Daylight Factor & Window Area Calculator
Compare windows, glass and wall finishes against your entered daylight and ratio targets.
Try before you buy.
- Free preview: the first room’s average daylight factor in full, with the first rows of each table (up to 3) and a watermarked chart; every other figure is hidden.
- Locked until you unlock it: download and copy.
- Unlock: Pro pass, ₹179 for 30 days or ₹79 for 1 week, a one-time payment that never renews. Unlock: Pro pass, $5.99 for 30 days or $2.99 for 1 week, a one-time payment that never renews.
Ways to unlock shows how to get the full result.
Printing this result is locked in the free preview.
Work from your own measurements and adopted rules. Every example value is illustrative; results are estimates that require professional verification.
Result exports
Computed copies and clean documents unlock with a pass. Changing the inputs makes a new result.
Your entered project
Input backups keep your measurements, fields and chosen assumptions; they contain no computed deliverable and remain available to everyone.
Inputs stay in this tab unless you choose local storage or download a backup.
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Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Daylight Factor & Window Area Calculator
Estimate a room's legacy average daylight factor from its geometry, net glazing, diffuse glass transmittance, maintenance factor, visible sky angle and area-weighted surface reflectance. Multiple windows retain their own values, and their transmitted-sky contributions add together. The room table checks the opening or glazing-to-floor ratio against the minimum and measurement basis you enter from your adopted code.
Compare a larger window, more transmissive glazing and lighter walls, or size proportionally enlarged windows within the area you make available. The tool updates weighted reflectance as glazing replaces opaque wall; it does not invent a national minimum or simulate annual daylight. A Pro pass opens the full room table, what-if working and PDF report.
How to use it
- Add each rectangular room with its dimensions, opaque-surface reflectances and available window-opening area.
- Assign each window to its room. Enter its gross opening and net glazing separately, diffuse transmittance, dirt factor, inward reflectance and visible vertical sky angle in degrees.
- Enter your ADF target, code ratio, measurement basis and source clause. Enter 0 when you want an estimate without a corresponding target check.
- Calculate to compare the room, individual window contributions and what-if scenarios. Window sizing scales existing windows proportionally and retains their glazing fractions.
- Inspect the preview and limitations, then export the clean PDF with a Pro pass. Your input-only project backup remains available.
Examples
4 × 3 × 3 m; 4 m² opening and 3 m² glazing; T 0.7, M 0.9, sky 65°; floor/ceiling/wall/window reflectances 0.2/0.8/0.5/0.1.
A 66 m²; weighted R 0.475758; ADF about 2.405936%.
These are explicit example inputs, not a code minimum or a measured building.
The legacy average-daylight-factor formula
ADF (%) = Σ(M × W × θ × T) / [A(1−R²)]. W is net glazing area, M the dirt / maintenance factor, θ visible sky angle in degrees, T diffuse transmittance, A all internal surface area including windows, and R area-weighted reflectance. The BRE-authored historical technical guide documents this equation. It already returns a percentage: do not multiply by 100 again.
Current targets are your inputs
The historical scheme is withdrawn and supplies equation evidence here, not current certification criteria. Obtain the adopted code from your authority or the Bureau of Indian Standards as applicable. Window-to-floor ratios may concern gross opening, glazing or ventilation area; this tool offers the first two only and labels the chosen basis. It supplies no numeric NBC or other code minimum.
How the what-if sizing works
A window increase keeps each existing window's glass fraction, sky angle, transmittance and dirt factor while scaling its area. The solver also replaces the corresponding opaque-wall reflectance in the area-weighted average. It finds no answer when the entered available area or transmitted-sky contribution cannot reach the target. Glass and wall-colour scenarios change only the named variable.
Limitations
- Estimate — not a professional design or daylight simulation. The legacy method assumes a standard overcast sky, vertical windows, a rectangular room and uniform opaque-wall reflectance.
- No current code minimum or certification target is supplied. You enter the authority, clause, ratio basis and design target.
- Annual daylight, direct sun, glare, point distribution, rooflights, external reflections and changing obstructions are outside this model.
- Window sizing retains the existing mix of windows and cannot decide structural feasibility, opening geometry or changes to reveals and shading.
Privacy
On this device; its files download once
Project inputs stay in this tab, or in this browser's local storage only when you choose it. Backups and generated files are made on your device. No project data is uploaded.
Frequently asked questions
What do I get without a pass?
Without a pass, Daylight Factor & Window Area Calculator shows the first room’s average daylight factor in full, with the first rows of each table (up to 3) and a watermarked chart; every other figure is hidden. Until you unlock it, the result can’t be downloaded or copied. A Pro, Premium or Ultimate pass, a one-time payment that never renews, unlocks the full result. The pricing page lists the passes and their prices.
Is the output already a percentage?
Yes. The historical equation uses a visible sky angle in degrees and produces ADF as a percentage. There is no additional ×100 factor.
Can different windows have different glazing or obstructions?
Yes. Each window has its own net glazing, diffuse transmittance, dirt factor and sky angle. Their contributions are summed for the room.
Does meeting a window ratio prove adequate daylight?
No. A ratio alone ignores transmittance, obstructions, maintenance, distribution and annual conditions. This is an early estimate against your own inputs.