Inputs
3 columns large • 2 columns medium • 1 column smallExample Data
| Case | Room (L×W×H) | Windows (W×H × n) | VT | Sky | UF | WFR | DF% | Indoor Lux |
|---|---|---|---|---|---|---|---|---|
| A | 8×6×3 m | 1.8×1.5 m × 2 | 0.65 | 0.20 | 0.55 | 11.25% | 1.31% | 131 lux |
| B | 10×7×3 m | 2.1×1.8 m × 3 | 0.70 | 0.22 | 0.60 | 16.20% | 2.33% | 233 lux |
| C | 6×5×3 m | 1.5×1.2 m × 2 | 0.55 | 0.18 | 0.50 | 12.00% | 1.01% | 101 lux |
Formula Used
- Floor area: Af = L × W
- Total window area: Aw = (Ww × Hw) × n
- Window-to-floor ratio: WFR% = 100 × Aw / Af
- Effective transmittance: Teff = VT × Framing × Obstruction × Maintenance
- Average indoor illuminance: Ein approx Eout × Sky × (Aw/Af) × Teff × UF × (1 + 0.6R)
- Daylight factor: DF% approx 100 × Sky × (Aw/Af) × Teff × UF × (1 + 0.6R)
- Effective aperture: EA% = 100 × (Aw/Af) × VT × Obstruction × Framing
How to Use This Calculator
- Enter room and window dimensions using your selected units.
- Set glazing VT and practical reduction factors for frames and dirt.
- Adjust sky, utilization, and obstruction factors for site reality.
- Choose an outdoor illuminance reference and indoor target lux level.
- Press Calculate to view results above the input form.
- Download CSV for spreadsheets or PDF for sharing and review.
Daylight metrics for early design
This calculator estimates average indoor illuminance and daylight factor using window area, visible transmittance, and practical loss factors. It supports quick feasibility checks before detailed modeling. Treat the numbers as planning outputs: they are most reliable for comparing options built from the same assumptions, on the same project. Use consistent inputs to track progress across revisions.
Window-to-floor ratio benchmarks
Window-to-floor ratio (WFR) is a fast indicator of daylight potential. Many general-use spaces begin to perform better when WFR is near 10–20%, while values above 30–35% may raise glare and cooling risk. For deep rooms, raise window head height or introduce secondary daylight paths, because WFR alone cannot describe distribution along the back wall.
Selecting practical reduction factors
Effective transmittance combines glazing VT with framing, obstruction, and maintenance factors. VT for clear glazing often ranges from 0.55 to 0.75, while heavy tint can fall below 0.50. Maintenance factors around 0.75–0.90 represent soiling and aging. Obstruction factors near 0.60–0.85 reflect nearby buildings, overhangs, or vegetation. When uncertain, select the lower end to avoid optimistic daylight claims during budgeting and approvals.
Interpreting daylight factor and lux
Daylight factor expresses indoor daylight as a percentage of outdoor illuminance under diffuse sky. As a planning reference, around 2% is frequently associated with usable daylight in offices, while circulation spaces may accept lower values. The lux estimate helps compare against a target such as 300–500 lux for general tasks. If lux is below target, you can estimate the remaining electric lighting demand and prioritize the zones with the highest occupancy.
Using results for option comparison
Run multiple scenarios by adjusting window size, count, and factors to test design moves. Increasing reflectance with lighter walls and ceilings can improve spread, while better framing and reduced obstructions raise effective aperture. If results are short of target, iterate with larger openings, added clerestories, light shelves, or redistributed glazing across facades. Keep notes for each run so teams can justify why an option was selected.
FAQs
1) What does the sky factor represent?
It is a simplified multiplier for diffuse daylight availability at the facade. Use higher values for clearer, brighter conditions and lower values for constrained sky views or conservative assumptions.
2) Why can my WFR be high but lux still low?
Low VT, strong obstructions, heavy framing, or poor utilization can offset larger glazing. Check effective transmittance and utilization factor, then adjust shading, geometry, or surface reflectance.
3) How should I choose utilization factor?
Use a mid-range value for typical rooms, then refine based on depth, window head height, and distribution. Deep rooms often have lower utilization than shallow rooms with higher openings.
4) Is daylight factor enough for final decisions?
No. It is an early indicator under diffuse conditions and does not capture sun, glare, or dynamic shading performance. Use it to rank options, then validate with detailed analysis.
5) What targets should I use for indoor lux?
General task areas commonly target 300–500 lux, while corridors may be lower. Pick a target aligned with your space use, then assess how much electric lighting supplementation is needed.
6) Why include maintenance and framing factors?
They prevent overestimation by accounting for dirt accumulation and frame area that reduces clear opening. Using realistic factors helps early designs avoid optimistic daylight claims.