How to use the daylight factor calculator
Daylight shapes how a room feels and how much electric light it needs, and planners and clients increasingly ask for evidence of it early. This calculator gives the average daylight factor for a side-lit room using the standard BRE formula, compares it with a target for the room type, and runs the no-sky-line room depth check, so you can test window sizes and room proportions at sketch stage before anything is modelled.
- Pick the units, metres or feet, and enter the room width (along the window wall), depth and ceiling height.
- Enter the glazed area of all windows in the room, measured as the glass, not the frame, and the height of the window head above the floor.
- Choose the glass type, then enter the visible sky angle: 90° for an unobstructed vertical window, less if a building or tree blocks the lower sky.
- Set the finishes and cleaning with the reflectance and maintenance factor, then pick the room type to set the target.
The results show the average daylight factor, the glazing as a share of the floor area, the maximum room depth for good daylight, the rule-of-thumb daylight penetration, and whether the room meets its target.
Daylight factor formula
Average DF = (W × T × θ × M) ÷ (A × (1 − R²))
A = 2 × (width × depth + width × height + depth × height)
Maximum depth: L/W + L/H ≤ 2 ÷ (1 − Rb)
Daylight penetration ≈ 2.5 × window head height
W is the net glazed area, T the visible light transmittance, θ the visible sky angle in degrees, M the maintenance factor, A the total area of every room surface including the windows, and R the average reflectance. The calculator uses the same reflectance for the back of the room (Rb) in the depth check. Results are averages: points near the window will be much brighter, and the back corners darker.
| Average daylight factor | How the room reads |
|---|---|
| Under 2% | Looks gloomy; electric lighting likely needed for much of the day |
| 2% to 5% | Adequately daylit; some supplementary lighting at times |
| 5% and above | Well daylit; watch for glare and solar gain |
Worked examples
A typical living room
A 4 m wide, 5 m deep room with a 2.7 m ceiling has 88.6 m² of surfaces. With 3.6 m² of low-e double glazing (T 0.70), an open view (θ 90°), clean glass (M 0.9) and typical finishes (R 0.5), the average daylight factor is about 3.1%. The window head at 2.2 m allows a depth of about 5.7 m, so the 5 m room passes both checks.
A deep open-plan room in feet
A room 14 ft wide, 20 ft deep and 9 ft high has 1,172 ft² of surfaces and 30 ft² of glass with a 7 ft head height. The average comes out at about 1.9%, and the depth limit is about 18.7 ft, so the back 1.3 ft will feel dim. A taller window head or a second window on another wall would fix both.
An obstructed city bedroom
A 3.5 × 4.5 m bedroom, 2.6 m high, faces a building that cuts the visible sky to 60°. With 2.4 m² of triple glazing (T 0.60) the average daylight factor is about 1.4%, which clears the 1% bedroom target but would fail the 2% figure for a kitchen.
Who this calculator helps
- Architects sizing windows and setting room depths at concept stage.
- Interior designers judging whether a room needs lighter finishes or more artificial light.
- Planning consultants preparing quick daylight evidence before a full study.
- Developers and self-builders comparing layouts for deep-plan apartments.
- Students learning how glazing, obstruction and room shape affect daylight.
Tips for better daylight
- Raise the window head: glass high on the wall throws light deeper than the same area low down.
- Use light finishes on ceilings and the back wall; moving R from 0.4 to 0.6 makes a noticeable difference.
- Balance daylight against heat loss and gain with the window-to-wall ratio calculator.
- Check room areas with the square footage calculator, and remember that obstructions set by the setback calculator also limit the sky angle.
- For planning or certification, follow up with a full study under BS EN 17037 or climate-based modelling.
Want to show a client how the room looks in different light? Try a new lighting mood on a photo or render with Lighting Transfer.
