Lumens per square foot

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In a nutshell…

Lumens per square foot:

Minimum amount of lighting needed is around 2000 lumens per square foot.
Mid range is around 5000 lumens per square foot.
Optimal is 7000-7500, or higher, lumens per square foot.

Watts per square foot (wattage per sq ft WILL vary by light source):

Minimum wattage per sq ft.: 30w
Mid Range wattage per sq ft.: 50w
Optimal Range wattage per sq ft.: 50-80w

To most accurately ascertain the proper amount of lighting needed for your space, there are several things that you have to know. At this point, a couple of definitions are in order.

Lumens - one lumen is equal to the amount of light emitted by one candle that falls on one square foot of surface one foot away.
Watts – A measure of the amount of electricity flowing through a wire. Watt hours measure the amount of watts used in one hour. A kilowatt/hour (KWH) is 1000 watt/hours.
Light Spectrum – In simplest terms – Blue (425nm – 460nm, think a 6500k CFL or MH) & Red (630 – 660nm 2700k CFL or HPS). Blue wavelengths are associated with vegetative growth, and reds with rooting & blooming.

What to do with it all
To determine how many lumens per square foot you have:

Find out the square footage of your space.
Width x Depth = Square feet. Divide the lumens available by your square footage.
This will give you lumens per square foot.

Example: Say your space is 3 feet deep by 4 feet wide, 12 square feet. The total lumens available from your light(s) is 45000 lumens. 45000/12 = 3750 lumens per square foot.

What if you want to determine how much light you need in watts?
The general rule of thumb for providing light for an area is a minimum of 30 watts per square foot. 50+ watts per square foot is optimal. You can determine the proper lighting for your area by using this formula:

50 watts (or the total watts you are using) x (your square footage). Example: You have an area of 10 sq. ft. – 50w x 10 s.f. = 500watts/sq.ft minimum or 50 watts x 10 s.f. = 500 watts/sq. ft. (optimal). Also, remember that fluorescent’s are weaker and emit less light than an HID. This means you will need 5 times the amount of wattage to equal the output of an HID. So, 30 watts of HID would equal 150 watts of fluorescent’s. This is why it is advised to provide a minimum of 30 watts per square foot for HID lights and aminimum of 150 watts per square foot for fluorescent’s.


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