Foot Candle Calculator

Foot-Candle Calculator

Understanding the illuminance levels in a space is crucial for comfort, productivity, and safety. Foot-candles (fc) are a standard unit of illuminance, measuring the amount of light falling on a surface. Specifically, one foot-candle is defined as one lumen per square foot. This calculator helps you estimate the average foot-candle level in a room based on your lighting setup and room characteristics.

Whether you're designing a new lighting system, evaluating an existing one, or simply curious about the brightness of your environment, this tool provides a practical way to quantify light levels. It uses the common "Lumen Method" for average illuminance calculations, taking into account the total light output, room dimensions, and efficiency factors.

Calculate Average Foot-Candles







function calculateFootCandles() { var numFixtures = parseFloat(document.getElementById("numFixtures").value); var lumensPerFixture = parseFloat(document.getElementById("lumensPerFixture").value); var roomLength = parseFloat(document.getElementById("roomLength").value); var roomWidth = parseFloat(document.getElementById("roomWidth").value); var cuFactor = parseFloat(document.getElementById("cuFactor").value); var llFactor = parseFloat(document.getElementById("llFactor").value); var resultDiv = document.getElementById("footCandleResult"); resultDiv.innerHTML = ""; // Clear previous result // Input validation if (isNaN(numFixtures) || numFixtures <= 0) { resultDiv.innerHTML = "Please enter a valid number of light fixtures (greater than 0)."; return; } if (isNaN(lumensPerFixture) || lumensPerFixture <= 0) { resultDiv.innerHTML = "Please enter valid lumens per fixture (greater than 0)."; return; } if (isNaN(roomLength) || roomLength <= 0) { resultDiv.innerHTML = "Please enter a valid room length (greater than 0)."; return; } if (isNaN(roomWidth) || roomWidth <= 0) { resultDiv.innerHTML = "Please enter a valid room width (greater than 0)."; return; } if (isNaN(cuFactor) || cuFactor 1) { resultDiv.innerHTML = "Please enter a valid Coefficient of Utilization (CU) between 0 and 1."; return; } if (isNaN(llFactor) || llFactor 1) { resultDiv.innerHTML = "Please enter a valid Light Loss Factor (LLF) between 0 and 1."; return; } // Calculations var totalLumens = numFixtures * lumensPerFixture; var roomArea = roomLength * roomWidth; if (roomArea === 0) { resultDiv.innerHTML = "Room area cannot be zero. Please check room dimensions."; return; } var averageFootCandles = (totalLumens * cuFactor * llFactor) / roomArea; // Display result resultDiv.innerHTML = "

Calculation Result:

"; resultDiv.innerHTML += "Total Lumens: " + totalLumens.toFixed(2) + " lm"; resultDiv.innerHTML += "Room Area: " + roomArea.toFixed(2) + " sq ft"; resultDiv.innerHTML += "Average Foot-candles: " + averageFootCandles.toFixed(2) + " fc"; }

What are Foot-Candles?

A foot-candle (fc) is a unit of illuminance, which measures the intensity of light incident on a surface. It quantifies how much light actually reaches a specific area. One foot-candle is equivalent to one lumen of light distributed over one square foot. This differs from lumens, which measure the total light output of a source, regardless of how it's distributed.

Why are Foot-Candles Important?

Proper illuminance levels are critical for various reasons:

  • Visual Comfort: Too little light can cause eye strain and fatigue, while too much can lead to glare and discomfort.
  • Productivity: Adequate lighting improves visibility, reduces errors, and enhances task performance in workplaces.
  • Safety: Well-lit areas prevent accidents and improve navigation, especially in industrial or public spaces.
  • Aesthetics: Lighting plays a significant role in setting the mood and highlighting features in architectural and interior design.

How This Calculator Works (Lumen Method)

This calculator uses the Lumen Method, a widely accepted technique for determining the average illuminance level in a space. The formula is:

Average Foot-candles = (Total Lumens * Coefficient of Utilization * Light Loss Factor) / Room Area (sq ft)

Understanding the Inputs:

  • Number of Light Fixtures: The total count of light sources in the room.
  • Lumens per Fixture: The light output of a single fixture, typically provided by the manufacturer.
  • Room Length & Width (feet): These dimensions are used to calculate the total room area in square feet.
  • Coefficient of Utilization (CU): This factor represents the efficiency with which light from the fixtures reaches the work plane (the surface where tasks are performed). It accounts for room geometry, surface reflectances (walls, ceiling, floor), and fixture design. CU values typically range from 0.4 to 0.8. Higher values mean more light reaches the work plane.
  • Light Loss Factor (LLF): This factor accounts for the reduction in light output over time due to lamp depreciation, dirt accumulation on fixtures and room surfaces, and other environmental factors. LLF values typically range from 0.7 to 0.9. A lower LLF indicates more light loss.

Typical Foot-Candle Requirements

The ideal foot-candle level varies significantly depending on the activity performed in a space. Here are some general guidelines:

  • General Office Work: 30-50 fc
  • Detailed Task Work (e.g., drafting, inspection): 70-100+ fc
  • Corridors/Circulation Areas: 10-20 fc
  • Warehouses/Storage: 20-30 fc
  • Retail Sales Areas: 30-70 fc (depending on merchandise)
  • Classrooms: 30-50 fc
  • Residential (General): 10-20 fc
  • Residential (Task Lighting, e.g., kitchen counter): 30-50 fc

These are general recommendations; specific standards may vary by region and industry.

Example Calculation

Let's say you have a 15 ft x 10 ft office room with 4 LED panel lights, each producing 2000 lumens. You estimate a Coefficient of Utilization (CU) of 0.65 and a Light Loss Factor (LLF) of 0.8.

  • Number of Light Fixtures: 4
  • Lumens per Fixture: 2000 lm
  • Room Length: 15 ft
  • Room Width: 10 ft
  • Coefficient of Utilization (CU): 0.65
  • Light Loss Factor (LLF): 0.8

Using the calculator:

  1. Total Lumens = 4 fixtures * 2000 lm/fixture = 8000 lm
  2. Room Area = 15 ft * 10 ft = 150 sq ft
  3. Average Foot-candles = (8000 lm * 0.65 * 0.8) / 150 sq ft
  4. Average Foot-candles = (5200 * 0.8) / 150
  5. Average Foot-candles = 4160 / 150 = 27.73 fc

The calculated average illuminance is approximately 27.73 foot-candles. This might be slightly below the recommended range for general office work (30-50 fc), suggesting you might consider adding more fixtures or higher lumen output lights for optimal conditions.

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