ℹ️ About This Calculator
The lux level calculator uses the lumen method to determine how many luminaires are required to achieve a specified illuminance (in lux) in a space. The method accounts for room geometry through the Room Cavity Ratio (RCR) and light losses through the Maintenance Factor (MF). Recommended lux levels for different tasks are specified in SP 72 (NBC Handbook) and NBC 2016 Part 8.
SP 72 (National Building Code Handbook on Lighting) provides recommended illuminance levels: office general – 300–500 lux; conference – 500 lux; manufacturing – 300–750 lux; hospital ward – 100 lux; ICU – 300 lux; classroom – 300–500 lux; parking – 50 lux. NBC 2016 Part 8 mandates minimum lux levels for different building types. Energy compliance: NBC 2016 limits Lighting Power Density (LPD) which must be checked alongside lux levels.
🧮 Worked Example
An office measuring
10 m × 6 m with a 3 m ceiling (work plane at 0.85 m, so Hc = 2.15 m) requires 500 lux. RCR = 5 × 2.15 × (10+6) / (10×6) ≈
2.87, giving a coefficient of utilisation CU ≈ 0.65 from the manufacturer's table for typical office reflectances.
Using 4000-lumen LED luminaires at MF = 0.80: N = (500 × 60) / (4000 × 0.65 × 0.80) ≈ 14.4, rounded up to 15 luminaires - typically arranged in a 3 × 5 grid across the room for even coverage.
❓ Frequently Asked Questions
What is a good maintenance factor (MF) to use?
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MF accounts for lamp lumen depreciation, luminaire dirt accumulation, and room surface dirt. Typical values: clean environment with LED (annual cleaning): MF = 0.80; office with quarterly cleaning: MF = 0.75; industrial with dust: MF = 0.65–0.70. LED luminaires have much less lumen depreciation than fluorescent, so their MF is higher. Always use manufacturer's LLF (Light Loss Factor) data when available.
What lux levels are required for offices?
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Per SP 72: open-plan office (general) = 300–500 lux; computer workstations = 300–500 lux (with glare control); meeting rooms = 300–500 lux; corridors = 100–200 lux; reception = 200–300 lux; filing areas = 200 lux. NBC 2016 mandates minimum 300 lux for normal office work. Higher lux (500+) improves productivity for detailed tasks.
How do I calculate Room Cavity Ratio (RCR)?
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RCR = 5 × Hc × (L + W) / (L × W), where Hc is the height from the work plane (usually 0.85 m from floor) to the luminaire plane (usually the ceiling). A low RCR (under 3) means a shallow, wide room – high CU. A high RCR (over 6) means a tall, narrow room – low CU, more luminaires needed. Suspended luminaires reduce effective ceiling height and change RCR.
What is the Coefficient of Utilisation (CU)?
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CU is the fraction of luminaire output that reaches the work plane. It depends on room geometry (via RCR), ceiling reflectance (typically 0.7–0.8 for white), wall reflectance (0.5 for light coloured), and floor reflectance (0.2). Manufacturers provide CU tables for their luminaires. For direct downlights in a typical office: CU ≈ 0.55–0.70. For indirect/uplighting: CU ≈ 0.30–0.45.
Can I use the lux level calculator for outdoor lighting?
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The lumen method is for indoor uniform lighting. For outdoor: car park surface – use the point-by-point method with manufacturer isolux diagrams. Road lighting – use the CIE 115 method with luminance calculations. Sports facilities – use manufacturer's aiming diagrams. However, a simplified lux check using the lumen method works for covered car parks, loading bays, and semi-outdoor spaces.
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