Footcandle Calculator

A footcandle calculator estimates average illuminance across a defined area. It converts the usable light output from installed fixtures into footcandles (fc) and lux, providing an early lighting-design value for comparing a room, work area, aisle, exterior zone, or equipment space against the illuminance level required by the project documents or lighting criteria.

The calculator produces Average illuminance in footcandles and lux after applying the entered Coefficient of utilization and Light loss factor to the entered fixture lumens. It also shows Effective lumens and the combined CU x LLF factor used in the calculation.

Average footcandles are commonly used during preliminary fixture selection and quantity review. The result helps determine whether a lighting layout may provide adequate general illumination before a photometric layout verifies fixture spacing, mounting height, distribution pattern, uniformity, glare, and light levels at the actual task plane.

Average Illuminance

A footcandle is one lumen distributed over one square foot:

\(\displaystyle 1\text{ fc} = 1\text{ lumen per square foot}\)

The calculator estimates average illuminance rather than the light level at a specific point. A calculated average of 30 fc does not mean every location in the area receives 30 fc. Areas directly below fixtures can be higher, while locations between fixtures, near walls, behind obstructions, or outside the effective distribution pattern can be lower.

For projects using metric lighting criteria, the calculator also reports lux:

\(\displaystyle 1\text{ fc} = 10.7639\text{ lux}\)

Footcandles are commonly used in U.S. electrical drawings, lighting schedules, and owner performance requirements. Lux is useful when coordinating with manufacturer photometric information, international specifications, or metric project documentation.

Entered Lighting Values

Field	Electrical purpose
Lumens	Total initial fixture lumens assigned to the illuminated area
Area	Illuminated floor or task area in square feet
Coefficient of utilization	Planning factor representing the portion of fixture lumens expected to reach the work plane or target area
Light loss factor	Planning factor representing light reduction from lamp or LED depreciation, dirt accumulation, temperature effects, driver performance, and other maintained-light considerations
Average illuminance	Estimated maintained average light level in footcandles and lux
Effective lumens	Fixture lumens remaining after the entered CU and LLF factors
CU x LLF factor	Combined multiplier applied to the entered lumens

Lumens should represent the total rated output of the fixtures included in the calculation. If four fixtures each provide 4,000 lm, enter 16,000 lm. Do not substitute wattage for lumens; electrical input watts indicate power consumption, while lumens describe luminous output.

Area should match the area intended to receive the calculated illumination. For general room lighting, this may be the room floor area. For benches, machine stations, inspection zones, or work surfaces, use the defined task area when that is the lighting-design target.

The Coefficient of utilization and Light loss factor reduce raw fixture lumens to a planning estimate of usable maintained light. A CU or LLF of 1.0 applies no reduction. Values below 1.0 reduce the light available to the target area.

Footcandle Formula

The calculator uses the following average-illuminance relationship:

\(\displaystyle \text{Average illuminance (fc)} = \frac{\text{Lumens} \times \text{Coefficient of utilization} \times \text{Light loss factor}} {\text{Area}}\)

Effective lumens are calculated first:

\(\displaystyle \text{Effective lumens} = \text{Lumens} \times \text{Coefficient of utilization} \times \text{Light loss factor}\)

The calculator then converts footcandles to lux:

\(\displaystyle \text{Average illuminance (lux)} = \text{Average illuminance (fc)} \times 10.7639\)

The combined utilization and loss multiplier is:

\(\displaystyle \text{CU x LLF factor} = \text{Coefficient of utilization} \times \text{Light loss factor}\)

Calculation Example

Enter the following values:

Input	Value
Lumens	16,000 lm
Area	300 sq ft
Coefficient of utilization	0.70 CU
Light loss factor	0.80 LLF

First, calculate the combined factor:

\(\displaystyle 0.70 \times 0.80 = 0.56\)

Next, calculate effective lumens:

\(\displaystyle 16{,}000 \times 0.56 = 8{,}960\text{ lm}\)

Then divide effective lumens by the illuminated area:

\(\displaystyle \frac{8{,}960}{300} = 29.8667\text{ fc}\)

The calculated results are:

Result	Value
Average illuminance	29.8667 fc
Average illuminance	321.482 lux
Effective lumens	8,960 lm
CU x LLF factor	0.56 x

This result indicates an estimated maintained average illumination level of approximately 29.9 footcandles across the entered 300 sq ft area.

Lighting Layout Limits

The calculation assumes the entered lumens can be treated as a total source serving the entered area and that the selected CU and LLF reasonably represent the installation. It does not calculate fixture spacing, point-by-point footcandles, minimum illuminance, uniformity ratio, vertical illuminance, glare, reflected light, daylight contribution, emergency-lighting performance, or photometric distribution.

A final lighting layout should use the selected fixture’s published photometric data, mounting height, room geometry, surface reflectances, fixture orientation, obstructions, and task-plane elevation. Fixture wattage and quantity from the final layout must also be carried into the branch-circuit load review, feeder calculation, panel schedule, control design, and voltage-drop evaluation as applicable.

Project lighting criteria, energy-code requirements, manufacturer instructions, and AHJ requirements remain separate from this arithmetic.

Related calculations: Lumen Calculator, Energy Use Calculator.