How Much Light Per Square Foot?

Measuring an area’s light per square foot has a number of practical applications, from gardening to home illumination. Various units of measurement serve such purposes.

These units of measurement describe everything from the brightness of light to its diffusion and the energy needed to create light of certain intensities or brightness. Gardeners, homeowners, engineers and others determine spatial light requirements using formulas and light-per-square-foot standards created by scientists.


In the United States foot-candles measure light per square foot. One foot-candle equals one lumen per square foot. Lumens measure the flow of light energy in the air. The lumen is in turn based on a unit called candela, which measures luminous intensity.

One lumen equals the amount of light given out through a solid angle of one steradian by a point source of one candela worth of intensity radiating uniformly in all directions. Lumens can be used in place of foot-candles when measuring light per square feet because the square footage of a space cancels out the square foot division of lumens in a mathematical formula. For instance, if a 100-square-foot room requires 10 lumens per square foot of space, it is represented thus: (10 lumen/square feet) x 100 square feet = 10 lumens x 100 = 1,000.

Object Brightness

Describing the brightness of certain objects in units such as lumens creates a less abstract framework for understanding light measurement. For instance, one standard 100-watt incandescent light bulb produces 1,630 lumens -- a relatively small unit of measurement.

The sun, on the other hand, produces a brightness of 160,000 lumens per square centimeter, or approximately 172.2 lumens per square foot, spread across the surface of the entire Earth. Assuming the surface of the Earth is approximately 196,935,000 square miles, the intensity of the sun, if condensed to a one square foot space is 945,418,071,000,000,000 lumens per square foot.

Home and Gardening Applications

According to A. Varghese, author of the book “Home Management,” 70 lumens per square foot, or 70 foot-candles, is average for home lighting. Using this formula helps determine how many light bubs to place in a space. For instance, if a 350-square-foot room requires 70 foot-candles of light per square foot, that room requires 350 x 70, or 24,500 lumens of light.

Understanding how much light per square foot species of greenhouse plants require to flourish is integral in maintaining optimal plant health, as George F. Van Patten, author of “Gardening Indoors: The Indoor Gardener's Bible” explains.

Indoor Considerations

When measuring light per square foot requirements in indoor environments, factors beyond bulb brightness merit consideration. For instance, windows add light to indoor environments. However, they also diffuse the brightness of light bulbs by increasing the overall amount of light in a space. As Robert B. Hickey explains in “Electrical Engineer's Portable Handbook,” light per square foot considerations account for reflective surfaces such as glass and metal, and how those surfaces interact with one another, the source of the light and the source of the light's perception, i.e., the individual.