BLIND SOLUTIONS
Daylight Factor Calculator — Cheat Sheet
Quick-reference cheat sheet for the Daylight Factor Calculator tool.
Published 27 May 2026
CHEAT SHEET
Daylight Factor Calculator
Quick-reference companion for the free Portal tool
What This Tool Does
The Blind Solutions Daylight Factor Calculator is an essential tool for South African architects to assess the adequacy of natural light within interior spaces. It calculates the Daylight Factor (DF), a ratio that indicates the percentage of external illuminance available at a specific point indoors, relative to the unobstructed external illuminance on a horizontal plane.
This tool is crucial for designing energy-efficient buildings in line with SANS 10400-XA and SANS 204 requirements, promoting occupant well-being, and reducing reliance on artificial lighting during daylight hours. Use it during conceptual design to inform window sizing, placement, and shading strategies, or for performance verification of existing designs.
Architects, lighting designers, and sustainability consultants will find this calculator invaluable for ensuring compliance, optimizing visual comfort, and contributing to green building certifications like Green Star SA.
Step-by-Step Usage Guide
Step 1: Project Location & Orientation
Enter your project's geographic coordinates (Latitude & Longitude) for accurate solar geometry calculations. For a typical Johannesburg site, use Latitude: -26.2041 and Longitude: 28.0473. Specify the project's North orientation (e.g., 0 for True North, 90 for East, etc.).
Enter your project's geographic coordinates (Latitude & Longitude) for accurate solar geometry calculations. For a typical Johannesburg site, use Latitude: -26.2041 and Longitude: 28.0473. Specify the project's North orientation (e.g., 0 for True North, 90 for East, etc.).
Step 2: Room Dimensions & Reflectances
Input the room's length, width, and height. Then, specify the average reflectance values for the interior surfaces: Ceiling (e.g., 0.7 for white plaster), Walls (e.g., 0.5 for light paint), and Floor (e.g., 0.3 for light-coloured tiles). These values significantly impact internal light distribution.
Input the room's length, width, and height. Then, specify the average reflectance values for the interior surfaces: Ceiling (e.g., 0.7 for white plaster), Walls (e.g., 0.5 for light paint), and Floor (e.g., 0.3 for light-coloured tiles). These values significantly impact internal light distribution.
Step 3: Window Specifications
Add each window opening. For each window, provide its width, height, and the distance from the floor to the window sill. Crucially, input the Visible Light Transmittance (VLT) of the glazing (e.g., 0.65 for standard clear double glazing). Consider typical South African window-to-wall ratios (WWR) here, aiming for optimal daylight without excessive heat gain.
Add each window opening. For each window, provide its width, height, and the distance from the floor to the window sill. Crucially, input the Visible Light Transmittance (VLT) of the glazing (e.g., 0.65 for standard clear double glazing). Consider typical South African window-to-wall ratios (WWR) here, aiming for optimal daylight without excessive heat gain.
Step 4: External Obstructions
Account for any external elements that might block direct sunlight or sky view. This includes adjacent buildings, overhangs, or significant trees. For each obstruction, specify its height, distance from the window, and angular width. This is particularly important in dense urban environments like Sandton.
Account for any external elements that might block direct sunlight or sky view. This includes adjacent buildings, overhangs, or significant trees. For each obstruction, specify its height, distance from the window, and angular width. This is particularly important in dense urban environments like Sandton.
Step 5: Define Calculation Points
Specify the grid of points within the room where the Daylight Factor will be calculated. You can typically define a grid spacing (e.g., 0.5m x 0.5m) and the height of the work plane (e.g., 0.85m for a typical desk height). Ensure points cover the entire usable area of the room.
Specify the grid of points within the room where the Daylight Factor will be calculated. You can typically define a grid spacing (e.g., 0.5m x 0.5m) and the height of the work plane (e.g., 0.85m for a typical desk height). Ensure points cover the entire usable area of the room.
Step 6: Calculate & Interpret Results
Click the "Calculate Daylight Factor" button. The tool will display a visual representation (e.g., a heatmap) of DF values across your chosen work plane, along with average, minimum, and maximum DF values. Compare these results against SANS 10400-XA recommendations (e.g., minimum 2% average DF for habitable rooms) and your project's specific daylighting goals.
Click the "Calculate Daylight Factor" button. The tool will display a visual representation (e.g., a heatmap) of DF values across your chosen work plane, along with average, minimum, and maximum DF values. Compare these results against SANS 10400-XA recommendations (e.g., minimum 2% average DF for habitable rooms) and your project's specific daylighting goals.
Common Scenarios & Expected Results
| Scenario | Input (Key Parameters) | Expected Output (Average DF) |
|---|---|---|
| Small Office, North-facing | Room: 3x4x2.7m (LxWxH), Ceiling: 0.7, Walls: 0.5, Floor: 0.3. Window: 1.5m W x 1.2m H (sill 0.9m), VLT: 0.65. No obstructions. |
~3.5% - 4.5% (Good for general office work, meeting SANS 10400-XA) |
| Classroom, East-facing, Overhang | Room: 8x7x3m (LxWxH), Ceiling: 0.7, Walls: 0.6, Floor: 0.4. Windows (3x): 2m W x 1.5m H (sill 0.9m), VLT: 0.6. Overhang: 1m deep, 0.5m above window. |
~2.0% - 3.0% (Acceptable, overhang reduces direct gain but impacts DF) |
| Residential Living Room, West-facing | Room: 6x5x2.5m (LxWxH), Ceiling: 0.8, Walls: 0.7, Floor: 0.5. Sliding Door: 3m W x 2.1m H (sill 0m), VLT: 0.7. Adjacent building: 10m high, 5m away. |
~1.5% - 2.5% (May be lower due to obstruction, requires careful analysis for visual comfort) |
Pro Tips
Optimize for SANS 10400-XA Compliance: Aim for an average Daylight Factor of at least 2% in habitable rooms. For critical task areas, consider higher targets (e.g., 3-5%) to minimize artificial lighting during peak daylight hours.
Consider Climate Zone & Glare: While maximizing DF, be mindful of glare, especially in sunny South African climates. North-facing windows generally provide more even, glare-free light. East and West facades require careful shading strategies (e.g., external louvres, deep reveals) to mitigate discomfort and excessive heat gain.
Reflectance Matters: Don't underestimate the impact of interior surface reflectances. Lighter ceilings and walls (high reflectance values) can significantly bounce light deeper into a room, improving DF uniformity and reducing the need for larger windows.
Iterate Your Design: Use this calculator as an iterative design tool. Experiment with different window sizes, placements, glazing VLTs, and shading elements. Small adjustments can lead to significant improvements in natural light distribution and energy performance.
Related Tools & Resources:
- Solar Shading Calculator: For designing effective external shading devices.
- SANS 10400-XA & SANS 204 Compliance Guide: Understand energy efficiency regulations.
- Glazing Performance Database: Find VLT and U-values for common SA glazing types.