Field calculator
Footcandle lighting calculator (fixture count)
This calculator gives a fast, average-level estimate of how many fixtures a space needs to hit a target light level, using the lumen method. A footcandle is one lumen per square foot, so the target lumens equal the area times the target footcandles, and the fixture count is that total divided by the lumens each fixture actually delivers after losses. Enter the area in square feet, the target footcandles for the task (the IES recommends levels by space and activity), the rated lumens per fixture, and a combined factor that rolls light-loss and room utilization into one number, often around 0.7 and lower in tall or dark-finished rooms. The result is a planning ballpark for fixture quantity and budget. A real lighting design uses the fixture coefficient of utilization, room cavity ratios, spacing criteria, and a point-by-point photometric layout to check uniformity and glare, so confirm the final design with a photometric study against the IES target.
Result
Fixture count by the lumen method: target lumens = area × the target footcandles (one footcandle is one lumen per square foot), and the fixture count is the target lumens divided by the lumens each fixture actually delivers. Enter the area in square feet, the target footcandles for the task (per the IES), the rated lumens per fixture, and a combined factor that rolls in light loss and utilization (often around 0.7, lower for high or dark rooms). This gives an average-level ballpark for fixture quantity; a real design uses the fixture coefficient of utilization, room cavity ratios, spacing criteria, and a point-by-point photometric layout, so confirm with a photometric study.
anvilfield.com/calculators/footcandle-lighting-fixture-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Footcandle FAQ
How many footcandles do you need for an office?
An office commonly targets 30 to 50 footcandles (about 300 to 500 lux) on the desk for reading and screen work, per IES recommendations. Circulation around it needs only 5 to 10 fc. Those are the standard of care, not a mandate. The project lighting design and the IES Lighting Handbook set the value for a given space.
What is lighting power density (LPD)?
Lighting power density is the connected lighting wattage divided by floor area, in watts per square foot, and the energy code caps it. ASHRAE 90.1, the IECC, and California Title 24 set the limits, which drop most editions as LED efficacy rises. The adopted code edition and local amendments control the actual allowable number.
What lighting controls does code require?
Energy codes commonly require automatic shutoff, occupancy or vacancy sensing in enclosed intermittent rooms, daylight-responsive dimming near windows and skylights, and multilevel or dimming control, with receptacle control in recent editions. A manual switch alone usually does not comply. The exact triggers vary, so confirm them against the adopted code edition and the AHJ.
What is the lumen method in lighting design?
The lumen method calculates average illuminance for a room: footcandles equal lamp lumens times the coefficient of utilization times the light loss factor, divided by area. The CU is the fraction of light reaching the work plane, and the LLF derates for aging and dirt, commonly 0.7 to 0.9. Use point-by-point for uniformity.
What is the difference between footcandles, lux, and lumens?
Lumens measure light leaving the source; footcandles and lux measure light arriving on a surface. A footcandle is one lumen per square foot, lux is one lumen per square meter, and 1 fc is about 10.76 lux. A high lumen rating does not tell you the footcandles on the floor without the room calculation.