Lux to Lumen Calculator
Convert lux (illuminance) to lumens (luminous flux) based on room area. Calculate the total lumens needed to achieve your target lux level and determine how many bulbs are required. See also our Lumen to Lux Calculator and Lux to Watt Calculator.
How to Convert Lux to Lumens
Converting lux to lumens is essential for lighting design — it tells you the total light output needed to achieve a desired illumination level in a room. Lux measures how much light reaches a surface, while lumens measure the total light emitted by a source. The area of the space connects these two measurements. Follow these steps:
- Determine the required lux level: Use lighting standards for your application — 500 lux for offices, 300 lux for general living areas, 150 lux for corridors, or 750 lux for detailed work.
- Measure the room area: Calculate the floor area in square meters (length × width). If you have the area in square feet, multiply by 0.0929 to convert to m².
- Multiply lux by area: Total Lumens = Lux × Area (m²). This gives the total luminous flux needed to uniformly illuminate the entire area at the target lux level.
- Apply a maintenance factor: Multiply by 1.25–1.5 to account for light depreciation over time, dirt accumulation on fixtures, and room surface absorption.
- Calculate number of fixtures: Divide total lumens by the lumen output of each fixture or bulb to determine how many you need.
This method assumes perfectly uniform light distribution across the entire area, which is an idealization. In practice, light distribution depends on fixture placement, beam angles, ceiling height, and room geometry. Professional lighting designers use software like DIALux or Relux to model these factors accurately. However, the lux × area formula provides an excellent starting estimate for most residential and commercial applications.
Lux to Lumen Formula
Lumens = Lux × Area (m²)
Where:
Lumens (lm) = Total luminous flux required
Lux (lx) = Target illuminance level (lumens per m²)
Area (m²) = Floor area of the space
Unit Conversions:
1 m² = 10.764 ft²
Area_m² = Area_ft² × 0.0929
1 lux = 1 lumen per m²
1 foot-candle = 10.764 lux
Number of Bulbs:
Bulbs = Total Lumens / Lumens per Bulb (rounded up)
With Maintenance Factor:
Adjusted Lumens = Lumens × Maintenance Factor (1.25–1.5)
Utilization Factor (advanced):
Actual Lumens = Lumens / (UF × MF)
UF = 0.4–0.8 depending on room geometryThe formula is derived from the definition of lux: 1 lux = 1 lumen per square meter. Therefore, to find total lumens, simply multiply the desired lux by the area in square meters. The utilization factor (UF) accounts for the fact that not all light from a fixture reaches the work plane — some is absorbed by walls, ceiling, and furniture. Typical UF values range from 0.4 for rooms with dark surfaces and indirect lighting to 0.8 for rooms with light surfaces and direct downlighting.
Example Calculation
Problem: A 15 m² home office needs 300 lux of illumination. How many lumens are needed, and how many 800-lumen LED bulbs should be installed?
Step 1: Apply the formula
Lumens = Lux × Area
Lumens = 300 × 15
Lumens = 4,500 lm
Step 2: Apply maintenance factor (1.25)
Adjusted Lumens = 4,500 × 1.25 = 5,625 lm
Step 3: Calculate number of bulbs
Bulbs = 5,625 / 800 = 7.03 → round up to 8 bulbs
Step 4: Verify actual lux achieved
Actual Lumens = 8 × 800 = 6,400 lm
Actual Lux = 6,400 / 15 = 427 lux (exceeds 300 lux target) ✓
Answer: You need approximately 4,500 lumens (5,625 with maintenance factor) for a 15 m² office at 300 lux. Install 8 LED bulbs rated at 800 lumens each, which provides comfortable margin above the minimum requirement.
Lux to Lumens Reference Table
Quick reference showing total lumens needed for common room sizes and lux levels.
| Room Area | 150 lux | 300 lux | 500 lux | 750 lux |
|---|---|---|---|---|
| 5 m² | 750 lm | 1,500 lm | 2,500 lm | 3,750 lm |
| 10 m² | 1,500 lm | 3,000 lm | 5,000 lm | 7,500 lm |
| 15 m² | 2,250 lm | 4,500 lm | 7,500 lm | 11,250 lm |
| 20 m² | 3,000 lm | 6,000 lm | 10,000 lm | 15,000 lm |
| 25 m² | 3,750 lm | 7,500 lm | 12,500 lm | 18,750 lm |
| 30 m² | 4,500 lm | 9,000 lm | 15,000 lm | 22,500 lm |
| 50 m² | 7,500 lm | 15,000 lm | 25,000 lm | 37,500 lm |
| 100 m² | 15,000 lm | 30,000 lm | 50,000 lm | 75,000 lm |
These values represent the minimum total lumens needed without maintenance factor. For practical installations, multiply by 1.25–1.5 depending on room surface colors and fixture type. Rooms with dark walls and ceilings may need up to 2× the calculated lumens to achieve the target lux at the work plane.
Recommended Lux Levels by Room Type
Choosing the right lux level is the first step in any lighting calculation. Too little light causes eye strain and reduced productivity, while too much creates glare and wastes energy. These recommendations are based on international standards and best practices for residential and commercial spaces.
- Hallways and stairs: 100–150 lux — enough for safe navigation without excessive brightness
- Living rooms: 150–300 lux — comfortable ambient lighting with task lights for reading
- Bedrooms: 100–200 lux — relaxing ambient level with bedside task lighting at 300+ lux
- Kitchens: 300–500 lux — higher levels needed for food preparation safety
- Home offices: 300–500 lux — matches commercial office standards for screen work
- Bathrooms: 300–500 lux — higher at mirrors (500–700 lux) for grooming tasks
- Workshops and garages: 300–750 lux — detailed work requires higher illumination
- Art studios: 750–1,000 lux — color-critical work needs high, uniform illumination
Frequently Asked Questions
How many lumens do I need for a 20 m² room?
For a 20 m² room, the lumens needed depend on the purpose: Living room (150–300 lux) = 3,000–6,000 lumens. Office (500 lux) = 10,000 lumens. Kitchen (300–500 lux) = 6,000–10,000 lumens. Add 25% for maintenance factor, so a 20 m² office actually needs about 12,500 lumens. This could be achieved with 16 LED bulbs at 800 lumens each, or 4 LED panel lights at 3,200 lumens each.
What is the difference between lux and lumens?
Lumens measure the total light output of a source (how much light a bulb produces), while lux measures how much of that light reaches a surface (illuminance). One lux equals one lumen per square meter. The same 1,000-lumen bulb creates 1,000 lux on a 1 m² surface, 500 lux on a 2 m² surface, or 100 lux on a 10 m² surface. Lumens are a property of the light source; lux depends on both the source and the geometry of the space.
Why do I need more lumens than the formula suggests?
The basic formula (Lumens = Lux × Area) assumes 100% of the light reaches the work surface uniformly. In reality, light is lost to wall absorption (dark walls absorb 50–70%), ceiling bounce, fixture inefficiency, and aging. The utilization factor (UF) typically ranges from 0.4 to 0.8. Additionally, lumen depreciation over the bulb's life means initial output should exceed the minimum requirement. Professional designs use a maintenance factor of 0.7–0.8, effectively requiring 25–43% more initial lumens.
How do I convert foot-candles to lux?
Multiply foot-candles by 10.764 to get lux. One foot-candle equals one lumen per square foot, while one lux equals one lumen per square meter. Since 1 m² = 10.764 ft², the conversion factor is 10.764. For example, 50 foot-candles = 538 lux. The IESNA (Illuminating Engineering Society of North America) uses foot-candles in their standards, while ISO and European standards use lux. Both measure the same physical quantity (illuminance) in different unit systems.
Does ceiling height affect the lux calculation?
Ceiling height does not directly appear in the Lux = Lumens/Area formula, but it significantly affects practical results. Higher ceilings mean light travels farther before reaching the work plane, spreading over a larger area and losing intensity. The utilization factor decreases with ceiling height. A room with 3m ceilings might have UF = 0.7, while the same room with 5m ceilings might have UF = 0.5, requiring 40% more lumens. For ceilings above 3m, use narrower beam angle fixtures to direct more light downward.
How many 800-lumen bulbs do I need for a room?
Divide your total lumen requirement by 800 and round up. For example: A 12 m² bedroom at 150 lux needs 1,800 lumens → 3 bulbs. A 15 m² office at 500 lux needs 7,500 lumens → 10 bulbs. A 20 m² kitchen at 400 lux needs 8,000 lumens → 10 bulbs. Add 25% for maintenance: bedroom → 3 bulbs, office → 12 bulbs, kitchen → 13 bulbs. Consider using higher-output fixtures (1,600+ lumens) to reduce the number of fixtures needed for larger spaces.