When clients ask, “What is a foot-candle, and why does it matter?”, they are really asking how bright a space will feel in real life. Architects, engineers, and lighting designers use foot-candles every day to size fixtures, check codes, and prove that a lighting layout works.
This guide explains what a foot-candle is in simple terms, how it relates to lumens and lux, and how to use foot-candle levels on photometric plans. If you work with IES files, lighting standards, or permit drawings, this is your quick reference.
What Is a Foot-Candle in Lighting?
A foot-candle (fc) is a unit of illuminance. It tells you how much light reaches a surface. One foot-candle means one lumen of light spread evenly over one square foot of area. In other words, foot-candles describe how bright a working surface, floor, or task area will be, not how powerful the fixture is.
This makes foot-candles very useful for architects and engineers, because they connect the photometric data inside an IES file with what people actually see and experience in a space.
Why Foot-Candles Matter for Real Projects
Foot-candles are the common language between architects, engineers, contractors, and inspectors. They show how much usable light reaches floors, desks, and task areas.
- They translate lumens and optics into light levels on the actual work plane.
- They help verify that lighting meets U.S. standards and owner requirements for safety and comfort.
- They allow fair comparison between different fixtures, distributions, and layouts on a photometric plan.
How Foot-Candles Work: Lumens, Lux, and Distance
Foot-candles link three things: lumens from the fixture, distance to the surface, and how the light is distributed. The more lumens that hit a square foot, the higher the foot-candle level. The further away you move from the source, the lower the foot-candle value becomes.
- Basic definition: 1 foot-candle = 1 lumen per square foot (lm/ft²).
- Relationship to lux: 1 foot-candle ≈ 10.76 lux, and 1 lux ≈ 0.093 foot-candles.
- Distance effect: As distance doubles, illuminance (fc) drops roughly by a factor of four (inverse-square law).
Lighting software and manufacturers’ photometric tools do the heavy math for you. They read the IES file, apply the distribution type, and calculate foot-candles across a grid. Your job as a designer is to know what foot-candle levels are appropriate for each space and how to interpret the results.
Practical Foot-Candle Ranges by Space Type
Exact values depend on standards, codes, and client expectations, but some typical maintained foot-candle ranges appear again and again in U.S. practice. Use these as conversation starters, not as a replacement for the latest IES or local code documents.
- Parking lots and site lighting: about 1–5 fc on pavement for basic safety, often with uniformity ratios to control dark spots.
- Warehouses and open storage: roughly 20–30 fc on the floor or task plane in aisles and open areas.
- Offices and workstations: about 30–50 fc on the horizontal work plane, with some attention to vertical illuminance for faces and screens.
- Sports courts and fields: can range from 30 fc for simple recreational areas to 50+ fc or more for higher performance levels.
Example: Using Foot-Candles on a Photometric Plan
On real projects, you rarely talk about lumens per fixture in isolation. Instead, you look at the photometric plan and ask: what are the minimum, maximum, and average foot-candle values? How uniform is the lighting, and does it meet the owner’s and code requirements?
- Project / context: A small office layout where the client requests comfortable but efficient lighting, with clear visibility on desks and circulation paths.
- Challenge: The original concept uses decorative fixtures with uneven distribution. Some workstations show only 10–15 fc on the work plane, while others spike above 60 fc.
- Result: After revising the layout with better optics and spacing, the final photometric plan shows 30–45 fc across the work plane, improved uniformity, and lower glare. The drawings clearly document average, minimum, and maximum foot-candles for review and permitting.
This same logic applies to parking lots, warehouses, sports fields, and other exterior areas. Foot-candles help you verify that the site is bright enough for safety without wasting energy. Uniformity ratios (for example, average-to-minimum or maximum-to-minimum) make sure that you do not have bright “hot spots” next to dark areas.
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Key Takeaways About Foot-Candles
Foot-candles are a simple but powerful way to describe how bright a space will be. They connect lumens, optics, and mounting height to what actually matters for your client: visibility, safety, and comfort. Once you understand what a foot-candle is, reading a photometric plan becomes much easier.
When you need a detailed, code-ready lighting study with clear foot-candle values, uniformity ratios, and documentation that contractors can trust, Stetra Lighting can prepare the complete photometric plan for you.