Lumen to Lux Calculator
Convert lumens (lm) to lux (lx) by dividing luminous flux by the illuminated area. Determine whether your lighting is adequate for any room or workspace. Use the area mode for room-level calculations or the distance and beam angle mode for spotlights and directional fixtures. Also try our Lux to Lumen Calculator and Lumen to Watt Calculator.
How to Convert Lumens to Lux
Converting lumens to lux is straightforward when you understand the relationship between these two photometric units. Lumens measure the total luminous flux emitted by a light source — essentially how much visible light the bulb produces in all directions. Lux, on the other hand, measures illuminance — how much of that light actually arrives at a specific surface per unit area. The conversion requires you to know the area over which the light is distributed.
To perform the conversion, follow these steps: First, determine the total luminous flux of your light source in lumens (this is printed on the bulb packaging or listed in the product specifications). Second, measure or calculate the area being illuminated in square meters. Third, divide the lumens by the area to get the illuminance in lux. If your area is in square feet, multiply by 0.092903 to convert to square meters before dividing.
For directional light sources like spotlights, recessed downlights, or track lighting, you can calculate the illuminated area from the beam angle and distance. The beam angle defines the cone of light, and the distance determines how far that cone has spread by the time it reaches the surface. A narrow beam angle concentrates light into a smaller area, producing higher lux from the same number of lumens. This is why a focused spotlight feels much brighter than a wide-angle flood light of the same wattage.
Professional lighting designers account for additional factors including the maintenance factor (light output decreases over time as bulbs age and fixtures accumulate dust), the coefficient of utilization (not all light from a fixture reaches the work plane — some is absorbed by walls, ceiling, and furniture), and the room cavity ratio (the proportions of the room affect how efficiently light bounces around). For home use, the basic formula provides an excellent starting estimate that gets you within 20-30% of the actual measured value.
Lumen to Lux Formula
Mode 1 — Area Method:
Lux = Lumens / Area (m²)
Mode 2 — Distance + Beam Angle Method:
Radius = Distance × tan(Beam Angle / 2)
Area = π × Radius²
Lux = Lumens / Area
Where:
Lux (lx) = Illuminance (light received per unit area)
Lumens (lm) = Total luminous flux from the light source
Area (m²) = Surface area being illuminated
Distance (m) = Distance from light source to surface
Beam Angle (°) = Full cone angle of the light beam
Unit Conversions:
Area (m²) = Area (ft²) × 0.092903
Foot-candles = Lux × 0.092903
1 foot-candle = 10.764 lux
Reverse formula:
Lumens = Lux × Area (m²)The area method formula is derived directly from the SI definition of lux: one lux equals one lumen per square meter. The beam angle method calculates the circular area illuminated by a cone of light at a given distance using basic trigonometry. Both methods assume uniform light distribution within the illuminated area. In reality, most light sources produce a brighter center with gradual falloff toward the edges, so the calculated lux represents an average across the illuminated surface.
Example Calculation
Problem: A ceiling light produces 800 lumens and illuminates a 10 m² room. What is the illuminance?
Step 1: Identify the values
Luminous flux = 800 lm
Area = 10 m²
Step 2: Apply the formula
Lux = Lumens / Area
Lux = 800 / 10
Lux = 80 lux
Step 3: Convert to foot-candles
Foot-candles = 80 × 0.092903 = 7.43 fc
Assessment: 80 lux is adequate for a living room or bedroom with ambient mood lighting but insufficient for reading or office work (which requires 300-500 lux). You would need approximately 3,000-5,000 total lumens for comfortable task lighting in this 10 m² space.
Beam Angle Example: A spotlight produces 1,000 lumens with a 60° beam angle at 3 meters distance.
Step 1: Calculate the illuminated radius
Radius = 3 × tan(60° / 2) = 3 × tan(30°) = 3 × 0.577 = 1.732 m
Step 2: Calculate the illuminated area
Area = π × 1.732² = π × 3.0 = 9.42 m²
Step 3: Calculate lux
Lux = 1,000 / 9.42 = 106.1 lux
Comparison: The same 1,000-lumen spotlight with a 30° beam angle at 3 meters would illuminate only 2.02 m², producing 495 lux — nearly 5× brighter on the surface due to the concentrated beam.
Lumens to Lux Reference Table
This table shows the resulting lux levels for common bulb outputs (400 lm, 800 lm, 1600 lm) across various room sizes, assuming uniform light distribution.
| Room Size | Area | 400 lm (40W eq.) | 800 lm (60W eq.) | 1600 lm (100W eq.) |
|---|---|---|---|---|
| Small Bathroom | 4 m² | 100 lux | 200 lux | 400 lux |
| Walk-in Closet | 6 m² | 67 lux | 133 lux | 267 lux |
| Small Bedroom | 9 m² | 44 lux | 89 lux | 178 lux |
| Standard Bedroom | 12 m² | 33 lux | 67 lux | 133 lux |
| Home Office | 10 m² | 40 lux | 80 lux | 160 lux |
| Living Room | 16 m² | 25 lux | 50 lux | 100 lux |
| Large Living Room | 20 m² | 20 lux | 40 lux | 80 lux |
| Kitchen + Dining | 25 m² | 16 lux | 32 lux | 64 lux |
| Open Plan | 35 m² | 11 lux | 23 lux | 46 lux |
| Garage / Workshop | 40 m² | 10 lux | 20 lux | 40 lux |
As the table demonstrates, a single bulb rarely provides sufficient illumination for an entire room. A standard 800-lumen LED bulb only delivers 80 lux in a 10 m² room — well below the 300-500 lux recommended for tasks like reading or cooking. This is why most rooms require multiple light fixtures or higher-output bulbs to achieve comfortable lighting levels. Use this table as a quick reference to estimate how many bulbs you need: simply divide your target lux by the single-bulb lux value shown above.
How Many Lumens Do I Need for My Room?
Determining the right amount of lumens for your room requires considering three key factors: the room size, the intended activity, and the room's surface colors. Darker walls and floors absorb more light, requiring additional lumens to achieve the same perceived brightness as a room with light-colored surfaces.
As a general rule of thumb, multiply your room area in square meters by the recommended lux level for your primary activity. For a living room where you watch TV and relax, 150-200 lux is comfortable, meaning a 20 m² living room needs 3,000-4,000 lumens total. For a home office where you read documents and work at a computer, 300-500 lux is recommended, so a 10 m² office needs 3,000-5,000 lumens. Kitchens require 300-500 lux on countertops for safe food preparation, while dining areas are pleasant at 200-300 lux.
For bedrooms, 100-150 lux provides comfortable ambient lighting for general activities, but you should add task lighting (300-500 lux) at reading positions. Bathrooms need 300-500 lux at the mirror for grooming tasks but can use lower ambient levels elsewhere. Hallways and corridors only need 50-100 lux for safe navigation.
When planning your lighting, consider layering: combine ambient lighting (ceiling fixtures providing general illumination), task lighting (desk lamps, under-cabinet lights for specific work areas), and accent lighting (spotlights highlighting artwork or architectural features). This approach lets you adjust the lighting level for different activities throughout the day without over-lighting the entire room.
Remember that the lumen values on bulb packaging represent the total output of the bulb, but not all of that light reaches your work surface. Fixture design, ceiling height, and room geometry all affect how efficiently lumens convert to lux at the task level. A good rule is to add 25-40% extra lumens beyond the theoretical calculation to account for these real-world losses. If your calculation says you need 4,000 lumens, plan for 5,000-5,600 lumens of installed capacity.
Frequently Asked Questions
How many lux is 1 lumen?
One lumen spread uniformly over one square meter equals exactly 1 lux. If that same lumen is spread over 2 square meters, it equals 0.5 lux. The relationship is always Lux = Lumens ÷ Area (m²). So 1 lumen per square meter = 1 lux, and 1 lumen per square foot = 1 foot-candle = 10.764 lux. The lux value depends entirely on the area over which the light is distributed.
How do I calculate lux from lumens?
Divide the total lumens by the illuminated area in square meters. For example, a 1,200-lumen bulb illuminating a 6 m² bathroom produces 1,200 ÷ 6 = 200 lux. If your area is in square feet, first convert to square meters by multiplying by 0.092903. For directional lights, calculate the illuminated area from the beam angle and distance using: Area = π × (Distance × tan(Beam Angle / 2))².
How many lumens do I need for a 20 m² room?
It depends on the room's purpose. For a living room (150-200 lux), you need 3,000-4,000 lumens. For a home office (400-500 lux), you need 8,000-10,000 lumens. For a bedroom (100-150 lux), you need 2,000-3,000 lumens. Multiply the desired lux level by 20 m² to get your target lumens, then add 25% for real-world light losses from walls and fixtures.
What is the difference between lumens and lux?
Lumens measure the total light output of a source (how much light the bulb produces in all directions), while lux measures how much light arrives at a specific surface (illuminance). Think of lumens as the total water flow from a hose, and lux as how wet a specific area of ground gets. The same 1,000-lumen bulb creates 1,000 lux on a 1 m² surface but only 50 lux on a 20 m² floor because the light spreads out.
How does distance affect lux?
For a point light source, lux decreases with the square of the distance (inverse square law). Doubling the distance reduces lux to one-quarter. However, for directional lights with a defined beam angle, the relationship is different — the illuminated area grows with distance, so lux decreases proportionally to the area increase. At 1 meter with a 60° beam, the area is 1.05 m²; at 2 meters, it grows to 4.19 m², reducing lux by 4×.
What lux level is recommended for reading?
For comfortable reading, 300-500 lux is recommended at the reading surface. The Illuminating Engineering Society (IES) recommends 300 lux minimum for casual reading and 500 lux for prolonged reading or studying. For older adults or those with vision impairments, 750-1,000 lux may be needed. A single 800-lumen desk lamp positioned 0.5 meters from the page can easily provide 400+ lux on the reading area.