Average daylight factor calculator

What is the daylight factor?

The daylight factor is the ratio of the daylight illuminance inside a room to the illuminance outdoors under an unobstructed overcast sky, expressed as a percentage. The average daylight factor describes the room as a whole and is the figure used in early-stage checks: a room with an average daylight factor of 2 per cent receives, on average, 2 per cent of the outdoor overcast-sky illuminance on its working plane.

Because it is a ratio under an overcast sky it does not depend on orientation or the time of year, which makes it simple to calculate and to compare between rooms. That simplicity is also its limit: it says nothing about sunlight, glare or how the daylight varies through the day, which is why BS EN 17037 and the 2022 edition of BRE 209 move towards target illuminance and climate-based modelling for detailed assessment.

How to use the daylight factor calculator

1. 01Enter the room length, width and height in metres.
2. 02Enter the window width and height and the number of windows, or the total glazing area.
3. 03Choose the glass type. The calculator applies typical visible transmittance values: single glazing 0.85, double clear 0.72, double low-E 0.65, triple 0.60, triple low-E 0.52, tinted double 0.45, or enter a custom value from the glazing datasheet.
4. 04Set the visible sky angle at the window (default 65°) and reduce it where buildings, trees or overhangs obstruct the view of the sky, or enter the obstruction angle.
5. 05Adjust the surface reflectances (defaults: ceiling 0.7, walls 0.5, floor 0.2) and the maintenance factor for dirt on the glass (default 0.9), then read the average daylight factor against the target for the room type.

The BRE average daylight factor formula

DF = (T × Aw × θ) ÷ (A × (1 − R²)), where T is the diffuse visible transmittance of the glazing (including a maintenance factor), Aw the net glazed area, θ the visible sky angle in degrees, A the total area of all interior surfaces and R their area-weighted average reflectance.

The numerator is the light that gets in: how much glass there is, how clear it is and how much sky it can see. The denominator is how the room absorbs it: a large room with dark surfaces needs far more glazing to reach the same daylight factor than a small, light-coloured one. The (1 − R²) term is why painting a ceiling white does more for daylight than most people expect.

The formula assumes a CIE overcast sky, side windows in a broadly rectangular room and no external reflections. It is the method given in BRE Report 209 and in CIBSE Lighting Guide LG10 for early-stage estimates.

Worked example

A kitchen 5 m long, 4 m wide and 2.7 m high has one window 1.5 m by 1.5 m (2.25 m² of glazing) in double clear glazing with a transmittance of 0.72 and a maintenance factor of 0.9, so T is 0.648. The window sees 65° of sky. The interior surface area A is 2 × (5 × 4) + 2 × (5 × 2.7) + 2 × (4 × 2.7) = 88.6 m². With a ceiling reflectance of 0.7, walls 0.5 and floor 0.2, the area-weighted average reflectance R is (20 × 0.7 + 48.6 × 0.5 + 20 × 0.2) ÷ 88.6 = 0.48, so (1 − R²) is 0.77.

DF = (0.648 × 2.25 × 65) ÷ (88.6 × 0.77) = 94.8 ÷ 68.2 = 1.4 per cent. Against the 2 per cent target the calculator uses for a kitchen, the room falls short: a second window, a larger pane or lighter wall finishes would be the first things to try.

Typical daylight factor targets

These are the traditional BRE targets and remain useful as a first check. For planning and formal assessment, BS EN 17037 and BRE 209 (2022) express daylight provision as a target illuminance achieved for half of daylight hours, which needs climate-based daylight modelling rather than a daylight factor.

Limitations, and when to involve a lighting designer

The calculator estimates an average value for a simple room under an overcast sky. It does not model rooflights, light wells, deep plans, external reflections, sunlight or the distribution of daylight across the room, and it is not a BS EN 17037 or BREEAM Hea 01 assessment. Use it to test ideas at sketch stage: window size, glass choice and surface finishes.

When daylight has to be demonstrated for planning, BREEAM or a client brief, I model the room and its surroundings and report the daylight metrics the relevant standard asks for, then design the electric lighting and its daylight-linked controls around the result so the two work together. Pro users can export the quick calculation as a PDF.