Daylight Density Calculator: How to Use It

A daylight density calculator works out how much natural light falls on a given area, usually expressed as a daylight factor (DF) or as illuminance in lux. You enter the room’s dimensions, window size, glass transmittance, and outdoor illuminance, and it returns the average daylight level, helping you assess whether a space feels bright or dim. This article explains the formula, how to use such a calculator, and what the numbers mean for real rooms.

Formula

The calculator uses the simplified average daylight factor formula:

DF = (Window area × Glass transmittance × 100) / (Total floor area × (1 – obstruction factor))

• DF = average daylight factor, expressed as a percentage
• Window area = window height × window width, in square meters (m²)
• Glass transmittance = fraction of visible light that passes through the glazing (typically 0.6–0.8 for double glazing)
• Total floor area = room width × room length, in square meters (m²)
• Obstruction factor = reduction due to external obstructions (buildings, trees, overhangs); 0.2 is a common default for unobstructed urban sites

The result is a percentage. A DF of 2% means the indoor daylight level is 2% of the outdoor level under an overcast sky.

Worked Example

Using the default values from the calculator:

• Room width = 4 m, room length = 5 m → floor area = 20 m²
• Window height = 1.5 m, window width = 2 m → window area = 3 m²
• Glass transmittance = 0.7
• Obstruction factor = 0.2

Plug into the formula:

DF = (3 × 0.7 × 100) / (20 × 0.8) = 210 / 16 = 13.125%

The calculator rounds to 13.13%. For illuminance, multiply the DF by the outdoor illuminance (typically 10,000 lux for an overcast sky): 13.125% × 10,000 = 1,312.5 lux, shown as 1313 lux.

What the Result Means

Daylight factor is a standard metric in building design. Here’s how to interpret the percentage:

• Below 2% – appears dim; artificial lighting will be needed most of the day.
• 2% to 5% – acceptable for living areas; daylight is noticeable but not overwhelming.
• Above 5% – bright; may cause glare or overheating in summer.

For comparison, a typical office aims for 2–3% DF, while a north-facing room in the UK might achieve only 1–2%. The average indoor illuminance (lux) gives a more intuitive feel: 300–500 lux is comfortable for reading, while 100 lux is enough to move around safely.

Choosing a Calculator

Not all daylight calculators are the same. Some are simple (like the one above) and give a rough average; others use ray-tracing or radiosity to simulate light distribution. When choosing, consider:

• Inputs – does it ask for window size, glass type, and room dimensions? More inputs usually mean more accurate results.
• Sky model – overcast vs. clear sky; overcast is standard for daylight factor.
• Output – does it give DF, lux, or both? For building code compliance, you often need DF.
• Complexity – a simple formula is fine for a quick estimate; for final design, use a validated simulation tool.

For most residential checks, the simplified formula is accurate to within 10–20% of a full simulation, which is usually enough to spot a daylight problem.

Common Mistakes and Limits

Unit errors are the most frequent issue. Always use meters for length and width; if you measure in feet, convert (1 ft = 0.3048 m). Also, the formula assumes a rectangular room with a single window and no internal obstructions. It ignores:

• Window frame thickness (use the glass area, not the frame opening)
• Reflectance of walls, floor, and ceiling (affects light distribution)
• External obstructions beyond the simple factor
• Skylights or multiple windows (treat as separate and add results)

Another common mistake is using the window’s total height when part of it is blocked by a curtain or overhang. Use the unobstructed glass area. Also, remember that daylight factor is measured under an overcast sky; on a sunny day, direct sunlight can make the space much brighter, but it also creates glare and heat.

Alternatives to Daylight Factor

If you need more detail than a simple average, consider:

• Daylight autonomy – the percentage of occupied hours when daylight alone meets a target illuminance (e.g., 300 lux). This is more useful for energy savings.
• Useful daylight illuminance (UDI) – the time when daylight is between 100 and 2000 lux, avoiding both dimness and glare.
• Point-in-time illuminance – a single lux value at a specific point, useful for task areas.
• Physical scale models – for complex geometries, a model under a sky simulator gives realistic results.

These require more advanced software or measurements, but they answer different questions. The simple calculator here is a starting point, not a substitute for a daylighting design.

Frequently Asked Questions

What is a good daylight factor for a living room?

Most guidelines suggest 2–5% for a living room. Below 2% feels gloomy; above 5% can cause glare and fading of furniture. For bedrooms, 1–2% is often acceptable since you don’t spend much time in them during the day.

How do I measure glass transmittance?

Check the window’s specification sheet. Double glazing typically has a visible transmittance of 0.6–0.8. If you don’t have the spec, use 0.7 as a default. For single glazing, use 0.85–0.9; for triple glazing, 0.5–0.7.

Can I use this calculator for a room with two windows?

Yes, but you need to run the calculation for each window separately and add the daylight factors. The total DF is the sum of the individual contributions, assuming the windows are not facing each other in a way that causes double-counting.

Does the calculator account for window orientation?

No, the simplified formula assumes an overcast sky, which is uniform in all directions. Orientation matters more for direct sunlight and for clear-sky conditions. For a rough average, orientation is less important; for a detailed analysis, use a simulation that includes latitude and orientation.

What is the difference between daylight factor and lux?

Daylight factor is a percentage that compares indoor to outdoor light; it is constant regardless of weather. Lux is an absolute measure of illuminance. For example, a DF of 5% gives 500 lux outdoors on a bright overcast day (10,000 lux), but only 250 lux on a darker day (5,000 lux).

Is a higher daylight factor always better?

Not necessarily. Too much daylight can cause glare, overheating, and UV damage to fabrics. The ideal DF depends on the room’s function and climate. In hot climates, you might want to limit daylight to reduce cooling loads, while in cold climates, more daylight can offset heating costs.

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in w | Room width | m | 4 | 0
in l | Room length | m | 5 | 0
in h | Window height | m | 1.5 | 0
in ww | Window width | m | 2 | 0
in g | Glass transmittance (0-1) | - | 0.7 | 0
out df | Average daylight factor | % | (ww * h * g * 100) / (w * l * (1 - 0.2)) | 2
out lux | Average indoor illuminance | lux | (ww * h * g * 10000) / (w * l * (1 - 0.2)) | 0