Daylight Factor Calculator
NBC 2016 Part 8 and IS 2440 compliant — check window-to-floor area ratio, compute average daylight factor using the Littlefair/BRE formula, and get room-type benchmarks for residential, office, classroom, and hospital spaces.
Room & Window
Room Dimensions
Window (each)
0 = clear, 45 = major
Room default 0.40
Daylight Results
Avg DF
0.82
%
WFR
11.1
%
WWR
13.3
%
Sky Angle
55
°
| Sky Condition | Outdoor (lux) | Indoor Avg (lux) | Reference |
|---|---|---|---|
| Overcast monsoon | 10,000 | 82 | IS 2440 reference |
| Clear post-monsoon | 35,000 | 286 | ECBC Appendix B |
Compliance Checks
NBC Window-to-Floor (min 10%)
NBC 2016 Part 8 Section 1 · your WFR 11.1%
11.1%
IS 2440 Avg DF (target 1%)
Task-specific DF per IS 2440 / CIBSE LG1
0.82%
Effective Daylight Depth
Within 2.5 × window head height
3.6 m
Recommendations
- •Raising mean surface reflectance from 0.40 to 0.50 (lighter walls/ceiling) lifts DF by ~-11%.
Living / Dining Room: NBC habitable room; comfortable daylight without glare.
Reference
NBC 2016 Part 8 & IS 2440 Benchmarks
NBC 2016 Part 8 Section 1 specifies prescriptive minimum Window-to-Floor area ratios. IS 2440:1975 and CIBSE LG1 provide performance benchmarks for average Daylight Factor.
| Room Type | Min WFR (NBC) | Target Avg DF (IS 2440) | Design Note |
|---|---|---|---|
| Living / Dining Room | 10% | 1% | NBC habitable room; comfortable daylight without glare. |
| Bedroom | 10% | 0.5% | Lower DF acceptable; glare control more important than illuminance. |
| Kitchen | 10% | 2% | Task area — DF 2% supports food-prep visibility per IS 3646 Part 2. |
| Bathroom / WC | 5% | 0.6% | NBC reduced minimum; ventilation often governs over daylight. |
| Staircase / Corridor | 7.5% | 1% | NBC 7.5% minimum; high DF needed for safe negotiation. |
| Study / Home Office | 10% | 2% | IS 2440 task area; DF 2% = ~200 lux under 10,000 lux overcast. |
| Office (General) | 12.5% | 2% | ECBC encourages DF ≥ 2% for 75% of regularly occupied area. |
| Classroom | 20% | 2.5% | NBC 20% min; IS 2440 recommends DF 2–3% for visual tasks. |
| Hospital Ward | 15% | 1.5% | Higher reflectance and DF support patient recovery (WHO 2007). |
| Examination / Treatment Room | 15% | 3% | High DF required for clinical observation; supplement with artificial. |
Source: NBC 2016 Part 8 Section 1 (Lighting and Ventilation); IS 2440:1975 Guide for Daylighting of Buildings; IS 3646 Part 2 (Schedule of illuminance values); CIBSE Lighting Guide LG1.
Glass
Diffuse Light Transmittance (T)
| Glass Type | T (diffuse) | U-value (W/m²K) | Notes |
|---|---|---|---|
| Clear Single-Pane Float | 0.88 | 5.8 | Baseline; poor thermal performance |
| Clear DGU (6mm + 12mm + 6mm) | 0.78 | 2.7 | Common in premium Indian residential |
| Low-E DGU (solar control) | 0.70 | 1.8 | ECBC-compliant; balances daylight and heat gain |
| Tinted Single-Pane (green/grey) | 0.55 | 5.8 | Reduces glare; sacrifices daylight |
| Tinted DGU | 0.50 | 2.7 | Good heat gain control; moderate daylight |
| Heat-Reflective Coated | 0.30 | 2.5 | Commercial facades; low daylight; verify manufacturer VLT |
| Laminated Safety (clear) | 0.82 | 5.7 | IS 2553 compliant; bathrooms, balconies, high-risk zones |
Sources: IS 2553 (safety glass); IS 1948 (aluminium windows); ECBC 2017 Appendix B; manufacturer datasheets (Saint-Gobain, Asahi, Sejal). Verify VLT for specific products.
Method
How Daylight Factor is computed
The calculator implements the Littlefair/BRE split-flux formula for average Daylight Factor, as adopted in IS 2440:1975 and CIBSE Lighting Guide LG1:
Where T is the diffuse glass transmittance, Aw is the net glazed area (gross window × 0.8 frame correction per IS 1948), θ is the visible sky angle in degrees reduced by external obstructions, Atotal is floor + ceiling + all walls, and R is the area-weighted mean reflectance of internal surfaces.
The result is multiplied by a maintenance factor (default 0.9) to account for dirt accumulation on glass — IS 3646 Part 1 recommends 0.70–0.95 depending on location and cleaning frequency.
The Window-to-Floor Area Ratio (WFR) check is prescriptive — the gross window area divided by floor area must meet or exceed NBC 2016 Part 8 Section 1 minimums (10% for habitable, 5% for bathrooms, 7.5% for staircases).
The effective daylight depth check uses the CIBSE rule of thumb: useful daylight penetrates to roughly 2.5 times the window head height. Rooms deeper than this are flagged as needing a second daylight source — clerestory, skylight, or light shelf.
FAQ
Common questions about daylight design
What is the Daylight Factor (DF) and why does it matter?
Daylight Factor is the ratio of indoor illuminance at a point to the simultaneous outdoor horizontal illuminance under a standard CIE overcast sky, expressed as a percentage. It is independent of time of day or season, making it a robust design metric. IS 2440:1975 and CIBSE LG1 recommend an average DF of 0.5–2% for general living spaces and 1.5–5% for tasks requiring good visual acuity. Unlike WFR, DF accounts for glass transmittance, external obstruction, and interior reflectance — not just window size.
What is the minimum window area required by NBC 2016?
NBC 2016 Part 8 Section 1 specifies minimum window area as a percentage of floor area: 10% for habitable rooms (living, dining, bedroom, kitchen), 5% for bathrooms and WCs, and 7.5% for staircases and corridors. At least 50% of this area must be openable for ventilation. This is a minimum compliance check; good daylight design typically needs 15–20% WFR for deep rooms or rooms with external obstructions.
Which formula does this calculator use?
The calculator implements the Littlefair/BRE split-flux formula adopted in IS 2440:1975 and CIBSE Lighting Guide LG1. Average DF = (T × A_w × θ) / (A_total × (1 − R²)), where T is the glass transmittance (diffuse), A_w is the net glazed area, θ is the visible sky angle in degrees, A_total is the total internal surface area, and R is the area-weighted mean reflectance. This gives a reliable average DF estimate for rectangular rooms with a single window wall.
How does external obstruction affect daylight?
External obstructions — neighbouring buildings, trees, balconies — reduce the visible sky angle from the room's reference point, which linearly reduces the sky component of DF. This calculator takes the obstruction angle above the horizontal (0° = clear horizon, 45° = major obstruction) and reduces θ accordingly. In dense Indian urban contexts (Mumbai, Delhi, Bangalore CBD), obstruction angles of 30–60° are typical and can halve the available daylight even with generous window sizing.
What glass transmittance should I use?
Typical diffuse-light transmittance values: clear single-pane float glass 0.88; tinted glass 0.50–0.70; low-E coated glass 0.65–0.75; insulated glass unit (DGU) clear 0.78; heat-reflective glass 0.20–0.40. This calculator uses diffuse transmittance, not visible light transmittance (VLT); for high-performance glazing consult manufacturer data sheets. ECBC 2017 recommends VLT ≥ 0.27 for daylit spaces in ECBC-compliant buildings.
What internal reflectance values are realistic?
Area-weighted mean reflectance (R) typical ranges: all-white small room 0.55–0.65; light pastel walls, white ceiling 0.40–0.50; medium-tone Indian finishes (cream walls, white ceiling, wood floor) 0.35–0.45; dark walls or timber panelling 0.20–0.30. The calculator defaults to 0.4 for residential and 0.5 for office/classroom. Higher R gives higher DF because internally reflected light adds to the sky component — but this benefit plateaus as R approaches 1.
Why is the WFR check separate from the DF check?
Window-to-Floor Area Ratio is a prescriptive rule in NBC 2016 Part 8 — a simple numerical minimum. DF is a performance metric that predicts actual daylight availability. A room can satisfy WFR and still have poor daylight (deep room, north-facing window, heavy obstruction), or fail WFR and have acceptable daylight (shallow room, light well). For regulatory approval, WFR must pass. For good design, DF must also pass the task-specific benchmark.
Does this tool account for different climate zones?
The DF formula itself is climate-independent because it uses a standard CIE overcast sky reference. However, the availability of daylight varies — India receives roughly 10,000–11,000 lux outdoor illuminance under overcast monsoon skies, rising to 30,000+ lux under clear post-monsoon skies. ECBC 2017 Appendix B provides climate-zone-specific daylight availability hours; this tool gives the DF, and you multiply by ambient illuminance for the relevant sky condition.
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