Lighting calculation: how many lumens and how many fixtures do you need?

Short answer: The total luminous flux needed is the target illuminance (lux) times the area (m²), divided by a factor for how much of the light reaches the working surface: Total lumens = Lux × m² ÷ (Utilisation factor × Maintenance factor). In light-coloured rooms of normal height with LED fixtures, the product of these two factors can be taken as about 0.5–0.6. Number of fixtures = total lumens ÷ lumens per fixture. Example: a 30 m² office at 500 lux → 500 × 30 ÷ 0.6 ≈ 25,000 lumens → about 7 panels of 3,600 lumens, or 8 for a symmetrical layout.

1. Choose the target lux

Approximate values (EN 12464-1 for offices and workplaces, common practice for homes):

• Office desk, computer work: 500 lux
• Meeting room: 500 lux
• Shop sales area: 300–500 lux
• Kitchen worktop: 300–500 lux
• Bathroom mirror: 300–500 lux
• Living room ambient: 100–150 lux
• Bedroom: 100 lux
• Corridors and stairs: 100 lux
• Storage, garage: 100–200 lux

2. Work out the area

Length × width. Split L-shaped rooms into rectangles and calculate areas that need different lux levels (e.g. living room and kitchen) separately.

3. Choose the factors

• Utilisation factor (UF/CU): how much of the fixture's light reaches the working surface. High (0.6–0.8) in square or wide rooms with light walls and ceilings; low (0.4–0.5) in dark, narrow or high-ceilinged rooms.
• Maintenance factor (MF): allowance for LED output loss and dirt over time; about 0.8 in clean interiors.

Shortcut: for a normal room, UF × MF ≈ 0.5–0.6.

4. Find total lumens and the number of fixtures

Total lumens = Lux × m² ÷ (UF × MF)

Number of fixtures = Total lumens ÷ lumens per fixture (round up and adjust for a symmetrical layout)

Take the fixture's lumen value from its datasheet; do not estimate it from wattage, because efficacy differs between fixtures.

Worked examples

• Office, 6 × 5 m = 30 m², 500 lux: 500 × 30 ÷ 0.6 = 25,000 lumens. With 3,600-lumen 600×600 panels, 25,000 ÷ 3,600 ≈ 7 → 2 rows × 4 = 8 panels.
• Living room, 25 m², 150 lux: 150 × 25 ÷ 0.55 ≈ 6,800 lumens. About 11 downlights of 600 lumens, or fewer downlights plus a linear fixture.
• Kitchen worktop, 3 m: under-cabinet light of about 300–500 lumens per metre; ambient light is calculated separately.
• Warehouse, 20 × 15 m = 300 m², 7 m ceiling, 200 lux: 200 × 300 ÷ 0.6 = 100,000 lumens. About 7 high bays of 15,000 lumens; in a 2 × 4 layout, 8.

Spacing between fixtures

• Rule of thumb: spacing should not be much more than the fixture's height above the working surface (about 1–1.5 times). Otherwise dark patches appear between fixtures.
• Distance to the wall is about half the spacing.
• Narrow-beam spots are placed closer together, wide-beam fixtures further apart.

How many watts?

LED fixtures typically deliver 90–130 lumens per watt. 25,000 lumens needs roughly 190–280 W of LED power. For energy calculations, take the fixture's actual power from the datasheet.

When to use software

This method gives an average; it does not show uniformity, glare (UGR) or wall brightness. For offices, shops and industrial projects, the calculation is done in software such as DIALux or Relux using the fixture's photometric file.

• → Lumens, watts and colour temperature
• → 600×600 LED panel wattage
• → Downlight sizes and spacing
• → Office and retail lighting
• → Weatherproof and high bay lights
• → LED floodlight wattage guide
• → LED panels
• → Industrial lighting