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    Building physics / Daylight

    Daylighting Calculator

    The average daylight factor of a room from the window area, its transmittance and the room surfaces. Above 2% reads as daylit; below 1% needs electric light most of the day.

    Daylight factor1.56%Glazing ratio5.0%Transmitted light4.2 m²
    Daylight factor
    1.56%
    Partial
    Glazing ratio
    5.0%
    Of internal surface
    Transmitted light
    4.2 m²
    Effective aperture
    Verdict
    Marginal
    BRE guidance
    Sensitivity to glazing area
    Split-flux estimate: DF = (A_glass · τ · θ) / (A_total · (1 − ρ²)).
    Sensitivity to glazing area
    Glazing (m²)Daylight factor
    20.52%
    41.04%
    61.56%
    82.07%
    123.11%
    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Daylight factor against glazing
    Linear in the glass area.
    Daylight factor against glazing0.0000.6971.392.092.793.482 m²2 m²4 m²4 m²6 m²6 m²8 m²8 m²12 m²12 m²Daylighting Calculator
    Canonical equations
    The exact relationships implemented by this engine.

    DF = (A_g·τ·θ)/(A·(1−ρ²)) · Split-flux, average over the room.

    2% average · BRE: 2% is a daylit space.

    engineering methodology

    A clear path from inputs to results.

    The average daylight factor of a room from the window area, its transmittance and the room surfaces. Above 2% reads as daylit; below 1% needs electric light most of the day.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Set glazing area (m²).
    2. Set visible transmittance.
    3. Set total room surface (m²).
    4. Set average reflectance.
    5. Read the headline metrics, then check the table for the detail.

    The formula

    DF=(Ag⋅τ⋅θ)/(A⋅(1−ρ2))DF = (A_g·τ·θ)/(A·(1−ρ²))

    Reference: Split-flux daylight factor, BRE guidance.

    Worked example

    At the default inputs

    Inputs
    • Glazing area (m²): 6; Visible transmittance: 0.7; Total room surface (m²): 120; Average reflectance: 0.5; Sky angle (degrees): 60
    Calculation path
    1. 1

      Daylight factor: 1.56% · Glazing ratio: 5.0% · Transmitted light: 4.2 m² · Verdict: Marginal

    Result

    Daylight factor: 1.56% · Glazing ratio: 5.0% · Transmitted light: 4.2 m²

    Interpreting the result

    6.0 m² of glass in 120 m² of surface gives a 1.56% daylight factor.

    Assumptions & limitations

    • Average over the room, not a point value.
    • One window wall, unobstructed.

    Frequently asked questions

    What is a good daylight factor?

    2% average is the usual target for a daylit space; 5% is brightly daylit.

    Does orientation matter?

    Not in the split-flux average, which is why it is only an estimate.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.