Room Lighting Lux Layout & Reflectance Calculator

The lumen (lm) measures total light output from a source — its luminous flux. The lux (lx) measures illuminance, how much of that flux lands on a surface: 1 lux = 1 lumen per square metre. Candela (cd) measures luminous intensity in a specific direction, relevant for point-by-point calculations and glare assessment but not needed for the average-illuminance lumen method used here. A very bright source can still produce low lux on a distant or angled surface, which is exactly why illuminance depends on geometry as well as source output.

E = (N × F × UF × MF) / A, where E is average illuminance (lux), N is the number of luminaires, F is lumens per luminaire, UF is utilization factor, MF is maintenance factor, and A is room area. Rearranged for design: N = (E × A) / (F × UF × MF) — the form this calculator solves. This gives the average illuminance across the working plane, not the value at any single point; real rooms have some variation above and below this average.

Room Index K = LW / [H_m(L+W)], where L and W are room length and width and H_m is luminaire mounting height above the working plane. A large, low-ceilinged room has a high room index — most light launched sideways from a luminaire still lands somewhere useful on the floor before escaping past the walls. A small, tall room has a low room index — proportionally more light escapes upward past the ceiling plane or sideways past the walls before reaching the working plane, which is why utilization factor drops sharply as room index falls.

Utilization factor (UF, or utilance) is the fraction of luminaire flux reaching the working plane. It depends on the luminaire intensity distribution, room proportions and surface reflectances. Use the UF supplied in the luminaire photometric data for the calculated room index and chosen ceiling/wall reflectances; this calculator does not derive a universal UF from reflectance alone.

Utilization factor (UF, or utilance) is the fraction of luminaire flux reaching the working plane. It depends on the luminaire intensity distribution, room proportions and surface reflectances. Use the UF supplied in the luminaire photometric data for the calculated room index and chosen ceiling/wall reflectances; this calculator does not derive a universal UF from reflectance alone.

White or near-white paint: roughly 70–85%. Light pastel colors: roughly 50–70%. Medium tones (mid grey, tan): roughly 30–50%. Dark colors: roughly 10–30%. Exposed concrete or dark wood: roughly 10–20%. Real reflectance depends on the specific paint, finish and lighting angle — these are order-of-magnitude ranges for design estimation, not measured values for a specific product.

MF = LLMF × LSF × LMF × RSMF. Lamp Lumen Maintenance Factor (LLMF) accounts for a light source's gradual output decline with age. Lamp Survival Factor (LSF) accounts for the probability a lamp is still working (not yet failed) at a given point in its rated life, relevant for group-replacement maintenance schedules. Luminaire Maintenance Factor (LMF) accounts for dirt accumulating on and inside the fitting itself, reducing its light output. Room Surface Maintenance Factor (RSMF) accounts for dust and soiling on ceiling and wall surfaces reducing their reflectance over time. This calculator asks for one combined maintenance factor rather than all four components separately, since a full breakdown needs manufacturer lamp-life data and a defined cleaning schedule.

Maximum spacing-to-height ratio is luminaire-specific. Multiply the entered SHR by the luminaire height above the working plane to obtain a screening maximum center-to-center spacing. The suggested regular grid is only a layout aid; a point-by-point design is needed to confirm uniformity.

Utilization factor (UF, or utilance) is the fraction of luminaire flux reaching the working plane. It depends on the luminaire intensity distribution, room proportions and surface reflectances. Use the UF supplied in the luminaire photometric data for the calculated room index and chosen ceiling/wall reflectances; this calculator does not derive a universal UF from reflectance alone.

Room index is a compact measure of room proportions. For the same mounting height, larger plan dimensions generally increase room index. Luminaire photometric tables use room index together with surface reflectances because both room proportions and interreflection affect how much emitted flux reaches the working plane.

Yes — since a fractional luminaire can't be installed, the required count is rounded up to a whole number, which means achieved illuminance is generally slightly above (never below) the target. This calculator shows that actual achieved illuminance so you can see exactly how much margin the rounding creates.

Not directly. The room-index/lumen method assumes a simple room and predicts maintained average illuminance, not detailed point-by-point uniformity. Irregular geometry, partitions, tall equipment, shelving or glare constraints should be evaluated with lighting-design software using the actual IES/LDT photometric file.

A higher MF means less light is assumed to be lost to aging and dirt, which reduces the calculated number of luminaires needed to reach a target — but MF should reflect your actual planned maintenance regime, not be chosen to minimize luminaire count. Specifying an optimistic MF for a space that won't actually be cleaned or relamped on that schedule results in a real installation that under-delivers its design illuminance well before the next maintenance cycle.