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BRE MethodEarly-stage screeningRoom-average estimateFree

Daylight Factor
Calculator

Calculate average daylight factor from room and window geometry using the simplified formula below. Explore early-stage options; this tool does not determine LEED, BREEAM, code or planning compliance.

Who it's for: architects and daylighting designers comparing early-stage room and window options. Enter geometry, glazing transmittance and surface reflectance to obtain a screening estimate, then verify the applicable project criteria and required assessment method separately.

1
Enter room dimensions
Width × depth × ceiling height in metres
2
Enter window dimensions
Width × height × sill height, plus glazing transmittance
3
Read the screening result
Average DF% · window-to-wall ratio · assumptions
✓ Visible formula and assumptions✓ Average DF% and WWR outputs✓ No certification verdict
Daylight Factor Calculator
Simplified room-average screening
Room Dimensions
Window Dimensions (one window)

How to use this calculator

1
Enter the room dimensions
Width × depth × ceiling height in metres. These set the floor area and the total internal surface area used in the denominator.
2
Set the average surface reflectance
The area-weighted mean reflectance of walls, floor, and ceiling. 0.5–0.6 is typical for light-coloured interiors; darker finishes lower the daylight factor.
3
Enter the window size
Width × height of one window, the sill height above the floor, and how many windows sit on the wall.
4
Enter the glazing transmittance (Tvis)
Visible light transmittance of the glass. Clear double glazing is about 0.70–0.78; tinted or triple glazing is lower.
5
Read the average daylight factor
The result reports estimated average DF%, window-to-wall ratio and glazing area. Compare options using the same assumptions; verify any project target and assessment method separately.

The formula

The average daylight factor (DF) expresses what fraction of outdoor overcast-sky daylight reaches the working plane inside the room, as a percentage.

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

Example
A 4.0 m × 4.0 m room with a 2.7 m ceiling has one 2.0 m × 1.8 m window (sill 0.8 m, glazing Tvis = 0.75) and 0.60 average surface reflectance. Glazed area = 3.60 m²; total internal surface = 75.2 m²; the visible-sky angle gives θ/90 = 0.367. Average DF = (3.60 × 0.75 × 0.367) ÷ (75.2 × (1 − 0.60²)) × 100 = 2.06%. Window-to-wall ratio = 33.3%. These are arithmetic outputs for the stated assumptions, not a compliance or certification verdict.

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.
  • Dirt and frame factors are not included. Reconcile the entered glazing dimensions and transmittance with the intended calculation basis before comparing options.
  • No LEED, BREEAM, code or planning compliance result is returned. The tool does not calculate sDA, ASE, illuminance grids or daylight uniformity; assess the applicable scheme and edition separately.

Frequently asked questions

Treating this simplified room-average estimate as a compliance verdict. It does not model external obstructions or point-by-point daylight distribution, and it cannot establish a rating-system credit.

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. This calculator returns a simplified room-average estimate and does not model orientation, local weather or direct sunlight.

This tool uses room and window geometry, glazing visible transmittance, average surface reflectance and the stated sky-angle approximation. See the equation above for its normalization, units and assumptions. The output is a sensitivity estimate, not a point-by-point simulation.

There is no universal pass/fail threshold applied by this tool. Confirm the room use, jurisdiction, assessment scheme and edition, including any area, uniformity or illuminance requirements. A 2% room-average result alone does not establish compliance or visual comfort.

No. This tool returns a simplified screening estimate only. LEED and BREEAM assessments depend on the applicable scheme, edition and assessment route, with requirements this calculator does not implement. Consult the linked official guidance and the responsible project assessor.

This calculation omits external obstructions, internal partitions, orientation, local weather and direct sunlight. It reports one room average, not an illuminance grid, daylight uniformity or annual sDA/ASE result. Use the applicable assessment method and qualified review for project decisions.

Sources

This free Daylight Factor Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.