Daylight · Energy

Window-to-Wall Ratio Optimizer

Calculates daylight factor from window area, wall area, orientation, glazing U-value, and shading. Outputs the “useful daylight” verdict and an estimate of winter heat loss. Goes beyond a single WWR number — factors in direction, since a south-facing 40% WWR is not equivalent to a north-facing 40% WWR.

Inputs

Room dimensions

Wall length (m) Room depth (m) Floor-to-ceiling (m)

Window

Window area (m²) Sill height (m) Orientation

Glazing & shading

Glazing U-value (W/m²K) External shading

Heating degree days (rough) E.g. New York ≈ 2500, London ≈ 1900, Helsinki ≈ 3500, Singapore ≈ 0

What problem this solves

Window-to-wall ratio (WWR) is a useful single number for early-stage design, but the standard “aim for 30–40%” rule of thumb hides a lot. A 30% WWR on a north wall in a deep room can produce a 1% daylight factor (too dim); the same 30% on a south wall can produce a 6% daylight factor (potentially glare-y). The orientation factor and depth-to-window-height ratio are what actually drive usable daylight.

Who it’s for

Architects sizing glazing for new builds and renovations
Interior designers checking whether deep rooms need borrowed light
Sustainability consultants auditing envelope heat-loss

Limitations

Daylight factor is a single-hour average — doesn’t capture seasonal swing
External obstructions (other buildings, trees) not modelled
Heat loss is window-only — wall U-value and air leakage contribute more
Internal blinds reduce the “useful” daylight factor further

Common questions

What’s a good daylight factor?

2–5% is the typical target for residential and office spaces. Below 2% feels dim and requires artificial lighting most of the day. Above 5% risks glare and overheating; shading systems become essential.

Does U-value account for solar gain?

No — U-value is heat-loss only. Solar gain is separate (measured as SHGC or g-value). A low-e double pane typically has U=1.6, SHGC=0.4 — meaning it loses less heat than a single pane but still admits 40% of solar radiation. The trade-off depends on climate: cooling-dominated climates want low SHGC, heating-dominated want higher SHGC with low U.

My deep room has good windows but feels dark?

Rule of thumb: useful daylight reaches about 2.5× the window-head-height into the room. If your window head is 2.4 m and the room is 6 m deep, the back 2 m will rely on borrowed light (interior glazing, light shelves, clerestories). The calculator’s daylight factor at the back wall will confirm whether the room passes muster.

About this tool

One of 36 interior design calculators on InteriorLab. Calculations are client-side; no values stored.

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Disclaimer

Estimates only. Daylight modelling and energy calculations for code compliance require specialist software and on-site survey. See full Disclaimer.