Indoor Lighting Lumen Count
Enter room size, target lux and fixture data to get the fixture count, layout, achieved lux and power density.
Step-by-step
- Enter valid values to begin.
Recessed ceiling-mounted fixtures, reflectances fixed at a typical 70/50/20 (ceiling/walls/floor). Utilisation factor is interpolated from a typical recessed LED troffer table by room index K (0.6 to 5.0, values outside are clamped); replace it with manufacturer photometric data. The space-type menu only shows a typical value: the target lux box is what is calculated, and EN 12464-1 and the IES Handbook values vary with the task. Work-plane height is converted to metres from its own unit.
Indoor Lighting Lumen Count: How Many Fixtures a Room Needs
Core Engineering Principles
Keep three units apart. Lumens are the light a fixture emits, candela is intensity in one direction, and lux is lumens landing on each square metre of the work plane. The lumen method asks how many lumens must reach the task surface and divides by what each fixture delivers. The utilisation factor (UF) is the share of emitted light that reaches the work plane after the walls, ceiling and fixture take their cut, and it depends on the room index K. We assume 70/50/20 reflectances, a clean white ceiling; paint it dark and the real UF drops.
Then the maintenance factor: LED output fades, dust coats the optic and the room darkens. 0.80 is a common allowance, not a measurement, so set it from the cleaning schedule you can afford. Finally check spacing against mounting height. Beyond about 1.5 times the height above the work plane you get bright pools and dark seams, and average lux hides it. Average lux says nothing about glare, so check UGR separately.
N = (E × A) / (Φ × UF × MF) • Eachieved = N × Φ × UF × MF / A
LPD = N × P / A • S / hm ≤ 1.5 (typical)
NEC & Standard References
EN 12464-1 sets maintained illuminance, uniformity and UGR limits by task. The IES Lighting Handbook gives North American recommended levels and the lumen method. ASHRAE 90.1 limits lighting power density,. The CIBSE SLL Code for Lighting is the UK guidance. The UF table here is a typical recessed LED troffer curve; replace it with manufacturer photometric data and check the adopted edition.1. Area = 8 × 6 = 48 m², and hm = 2.8 − 0.85 = 1.95 m.
2. K = 48 / (1.95 × 14) = 1.758, and interpolating between K 1.5 (0.64) and K 2.0 (0.70) gives UF = 0.671.
3. N = (500 × 48) / (4000 × 0.671 × 0.80) = 11.18, rounded up to 12 fixtures.
4. Layout 3 rows × 4 columns at 2.0 m spacing, S / hm = 1.03.
5. Achieved = 12 × 4000 × 0.671 × 0.80 / 48 = 537 lx.
6. Power = 12 × 36 = 0.432 kW, which is 9.0 W/m² (0.84 W/ft²), at 111 lm/W.
- Nameplate lumens are not site lumens. Catalogue output is measured at 25 °C, and a hot plenum costs several percent.
- Dirt depreciation is real. Dusty plants and kitchens need a lower MF or a cleaning contract.
- Dark ceilings change UF. A grey tile or exposed deck makes this table optimistic.
- Uniform spacing is not task lighting. Add local light for detail work.
- Emergency lighting is separate. Escape-route luminaires follow their own rules and are not counted here.