–100
Metal halide lights	75–100
Mercury vapor lights	35–65
Halogen light bulbs	16–30
Tungsten incandescent bulbs	12–18

Manual Calculations

When you need to work out wattage from lumens by hand, use the formula:

$$P = \dfrac{\Phi_{V}}{\eta}$$

where:

• $P$ = power in watts,
• $\Phi_{V}$ = luminous flux in lumens,
• $\eta$ = luminous efficacy in lm/W.

Example 1 – finding wattage:
An LED with $\eta = 120$ lm/W must produce 1200 lm.

$$P = \dfrac{1200\ \text{lm}}{120\ \text{lm/W}} = 10\ \text{W}$$

Example 2 – finding efficacy:
A bulb consumes 15 W and emits 1000 lm.

$$\eta = \dfrac{1000\ \text{lm}}{15\ \text{W}} \approx 66.67\ \text{lm/W}$$

If the wattage is not marked, you can calculate it from voltage and current:

$$P = V \times I$$

Comparing LED Wattage with Traditional Bulbs

The following table shows how much more power older lighting technologies use to produce the same light output as a modern LED (reference: 12 W at 100 lm/W). Values are approximate.

Light bulb type	Efficacy (lm/W)	Power vs. LED (approx.)
Tungsten incandescent	15	7 times more
Halogen	20	5 times more
Mercury vapor	50	2 times more
Fluorescent tube	60	1.7 times more
Compact fluorescent (CFL)	70	1.4 times more
Sulfur plasma	75	1.3 times more
Metal halide	85	1.2 times more

Key takeaway: An LED bulb uses roughly 7–10 times less electricity than an incandescent bulb for the same brightness. Replacing old bulbs with LEDs can substantially lower your lighting energy costs.

Disclaimer: The values in the table are only rough guidelines. Actual luminous efficacy depends on the specific product and operating conditions.

By using this LED wattage calculator alongside the reference data, you can quickly evaluate any bulb’s efficiency and make informed purchasing decisions that save both energy and money.