Daylight Factor Calculator - Free Online Tool

Calculate your daylight factor with our free online tool.

Get accurate results instantly.

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How This Tool Works

The Daylight Factor Calculator uses the standard daylight factor relationship: DF = (indoor illuminance / outdoor illuminance) x 100. Enter the measured illuminance inside the room and the simultaneous outdoor illuminance, both in lux, and the tool returns the daylight factor as a percentage.

For example, 300 lux indoors with 10,000 lux outdoors gives a daylight factor of 3.00%.

  • Input: Indoor illuminance and outdoor illuminance in lux.
  • Formula: (Indoor lux / Outdoor lux) x 100.
  • Output: Daylight factor percentage with a practical quality rating.

Why This Matters for Building Design

Daylight factor helps compare how much available outdoor daylight reaches an indoor space under comparable sky conditions. It is useful for early checks of room daylight access, occupant comfort, and whether a space may need supplemental electric lighting.

A daylight factor around 2% or higher is commonly treated as a useful baseline for many occupied spaces, while the appropriate target depends on room type, task, glazing, obstructions, and local design standards.

Common Mistakes to Avoid

  • Use matching measurements: Indoor and outdoor lux should be measured at the same time or under the same assumed sky condition.
  • Do not mix units: Both inputs must be in lux before calculating the ratio.
  • Keep context in mind: The result is a quick daylight ratio, not a full daylight simulation with window geometry, orientation, shading, or room reflectance.

Frequently Asked Questions

Common questions about the Daylight Factor Calculator - Free Online Tool

Lumens measure total light output from a source, while lux measures light intensity at a surface. Lux = lumens per square meter.
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Sources & References

International System of Units (SI): luminous intensity and illuminance

Luminous intensity and illuminance is measured in the candela (cd) and lux (lx). Conversions between SI and other units use exact, internationally agreed factors maintained by NIST.
→Source: NIST — International System of Units (SI)

International System of Units (SI)

Authoritative definitions for luminous intensity and illuminance, from the BIPM SI Brochure (9th edition), the defining reference for the SI.
→Source: BIPM — SI Brochure (9th ed.)

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