Daylight Factor Calculator
Plan your architectural & design project with our free daylight factor calculator. Get precise measurements, material lists, and budgets.
Formula
DF = (Aw × T × M) / (Af × (1 - R²)) × 100
The average daylight factor equals the net glazed window area times glass transmittance times maintenance factor, divided by the total floor area times (1 minus the square of the average room surface reflectance), expressed as a percentage.
Worked Examples
Example 1: Open-Plan Office Daylighting
Problem: An office has 12 m² of double-glazed windows (transmittance 0.70) and a floor area of 80 m². Maintenance factor is 0.8. Check the daylight factor.
Solution: DF = (Aw × T × M) / (Af × (1 - R²)) × 100\nDF = (12 × 0.70 × 0.8) / (80 × (1 - 0.25)) × 100\nDF = 6.72 / 60 × 100 = 11.2%\nThis exceeds the 2% target — glare control may be needed.
Result: DF = 11.2% — Excellent daylight, consider shading
Example 2: Residential Bedroom Assessment
Problem: A bedroom has 2.5 m² of window area, 15 m² floor area, single glazing (T = 0.85), and a maintenance factor of 0.9.
Solution: DF = (2.5 × 0.85 × 0.9) / (15 × 0.75) × 100\nDF = 1.9125 / 11.25 × 100 = 17.0%\nNote: In practice, obstructions and frame deductions would lower this value significantly.
Result: DF = 17.0% — Adequate for residential use
Frequently Asked Questions
What is the daylight factor and why does it matter?
The daylight factor (DF) is the ratio of internal illuminance at a point inside a building to the unobstructed external horizontal illuminance, expressed as a percentage. It is measured under overcast sky conditions to provide a consistent benchmark. A daylight factor of 2% means the indoor light level is 2% of the outdoor level. This metric matters because adequate daylight reduces energy consumption from artificial lighting, improves occupant health and productivity, and is increasingly required by building regulations such as BREEAM and LEED. Designers use daylight factor calculations to ensure rooms receive sufficient natural light throughout the year.
What is a good daylight factor for different room types?
The recommended daylight factor varies by room function and building code. For residential living rooms and bedrooms, a minimum average DF of 1.5% to 2% is generally acceptable. Kitchens benefit from higher values of 2% or more since food preparation tasks need good visibility. Offices typically target an average DF of 2% to 5% for comfortable working conditions without excessive glare. Classrooms and educational spaces should aim for 3% to 5% to support reading and concentration. Hospitals and healthcare facilities often require 3% or above. Industrial spaces may need lower or higher values depending on the specific tasks performed. Values above 5% are considered excellent but may require glare control.
What is the depth rule for daylight in rooms?
The depth rule, sometimes called the 2.5 times rule, is a quick check to determine whether a room will be adequately daylit. It states that the room depth should not exceed 2.5 times the window head height for side-lit rooms to achieve an acceptable average daylight factor. For example, if the top of the window is 2.8 meters above the floor, the room should be no deeper than 7 meters. Beyond this depth, daylight levels drop significantly, and supplementary artificial lighting becomes necessary. Some standards use a ratio of 2.0 for more conservative designs. This rule applies primarily to single-sided daylighting; rooms with windows on opposite walls can be much deeper.
How do glass transmittance and maintenance factor affect calculations?
Glass transmittance is the fraction of light that passes through the glazing material. Clear single-pane glass has a transmittance of about 0.85 to 0.90, double glazing typically ranges from 0.65 to 0.75, and triple glazing or coated glass can drop to 0.50 to 0.60. Lower transmittance means less daylight enters the room. The maintenance factor accounts for dirt accumulation on glass surfaces over time, which reduces light transmission. Clean environments like offices might use a maintenance factor of 0.8 to 0.9, while industrial or urban settings may use 0.6 to 0.7. Both factors multiply together to reduce the effective daylight entering the space, making them critical inputs for accurate calculations.
What is a structural safety factor and why is it important?
A safety factor is the ratio of a structure's actual strength to the maximum expected load. Building codes typically require safety factors of 1.5 to 3.0 depending on the material and application. This accounts for material variations, unexpected loads, and degradation over time.
How accurate are the results from Daylight Factor Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.