Daylight Factor Calculator
Enter indoor work-plane illuminance and unobstructed outdoor horizontal illuminance in the same unit, measured at the same time.
A QUICK WALKTHROUGH
How to use this tool
- Enter both illuminance values in the same unit.
- Use measurements taken at the same time; the outdoor reading must be unobstructed.
- Calculate the indoor-to-outdoor illuminance ratio as a percentage.
Formula
Daylight factor (%) = indoor work-plane illuminance ÷ unobstructed outdoor horizontal illuminance × 100%. Use the same illuminance unit and simultaneous readings.
Scope and limits
This is a simple ratio of user-entered measurements. It does not assess code, standard, or design compliance, and does not account for changing sky conditions or measurement uncertainty.
Reference
University of Illinois, Energy Impact of Daylighting: https://smartenergy-form.arch.illinois.edu/pdf/Archive/EnergyImpactofDaylighting.pdf
GOOD TO KNOW
Common questions
Can the readings be taken at different times?
No. Use simultaneous readings because outdoor daylight changes over time.
Does this determine compliance?
No. It reports only the arithmetic ratio and does not determine compliance with any standard.