How to Calculate Daylight Factor
What is Daylight Factor?
Daylight factor measures indoor daylight availability as percentage of outdoor illuminance. It determines whether artificial lighting is required.
Formula
Daylight factor = (indoor illumination / outdoor illumination) × 100%
Step-by-Step Guide
- 1Daylight factor = (indoor illumination / outdoor illumination) × 100%
- 2Values: <2% (supplementary lighting needed), 2-5% (adequate), >5% (well-lit)
- 3Depends on window size, orientation, obstruction
Worked Examples
Input
Indoor illumination 100 lux, outdoor 5000 lux
Result
Daylight factor = 2% (borderline, likely needs supplementary lighting)
Typical office
Common Mistakes to Avoid
- ✕Over-sizing windows in climate with high cooling loads
- ✕Neglecting glare control in high daylight factor spaces
Frequently Asked Questions
Does orientation affect daylight factor?
Significantly; south-facing windows get stable light, north-facing less direct but more diffuse.
How do you improve daylight factor?
Increase window area, use reflective interior surfaces, reduce obstructions, add skylights.
Ready to calculate? Try the free Daylight Factor Calculator
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