Lighting layout calculator (lumen method)
How many fixtures does this room need? The lumen method answers it (footcandles and feet on Imperial, lux and metres on Metric): the light the room needs (level × area) divided by the useful light one fixture delivers. The target level and the fixture figures are yours — from the spec, the lighting design guide you work to, and the fixture's photometric sheet.
An average over the work plane. The target level, CU and LLF are yours — from the spec, your design guide and the fixture’s photometric data.
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How it works
- N = E × A ÷ (lumens × lamps × CU × LLF)fixtures needed
- E = N × lumens × lamps × CU × LLF ÷ Alevel you get
- 1 fc = 1 lm/ft² ≈ 10.76 luxunits
Light level is lumens spread over an area. The room needs level × area lumens landing on the work plane. Each fixture puts out its lamp lumens, but only the CU share reaches the work plane and only the LLF share survives ageing and dirt. Divide the first by the second and round up.
Worked examples
A 30 × 20 ft shop office at 50 fc
- Light needed on the work plane: 50 fc × 600 ft² = 30,000 lm.
- Useful light per fixture: 4,000 lm × 0.6 CU × 0.8 LLF = 1,920 lm.
- 30,000 ÷ 1,920 = 15.6 → 16 fixtures, a 4 × 4 grid.
- Check: 16 × 1,920 ÷ 600 = 51.2 fc average.
A 10 × 6 m classroom at 500 lux
- 500 lux × 60 m² = 30,000 lm needed.
- Two-lamp fixtures at 2,500 lm a lamp, CU 0.55, LLF 0.85: 2 × 2,500 × 0.55 × 0.85 = 2,337.5 lm each.
- 30,000 ÷ 2,337.5 = 12.8 → 13, but lay out 15 (3 rows of 5) to keep the grid even.
Questions people ask
What is the coefficient of utilization (CU)?
The share of a fixture’s lumens that actually reaches the work plane in this room. It depends on the fixture’s shape and the room’s proportions and surface reflectances, and it comes from the fixture maker’s photometric data (a CU table). Typical values fall somewhere between 0.4 and 0.8.
What is the light loss factor (LLF)?
An allowance for the light the system loses over time: lamps dimming with age, dirt on the lenses and the room. It is a number below 1 — design guides and manufacturers give the parts to multiply together. Using 1 gives the brand-new, freshly-cleaned figure.
Where does the target light level come from?
From the project specification or the lighting design guide you work to (the IES handbook in North America, EN 12464 in Europe), and sometimes a code or a building standard. This page does not supply it. Footcandles are lumens per square foot; lux are lumens per square metre; 1 fc ≈ 10.76 lux.
Does the lumen method tell me where to put them?
It gives the count for an even average. Spacing is the other half: keep the spacing-to-mounting-height ratio within what the fixture’s data allows, and lay the count out in a grid that suits the room — rounding up so the rows come out even.
Related
- Lighting circuits and controls — the lesson behind this tool.
- Ohm’s law & power calculator — watts and amps for the lighting circuit.
- Efficiency & percent
- Occupancy sensor wiring — switching the fixtures you just counted.
- Series vs parallel lighting
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