Daylight factor compares indoor daylight with outdoor daylight under defined conditions.
Daylight factor is a daylighting relationship between indoor illuminance and outdoor illuminance. It does not turn outdoor daylight ranges into guaranteed indoor task light.
Technical meaning
Daylight factor is the ratio of indoor daylight illuminance to outdoor reference illuminance, expressed as a percentage.
It is meaningful only when the indoor and outdoor readings or assumptions use the same daylight condition and a named indoor point or plane.
Paired daylight readingsDaylight factor needs paired indoor and outdoor readings from the same daylight condition.
Calculation use
Measured daylight factor equals indoor daylight lux divided by outdoor reference lux, multiplied by 100.
Estimated indoor daylight equals outdoor reference lux multiplied by daylight factor, divided by 100.
Not the same as
Daylight factor is not a daylight-code design result from this public calculator.
Daylight factor is not a guarantee of useful task light through the day. Orientation, sky condition, shading, glazing and room depth can change the result.
Australian context
Australian daylight notes should treat skylights, windows and shading as variable contributors and keep electric-light calculations available for the remaining target.
Examples
Example
Value
Planning note
Indoor 300 lx, outdoor 15,000 lx
2% daylight factor
Same-condition readings are required for the ratio to be meaningful.
Outdoor 20,000 lx at 1.5%
300 lx indoor estimate
An estimate for the selected point or plane, not the whole room.
Target 500 lx, daylight 300 lx
200 lx remaining
Electric lighting may still be needed depending on the task and time.
This glossary explains the daylight-factor relationship and calculator arithmetic. It does not replace climate-based daylight modelling, code assessment or measured commissioning evidence.