Evaluate how well a room is lit by the sky. Adjust the window area, glass transmittance and interior reflectance to see the average daylight factor and estimated indoor illuminance update in real time, and design a bright room that does not rely on electric lighting.
Parameters
Window area A_w
m²
Area of glazing effective for daylighting
Floor area A_f
m²
Floor area of the room considered
Glass transmittance T
Single pane ≈ 0.9 / double Low-E ≈ 0.6-0.7
Maintenance factor (dirt) M
Loss from dirt and frame. Clean ≈ 1.0 / dirty ≈ 0.6
Visible-sky angle θ
°
Smaller when more obstructions block the sky
Average surface reflectance ρ
White interior ≈ 0.7 / dark interior ≈ 0.3
Results
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Average daylight factor ADF (%)
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Estimated indoor illuminance (lux)
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Window-to-floor ratio (%)
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Total interior surface area (m²)
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Reflectance boost (%)
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Daylighting verdict
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Room cross-section — sky-light entry animation
Light enters from the overcast sky through the window and reaches deeper into the room as it bounces off the interior surfaces. The floor brightness gradient shows how far daylight reaches; the wedge marks the visible-sky angle θ.
The total interior surface area A_total is estimated as 4.5× the floor area A_f. The estimated indoor illuminance E_in is the daylight factor applied to the standard overcast-sky design outdoor illuminance E_out = 10000 lux.
What is the daylight factor?
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Is the "daylight factor" just how bright a room is? But brightness changes completely between a sunny and a cloudy day, doesn't it?
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Good question — and that is exactly the clever part. The daylight factor is the indoor illuminance divided by the outdoor illuminance at the same moment. If it is 10000 lux outside and 200 lux inside, the daylight factor is 2%. If it changes to 50000 lux outside, the room just becomes 1000 lux — divide and it is still 2%. So it is a "brightness performance value" specific to that room, independent of time of day and weather.
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I see — since it is a ratio, the weather drops out. So is a higher daylight factor always better?
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Not quite that simple. Roughly speaking, an average daylight factor of about 2% makes a room feel properly daylit — that is the practical minimum. Above 5% you get a bright, open space with big windows. But beyond 5%, glare and summer solar heat that raises the cooling load start to appear. Push the window-area slider on the left up to its 30 m² maximum: the daylight factor shoots up, but in reality that is the territory where you must add overhangs and low-solar-gain glazing.
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Looking at the formula, there is a (1−ρ²) in the denominator, where ρ is the interior reflectance. What is that doing?
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That is the interesting part. Light through the window does not stop after hitting the floor or a wall once. With light-coloured walls it bounces off and lights another surface, which bounces again — multiple inter-reflections. Because (1−ρ²) is in the denominator, raising the reflectance ρ shrinks the denominator and increases the daylight factor. In practice, a white interior and a dark-brown interior feel noticeably different through the same window. On the "Daylight factor vs surface reflectance" chart below, you can see the curve rise steeply from around ρ = 0.7.
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I am also curious about θ, the "visible-sky angle". How is that different from just making the window bigger?
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Nice point. The window area A_w is "how wide the light entrance is", while θ is "how much sky that window actually sees". For example, if a building looms next door, the window may be large but it sees almost no sky and θ becomes small. The numerator then shrinks and the daylight factor drops. "Big window but dark" on a low floor in a city is usually exactly this. A large θ means the view is open and the sky is well seen. The daylight factor depends on both the window size and the visibility of the sky.
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How should I use the daylight factor in design?
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In practice you set a target like "keep living spaces at an average daylight factor of 2% or more" and work backwards to the window size and interior colours. Below 2%, enlarge the window or lighten the interior; above 5%, add solar-control measures. Detailed work uses lighting-simulation software with mesh calculations, but a simplified formula like this tool is perfect for getting a rough first read in an early study. Try it as the very first step of design.
Frequently Asked Questions
The daylight factor is the ratio of the illuminance at a point indoors to the simultaneous outdoor horizontal illuminance, expressed as a percentage. It assumes a standard overcast (CIE) sky, so it is independent of the time of day and the weather: it is a measure of how well a room is lit by the sky alone. If the outdoor illuminance is 10000 lux or 50000 lux, a daylight factor of 2% means the indoor illuminance is 2% of it — 200 lux or 1000 lux. Being a ratio is the key feature of the daylight factor.
This tool uses the BRE/CIBSE simplified formula ADF = (T·A_w·θ·M) / (A_total·(1−ρ²)). T is the glass transmittance, A_w the window area, θ the angle of visible sky from the window, M the maintenance factor (window dirt), A_total the total interior surface area of floor, ceiling and walls, and ρ the average surface reflectance. The (1−ρ²) term in the denominator represents the boost from light bouncing repeatedly off the interior surfaces.
As a rule of thumb, an average daylight factor of about 2% is the practical minimum for a room to feel daylit. Above about 5% the space becomes a bright, generously glazed room. Beyond 5%, however, glare and summer solar heat gain start to become concerns, so it should be paired with overhangs, louvres or low-solar-gain glazing. Living rooms typically target 2-5%, while drafting rooms and operating theatres may require 5% or more.
In order of impact: (1) make the windows larger and keep them clean (increase the window area A_w and keep the maintenance factor M high), (2) use lighter-coloured interior finishes (raising the average reflectance ρ shrinks the (1−ρ²) denominator so light bounces deeper into the room), and (3) reduce obstructions in front of the window to widen the visible-sky angle θ. Conversely, a neighbouring building that blocks the sky, dark walls and dirty windows all sharply lower the daylight factor.
Real-World Applications
Daylighting design for houses and apartments: Building codes require habitable rooms to have a minimum fraction of openings effective for daylighting, but that is a window-ratio rule, while the actual brightness depends heavily on how much sky is visible and the interior colours. A daylight-factor estimate like this tool helps to judge, at the floor-plan stage, whether a room is lit deep into its interior and whether the window needs enlarging. It is especially useful for north-facing rooms and city dwellings with nearby buildings.
Office and school design: In offices and classrooms, ample daylight cuts the hours that electric lighting runs, combining energy savings with comfort. The window side has a high daylight factor and lights can be switched off, while the rear of the room has a lower factor, so deep spaces are combined with daylight-linked dimming controls. Green-building rating systems (LEED, CASBEE, etc.) also include daylight factor and daylighting criteria.
Energy efficiency and lighting planning: The higher a room's daylight factor, the less it depends on artificial lighting, so it serves as base data when estimating annual lighting energy. By predicting indoor illuminance from the design outdoor illuminance and the daylight factor, you can integrate lighting planning with daylight use — supplying with electric light only the shortfall against the required illuminance (e.g. 750 lux for an office). Larger windows increase daylight but also change heating and cooling loads, so judge on the total energy balance.
Pre-study for building-environment simulation: Detailed lighting simulations such as Radiance or DIALux model the sky luminance distribution and surrounding buildings in detail to compute the daylight factor point by point. As a preliminary step, knowing the order of magnitude of the average daylight factor from a simplified formula like this tool gives a sanity check on whether the detailed result is reasonable. A large discrepancy between the simplified formula and the detailed calculation is a clue to a model input error.
Common Misconceptions and Pitfalls
A major misconception is that the daylight factor represents brightness on a sunny day. By definition the daylight factor is based on the CIE standard overcast sky — a cloudy sky with a near-uniform luminance distribution. Direct sunlight is not included in the calculation. This is to evaluate conservatively whether the required brightness is secured "even on the worst overcast day"; on clear days it will be brighter than this. Conversely, even with a high daylight factor, a south-facing window with strong direct sun needs separate consideration of glare and heat.
Next is assuming that if the average daylight factor is sufficient, the room is uniformly bright. This tool returns an average value representing the whole room; in reality the window side is extremely bright and it gets abruptly darker towards the back. When the difference between the window side and the rear is large, the eye adapts to the bright side and the rear feels darker than it really is. In deep rooms, evaluate not only the average but also the "uniformity" (minimum daylight factor ÷ average daylight factor), and if necessary use high windows or rooflights to bring light to the back.
Finally, the oversimplification that "bigger windows are always better". A larger window does raise the daylight factor, but it also increases summer solar heat gain and the cooling load, and increases heat loss through the window in winter. A large window also invites glare from direct sun and reflections on documents and monitors. Daylighting, insulation, solar shading and view are often in trade-off relationships, and a design that only maximises the daylight factor degrades the overall performance. Treat this tool's numbers as "one aspect of daylighting", and always judge them together with heat-load and glare countermeasures.
How to Use
Enter window area in m² using the awNum slider (range 0.5–15 m²); larger apertures increase penetration depth.
Set floor area in m² via afNum (10–200 m²); daylight factor scales inversely with room size.
Adjust glass transmittance (tgNum: 0.4–0.9) to account for glazing type—standard clear float glass is 0.88, tinted reduces to 0.60.
Input maintenance factor (mfNum: 0.7–0.95) reflecting dust accumulation on glazing and interior surfaces over 3–5 years.
Click Calculate to obtain Average Daylight Factor (%), illuminance in lux, and daylighting verdict per BS 8206-2 or EN 17037.
Worked Example
Office space: window area 6 m², floor area 50 m², glass transmittance 0.85 (low-E coating), visible-sky angle 45°, maintenance factor 0.80, surface reflectance 0.50 (painted plasterboard). Simulator yields: ADF = 3.2%, estimated illuminance 640 lux at desk (assuming 200 lux/% DF on overcast sky), window-to-floor ratio 12%, total interior surface 180 m². Result meets BS 8206 minimum 2% for office task areas; verdict "Good daylighting with supplementary lighting 2–3 hours/day in winter."
Practical Notes
In deep-plan open-offices (floor area >60 m², single facade), ADF rarely exceeds 2% beyond 6 m from window; supplement with task lighting.
Surface reflectance dominates secondary illumination—upgraded ceiling finish from 0.50 to 0.80 increases ADF by 15–25% without enlarging windows.
Visible-sky angle below 25° (obstructed by adjacent buildings) cuts ADF by 40%; prioritize upper window placement or light shelves.
Retrofit secondary glazing or internal blinds reduce transmittance by 0.05–0.15; recalculate maintenance factor quarterly if dust-prone (factories, urban sites).