Lumens to watts free online calculator

Luminous flux in lumens (lm) to electric power in watts (W) calculator.

Enter the luminous flux in lumens, luminous efficacy in lumens per watt and press the Calculate button to get the power in watts:


* the predefined luminous efficacy values are typical / average values.

Watts to lumens calculator »

Lumens to watts calculation formula

Energy saving lamps have high luminous efficacy (more lumens per watt).

The power P in watts (W) is equal to the luminous flux ΦV in lumens (lm), divided by the luminous efficacy η in lumens per watt (lm/W):

P(W) = ΦV(lm) / η(lm/W)

Lumens to watts table

Lumens Incandescent
light bulb
 / LED
375 lm 25 W 6.23 W
600 lm 40 W 10 W
900 lm 60 W 15 W
1125 lm 75 W 18.75 W
1500 lm 100 W 25 W
2250 lm 150 W 37.5 W
3000 lm 200 W 50 W

What is luminous flux ?

In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

Luminous flux is often used as an objective measure of the useful light emitted by a light source, and is typically reported on the packaging for light bulbs, although it is not always prominent. Consumers commonly compare the luminous flux of different light bulbs since it provides an estimate of the apparent amount of light the bulb will produce, and a lightbulb with a higher ratio of luminous flux to consumed power is more efficient.

Luminous flux is not used to compare brightness, as this is a subjective perception which varies according to the distance from the light source and the angular spread of the light from the source.

Luminous Flux (Φv) is energy per unit time (dQ/dt) that is radiated from a source over visible wavelengths. More specifically, it is energy radiated over wavelengths sensitive to the human eye, from about 330 nm to 780 nm. Thus, luminous flux is a weighted average of the Radiant Flux in the visible spectrum. It is a weighted average because the human eye does not respond equally to all visible wavelengths.

The sensitivity of the eye peaks at 555 nm and falls off to approximately 10-4 at 380 and 750 nm. This constitutes the range of daylight sensitivity, or photopic vision. The eye's nighttime sensitivity, called scotopic vision, shifts toward the blue end of the visible, peaking at 507 nm and falling to 10-4 at 340 and 670 nm. This weighting factor, or luminous efficacy (Vλ), allows for conversion of Radiant Flux to Luminous Flux at any wavelength. In the photopic region, the peak at 555 nm is assigned a conversion value of 683 lumens per Watt. The lumen is the unit of luminous flux, and is defined in terms of the candela, an SI base unit like the meter or second. 1 lumen is defined to be 1/4π candela, the SI base unit of Luminous Intensity.

Since the eye does not see all wavelengths equally well, the efficacy curve is a very important way to determine the Luminous Flux from a source. The Luminous Flux from a monochromatic source producing light at a single wavelength is easiest to determine.

Φv = Φ *Vλ * (683 lm/W)

For instance, a 5 mW laser pointer using at a wavelength of 680 nm produces

0.005 W * 0.017 * 683 lm/W = 0.058 lm

While a 5 mW laser pointer at 630 nm produces

0.005 W * 0.265 * 683 lm/W = 0.905 lm, a significantly greater luminous flux.

What is lumen?

The lumen is the standard unit for the luminous flux of a light source. It is an SI derived unit based on the candela. It can be defined as the luminous flux emitted into unit solid angle (1 sr) by an isotropic point source having a luminous intensity of 1 candela. The unit lumen is then equal to cd x sr. The abbreviation is lm and the symbol is Φv. In terms of radiant power (also called radiant flux) it can be expressed as:

Luminous flux in lumens = Radiant power (watts) x 683 lumens/watt x luminous efficacy

The luminous flux is the part of the power which is perceived as light by the human eye, and the figure 683 lumens/watt is based upon the sensitivity of the eye at 555 nm, the peak efficiency of the photopic (daylight) vision curve. The luminous efficacy is 1 at that frequency.

A typical 100 watt incandescent bulb has a luminous flux of about 1700 lumens.

Units for other quantities in photometry contain the lumen, such as the lux (lumens/meter2

Lumens to watts calculation »

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