Daylight Factor Calculator

A daylight factor expresses how much of the measured outdoor illuminance is present at an indoor measurement point. The calculator produces a dimensionless ratio and a percentage from paired readings:

\(\displaystyle \text{daylight factor ratio} = \frac{\text{Indoor illuminance}}{\text{Outdoor illuminance}}\)

\(\displaystyle \text{daylight factor percent} = \text{daylight factor ratio} \times 100\)

The result is useful for preliminary daylight availability review near windows, skylights, atriums, and other daylight openings. Electrical designers and lighting professionals can use it to compare daylight conditions among interior locations, review locations where daylight-responsive controls may be considered, or document a lighting survey before evaluating electric-light contributions.

The calculation does not establish maintained illuminance, lighting power compliance, automatic daylighting-control requirements, energy-code compliance, or a lighting design approval.

Paired Illuminance Measurements

The calculator uses two measured quantities:

Input	Field purpose	Measurement requirement
Indoor illuminance	Illuminance measured at the indoor point being evaluated	Enter the measured indoor illuminance
Outdoor illuminance	Illuminance measured outdoors for the same measurement event	Enter the paired measured outdoor illuminance

Both readings must use a clearly documented common unit and measurement condition. Lux is shown in the example, but the ratio remains valid in footcandles if both the indoor and outdoor readings use footcandles under the same documented condition.

Do not combine an indoor lux reading with an outdoor footcandle reading. Do not pair readings taken at materially different times, under changing cloud cover, or with different meter placement conditions unless the measurement method explicitly accounts for those differences.

For practical lighting measurement, record the date and time, indoor location, work-plane height, meter orientation, outdoor sensor location, weather or sky condition, whether electric lighting was operating, and any shading position that affects the space. Those notes make the daylight-factor result traceable when the same room is reviewed later.

Calculation Result

The calculator reports:

• Daylight factor as a decimal ratio
• Daylight factor as a percentage
• Indoor illuminance used
• Outdoor illuminance used

A higher daylight factor means the indoor point received a larger fraction of the outdoor illuminance during the paired measurement. It does not mean the indoor space necessarily has adequate task lighting. A space can show a comparatively high daylight factor during bright exterior conditions while still requiring properly designed branch-circuit lighting for nighttime operation, overcast conditions, glare control, or uniform work-plane illuminance.

The daylight factor is also separate from voltage-drop review, branch-circuit ampacity, feeder sizing, raceway fill, and conductor selection. Those electrical calculations address the distribution system supplying luminaires and controls; daylight factor describes measured light availability at a location.

Calculation Example

Use the following paired illuminance measurements:

Field	Value
Indoor illuminance	300 lux
Outdoor illuminance	10,000 lux

First, calculate the ratio:

\(\displaystyle \text{daylight factor ratio} = \frac{300}{10{,}000} = 0.03\)

Then convert the ratio to percent:

\(\displaystyle \text{daylight factor percent} = 0.03 \times 100 = 3\%\)

Result:

Result	Value
Daylight factor	0.03 ratio
Daylight factor	3%
Indoor illuminance used	300 lux
Outdoor illuminance used	10,000 lux

The measured indoor point received illuminance equal to 3% of the measured outdoor illuminance under the documented measurement conditions.

Field Conditions and Limits

Daylight factor arithmetic assumes that the two readings represent a paired condition. The result is not normalized for solar position, cloud cover, seasonal changes, adjacent building reflections, exterior obstructions, glazing characteristics, blinds, shades, window geometry, room reflectance, or changing weather.

The calculation also does not apply a sky model, window model, target illuminance, electric-light contribution, photometric calculation, or energy-code analysis. A daylight-factor percentage alone cannot determine whether luminaires, daylight-responsive controls, occupancy sensors, lighting control zones, branch circuits, feeders, or control wiring meet a project requirement.

Use the recorded value as a measurement-based daylight screening result. Complete electrical and lighting decisions separately using the project lighting design, luminaire photometrics, control sequence, applicable adopted code, manufacturer instructions, commissioning requirements, and the requirements of the AHJ.