Daylight Factor
Calculator
Calculate average daylight factor using the BRE simplified formula. Check compliance against the 2% habitable room target, LEED daylighting credits, and BREEAM Hea 01.
Who it's for: architects and daylighting designers checking early-stage daylight provision. You enter room and window dimensions and get the average daylight factor by the BRE method — accessible, but precise enough to flag rooms that miss the 2% target before simulation.
How to use this calculator
The formula
DF = (W × T × θ/90) ÷ (S × (1 − R²)) × 100
W — total glazing area in m² (window width × height × number of windows)
T — visible light transmittance of the glass (Tvis); clear double glazing is typically 0.70–0.78
θ — sky angle in degrees: arctan((sill height + window height) ÷ room depth) × 180/π
S — total internal surface area of all enclosing surfaces in m²: 2(depth × height) + 2(width × height) + 2(width × depth)
R — area-weighted average surface reflectance of walls, floor, and ceiling
Worked example
When this estimate will be off
- Assumes no external obstructions — a building opposite, deep reveals, or roof overhangs will reduce the actual sky angle and lower real DF significantly.
- Returns a single room-average; daylight close to the window can be several times the back-of-room value. Point illuminance simulation is needed to assess glare or task-level adequacy.
- Does not apply a dirt or frame factor — accumulated glass dirt and window frame obstruction typically reduce real DF by 10–20% compared to the calculated figure.
- Uses a standard CIE overcast sky with no orientation or seasonal variation. LEED v4 Option 1 requires spatial daylight autonomy (sDA) and annual sunlight exposure (ASE) from full climate-based simulation.
Frequently asked questions
A common mistake: Assumes no external obstructions — a building opposite, deep reveals, or roof overhangs will reduce the actual sky angle and lower real DF significantly.
Daylight factor is the ratio of indoor daylight illuminance to the unobstructed outdoor illuminance under a standard overcast sky, expressed as a percentage. A DF of 2% means the working plane receives 2% of the available exterior daylight. It rates daylight provision independent of orientation, because the overcast sky is assumed uniform.
The BRE average daylight factor uses four inputs: total glazing area, the glazing visible transmittance (Tvis), the total internal surface area of the room, and the area-weighted average surface reflectance. DF = (T × glazing area × sky angle) ÷ (surface area × (1 − reflectance²)). Larger or clearer glazing raises DF; more surface area or lower reflectance lowers it.
A daylight factor of 2% is the long-standing target for habitable rooms and general office work, giving usable daylight without constant electric light. Below 1% a space reads as poorly lit and needs supplementary lighting most of the day. Above 5% a room is well daylit but can risk glare and summer overheating.
The average daylight factor is a simplified overcast-sky estimate. It ignores external obstructions, internal partitions, window orientation, and direct sunlight, and it reports one room average rather than a point grid. LEED v4 Option 1 instead requires spatial daylight autonomy (sDA) and annual sunlight exposure (ASE) from Radiance or EnergyPlus simulation.
Sources
- CIBSE Lighting Guide 10 (LG10): Daylighting and Window Design — describes the BRE simplified average daylight factor formula used in this calculator and the standard 2% habitable-room target
- BREEAM Technical Standards — Hea 01 Visual Comfort — references average daylight factor as one route to daylighting credits for occupied spaces
This free Daylight Factor Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.