Total flux from the fixture, e.g. an 800 lm LED bulb.
Surface the light spreads over.
If length and width are both filled, area is computed from them (m).
—
lux (lx)
—
lumens needed (total, all fixtures)
1 fc = 10.7639 lux — the imperial sibling of lux.
—
foot-candles (fc)
—
lumens needed
Watts depend on bulb technology — see our lighting converter for lumens ↔ watts by bulb type.
01 — The formulas
Lumens → lux
E (lux) = Φ (lumens) / A (m²)
Illuminance equals luminous flux divided by area. This is the definition of lux: 1 lux = 1 lumen per square meter.
Lux → lumens
Φ (lumens) = E (lux) × A (m²)
Reversed. You need the area — converting lux to lumens without an area is impossible, which is why thin converters that don't ask for area are lying to you.
Foot-candles
E (fc) = Φ (lumens) / A (ft²) · 1 fc = 10.7639 lux
A foot-candle is the imperial sibling of lux: 1 fc = 1 lm/ft², exactly as 1 lux = 1 lm/m².
02 — Worked examples
Example 1 — lumens to lux. A 1,200-lumen lamp lights a 20 m² room evenly.
E = 1,200 / 20 = 60 lux
E = 1,200 / 20 = 60 lux
Example 2 — lux to lumens (room sizing). You want 300 lux in a 20 m² room.
Φ = 300 × 20 = 6,000 lumens — about eight 800-lm bulbs or four 1,600-lm bulbs.
Φ = 300 × 20 = 6,000 lumens — about eight 800-lm bulbs or four 1,600-lm bulbs.
Example 3 — square feet. 500 lumens spread over 100 ft².
100 ft² = 100 × 0.09290304 = 9.290304 m² → E = 500 / 9.290304 = 53.82 lux.
Shortcut: 500 / 100 = 5 foot-candles, and 5 × 10.7639 = 53.82 lux — both routes agree.
100 ft² = 100 × 0.09290304 = 9.290304 m² → E = 500 / 9.290304 = 53.82 lux.
Shortcut: 500 / 100 = 5 foot-candles, and 5 × 10.7639 = 53.82 lux — both routes agree.
03 — Lumens vs lux vs candela vs foot-candle vs watts
| Unit | Measures | Definition |
|---|---|---|
| Lumen (lm) | Luminous flux — total light a source emits | Base unit here |
| Lux (lx) | Illuminance — light landing on a surface | 1 lux = 1 lm/m² |
| Foot-candle (fc) | Illuminance (imperial) | 1 fc = 1 lm/ft² = 10.764 lux |
| Candela (cd) | Luminous intensity — light in one direction | 1 cd = 1 lm/sr · candela to lux calculator |
| Watt (W) | Power consumed | W = lm / efficacy (lm/W) · lumens ↔ watts |
04 — Recommended lux by room
Typical design guidelines (IESNA / ISO 8995-1 based practice). Ranges show low–mid–high; codes vary by country — these are planning values, not legal requirements.
| Room | Low | Typical | High |
|---|---|---|---|
| Bedroom | 100 | 150 | 200 |
| Living room | 100 | 150 | 300 |
| Kitchen | 200 | 300 | 500 |
| Dining room | 150 | 200 | 300 |
| Bathroom | 200 | 300 | 500 |
| Hallway / stairs | 100 | 150 | 200 |
| Home office / study | 300 | 400 | 500 |
| Office workspace | 300 | 500 | 750 |
| Classroom | 300 | 400 | 500 |
| Retail / showroom | 500 | 650 | 800 |
| Restaurant | 100 | 150 | 200 |
| Garage | 150 | 200 | 300 |
| Gym / fitness | 200 | 300 | 400 |
| Laundry / utility | 200 | 300 | 500 |
| Workshop | 300 | 500 | 750 |
| Warehouse / storage | 100 | 150 | 200 |
05 — Quick chart: lumens to lux by area
| Lumens \ Area | 10 m² | 20 m² | 50 m² |
|---|---|---|---|
| 800 lm | 80 | 40 | 16 |
| 1,600 lm | 160 | 80 | 32 |
| 6,000 lm | 600 | 300 | 120 |
| 10,000 lm | 1,000 | 500 | 200 |
Values are lux = lumens / m². For other numbers, use the calculator above.
06 — Spreadsheet formula
Lumens to lux in Excel / Google Sheets (A1 = lumens, A2 = area in m²):
=A1/A2
Lux to lumens (B1 = target lux, B2 = area in m²):
=B1*B2
Square feet to m² first if your area is imperial:
=A1/(A2*0.09290304)
07 — Scope boundary
This tool is area-based (total flux ÷ surface area: E = Φ/A). Related tools for adjacent physics:
- Candela to lux — point-source physics: inverse-square law E = I/d², beam angle and solid angle (cd ↔ lm).
- Lighting converter — the hub: 19 units across flux, intensity, illuminance, luminance, efficacy and energy, plus lumens ↔ watts by bulb type.
- Grow light lab — lux/lumen/foot-candle → PPFD/DLI for horticulture lighting.
08 — Methodology & honest limitations
Method. Every result on this page is computed from the two definitions of illuminance: E(lux) = lumens / m² and E(fc) = lumens / ft², with the exact area conversion 1 ft² = 0.09290304 m². The engine behind the calculator is verified by an independent Python oracle (352 test vectors, hand-computed).
Honest limitations. The formula assumes light spreads perfectly evenly and ignores losses — real rooms lose 20–50% of fixture lumens to absorption, reflector inefficiency and dirt, and create hot spots and dark corners. Professional lighting designers add a coefficient of utilization and a light-loss factor on top of this math. This calculator gives the correct theoretical average: perfect for planning, not a substitute for a lux meter reading or a lighting designer.
09 — FAQ
What is the formula to convert lumens to lux?
The formula is E(lux) = Φ(lumens) / A(square meters). Illuminance in lux equals luminous flux in lumens divided by the area in square meters. For example, a 1,200-lumen lamp lighting 20 m² gives 1,200 / 20 = 60 lux. This is the exact definition: 1 lux = 1 lumen per square meter.
Can you convert lumens to lux without knowing the area?
No — and any converter that asks only for lumens is misleading you. Lumens measure total light output (flux); lux measures light landing on a surface (illuminance). The same 800-lumen bulb gives 800 lux over 1 m² but only 80 lux over 10 m². You must know (or estimate) the area. If all you know is a room's length and width, multiply them to get the area first.
How do I convert lux to lumens?
Reverse the formula: Φ(lumens) = E(lux) × A(m²). Multiply your target illuminance by the area in square meters. Example: you want 300 lux in a 20 m² room, so you need 300 × 20 = 6,000 lumens total from all fixtures combined.
What is the difference between lumens and lux?
Lumens (lm) measure luminous flux: the total amount of light a source emits in all directions. Lux (lx) measure illuminance: how much of that light actually lands on a surface, per square meter. Think of lumens as the total water coming out of a hose and lux as the water depth in the bucket it fills — spread the same water over a bigger bucket and the depth drops.
How many lumens do I need for a 20 m² room at 300 lux?
6,000 lumens total: 300 lux × 20 m² = 6,000 lm. In practice that could be eight 800-lumen bulbs (a common 60 W-equivalent LED size) or four 1,600-lumen bulbs. Use the room planner above with your actual room type — kitchens and offices want around 300–500 lux, bedrooms only 100–200.
What is 1 lux in lumens?
1 lux = 1 lumen per square meter, so 1 lux equals 1 lumen only over exactly 1 m². Over 10 m², 1 lux requires 10 lumens. There is no single fixed answer — lumens and lux are linked by area, and the area is what decides.
How do I convert lumens to lux with area in square feet?
Two steps: first compute foot-candles by dividing lumens by the area in square feet (1 foot-candle = 1 lumen/ft²), then multiply by 10.764 to get lux. Example: 500 lumens over 100 ft² = 5 foot-candles = 53.8 lux. Equivalently, convert the area first (1 ft² = 0.09290304 m²) and divide lumens by the m² area — both routes give the same answer.
What is the relationship between lux and foot-candles?
1 foot-candle (fc) = 10.7639 lux. A foot-candle is the imperial sibling of lux: 1 fc = 1 lumen per square foot, exactly as 1 lux = 1 lumen per square meter. Foot-candles are common in US lighting specs and theater/cinema contexts; lux is the SI unit used almost everywhere else.
How do ANSI lumens relate to lux? (projector question)
ANSI lumens are a standardized way of measuring a projector's total light output — the number is directly usable as lumens in E = Φ/A. The lux on your screen equals the projector's ANSI lumens divided by the screen area in m². Example: a 2,000 ANSI-lumen projector on a 4 m² screen delivers 500 lux at the screen. That said, lux at the screen does not determine perceived image quality alone — ambient light and screen gain matter too.
How many lux are recommended for a kitchen, office, or bedroom?
Common design guidelines: kitchen 200–500 lux (higher over task counters), office/workspace 300–500 lux (500 for detailed work), living room 100–300 lux, bedroom 100–200 lux, bathroom 200–300 lux, hallway 100–200 lux, workshop 300–750 lux, retail 500–800 lux. These are typical practice values (IESNA/ISO 8995-1 based), not legal building codes, which vary by country.
Why doesn't my lux meter reading match the calculator?
A calculator assumes light spreads perfectly evenly; real rooms don't. Typical causes: the meter is not at the reference plane (readings are usually quoted at desk height, ~0.75 m), the source is closer than the average distance (inverse-square falloff), walls absorb light instead of reflecting it, the bulb's rated lumens are initial (not aged) values, and fixtures lose light in the reflector. Differences of 20–40% are normal; the calculator gives the theoretical average, the meter gives reality at one spot.
Does distance matter when converting lumens to lux?
Not in the area-based formula E = Φ/A — distance is already baked in, because the area is the surface the light spreads over. Distance matters in the point-source formula E = I/d² (intensity in candela divided by distance squared), which is a different model for a single spot beam hitting a small target. For that physics use our candela-to-lux calculator; for total flux over a room or surface, use this one.
How do lumens relate to watts?
Watts measure power consumed; lumens measure light produced. They are linked by luminous efficacy (lumens per watt), which depends on the technology: modern LEDs make roughly 80–120 lm/W, old incandescents only ~15 lm/W. So an 800-lumen LED uses about 8 W while an 800-lumen incandescent needs ~53 W. For watts ↔ lumens conversions by bulb type, use our lighting converter hub.
What is the difference between a lux meter and this calculator?
A lux meter physically measures illuminance at one point with a sensor. This calculator computes the theoretical average illuminance from a known lumen output and area. Use the calculator when planning lighting (buying fixtures, sizing a room); use a meter when verifying what was actually installed. They complement each other, and a 20–40% gap between them is usually fine.
How do I calculate lux for a projector screen?
Divide the projector's ANSI lumens by the screen area in square meters. A 2,000 ANSI-lumen projector on a 100-inch 16:9 screen (about 2.77 m²) gives roughly 722 lux at the screen. Bigger screen, same projector — dimmer image. This is the same E = Φ/A math this calculator performs, just applied to a screen.
What are the limitations of the lumens-to-lux formula?
Three honest limitations. (1) It assumes perfectly uniform light distribution; real fixtures create hot spots and dark corners. (2) It ignores losses — real rooms lose 20–50% of fixture lumens to absorption, reflector inefficiency, and dirt (lighting designers use a coefficient of utilization and light-loss factor for this). (3) It averages over the whole area, so it can't predict the lux at your desk specifically. For a quick planning estimate it is exactly right; for a guaranteed design, measure or consult a lighting designer.