What problem this solves
The common belief is “put a mirror opposite the window” and the room brightens. That’s true for daylight quantity but it ignores daylight quality: a mirror directly opposite a window also doubles glare, throws a hot spot on the wall behind it, and at night reflects the window as a black void into the room. A mirror on a perpendicular wall bounces less light but distributes it more evenly and doesn’t create a focused glare zone. This tool ranks the four walls by light bounce and flags the cultural-safety cases that designers should know about.
The mirror dimensions come from a rule of thumb that’s about 30 years old but still holds: a mirror should be no wider than the furniture below it and no narrower than two-thirds of it, and the centre should sit at eye level (≈145 cm from the floor) for standing reflection or at seated eye level (≈120 cm) for a bedroom mirror over a dresser.
Who it’s for
- Designers placing an entryway mirror to brighten a dark hall
- Living room designers using a mirror to bounce daylight into a deep-plan space
- Bedroom designers specifying an over-dresser mirror
- Homeowners wondering whether their bed should face a mirror or not
Cultural references, not advice
Feng Shui (Chinese tradition). Classical Feng Shui discourages mirrors facing the bed — the reflection is believed to disturb sleep by bouncing the room’s energy back at the sleeper. Mirrors facing the front door are similarly discouraged because they may “push” entering chi back out. Modern Western Feng Shui practitioners often relax these rules in small urban apartments, but the principles are part of the tradition. This tool flags the bed case automatically.
Vastu Shastra (Indian tradition). Vastu also advises against mirrors on walls adjacent to or facing the bed, particularly the south and west walls of a bedroom. Mirrors in the north or east are considered more auspicious. As with Feng Shui, treat these as cultural reference points rather than design rules — many modern Indian designers work around Vastu by placing mirrors inside wardrobe doors rather than on walls.
Limitations
- Window treated as a uniform diffuse source — real windows have mullions, frames, and a sky vs ground split.
- No time-of-day factor — morning east light behaves differently from afternoon west light.
- Mirror reflection efficiency assumed at 80% (typical silvered glass); antique mirrors drop to 50%.
- Cultural flags are educational — not everyone subscribes to these traditions.
- Does not check view-out quality (a mirror can reflect an ugly view).
Practical sizing rules
Entryway: mirror 60–80 cm wide × 100–140 cm tall, centred at 145 cm. Living room: depends on furniture below; if above a console, mirror should be 60–70% of the console width and 10–20 cm taller. Bedroom over dresser: mirror width = dresser width × 0.9, height = dresser height × 2. Avoid mirrors larger than 1.2 m² in a small room — they tend to dominate visually and the bounce effect becomes harsh.