💡 Illuminance Conversion Calculator 2026
Instantly convert between lux (lx), footcandles (fc), phots, millilux, nox, lumen/m², lumen/ft² and 6 more units. Trusted reference for lighting designers, photographers, architects, agronomists, and display engineers — with MathJax inverse-square law formulas, ISO/EN/IESNA recommended illuminance standards, and a 2,500+ word photometry guide.
📊 All Units — Simultaneous Conversion
📖 How to Use This Illuminance Converter
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1Enter Your Illuminance Value
Type any illuminance level in the Value field. The tool accepts decimals and very large or very small numbers — from starlight (0.0001 lx) to direct sunlight (100,000 lx). You can also click any Quick Preset button to instantly load a real-world reference value.
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2Select Source and Target Units
Choose "From Unit" (e.g., lux for international SI measurements or footcandle for US lighting specifications) and "To Unit". All 13 photometric illuminance units are available — covering SI, CGS, imperial, and derived flux-per-area formats.
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3Read Instant Results in All Units
The indigo result box shows your primary conversion instantly. The "All Units" panel simultaneously displays your value in every available unit — ideal for technical lighting reports, architectural specifications, or photographic exposure planning that need multiple format references.
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4Use Quick Presets for Real-World Reference
Six presets cover the most common illuminance scenarios — direct sunlight (100,000 lx), overcast sky (10,000 lx), office standard (500 lx), reading level (300 lx), cinema projection (100 lx), and full moonlight (0.25 lx). These let you instantly benchmark any measurement against known real-world values.
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5Apply the Inverse Square Law Formula
Use the MathJax formula section below to calculate how illuminance changes with distance from a point source: E = I ÷ d². If you know your luminous intensity (candela) and distance, plug into the formula to find the illuminance on a surface — crucial for architectural lighting layout, photography, and plant growth lamp positioning.
📐 Illuminance Formulas — MathJax Rendered
\[ E_v = \frac{d\Phi_v}{dA} \quad \text{(lux, lm/m²)} \]
\( E_v = \) Illuminance (lux) · \( \Phi_v = \) Luminous flux (lumens) · \( A = \) Surface area (m²)
\( \text{Equivalently: } E_v = \frac{\Phi_v}{A} \text{ (for uniform illumination over area } A \text{)} \)
\( \text{Example: 1000 lm uniformly spread over 2 m}^2 \Rightarrow E_v = \frac{1000}{2} = 500 \text{ lux} \)
\[ E_v = \frac{I_v}{d^2} \quad \text{(lux, for a point source)} \]
\( E_v = \) Illuminance (lux) · \( I_v = \) Luminous intensity (candela, cd) · \( d = \) Distance (metres)
\( \text{With angle: } E_v = \frac{I_v \cos\theta}{d^2} \quad (\theta = \text{angle between beam and surface normal}) \)
\( \text{Ratio law: } \frac{E_2}{E_1} = \left(\frac{d_1}{d_2}\right)^2 \quad \text{(double distance → quarter illuminance)} \)
\( 1 \text{ fc (footcandle)} = 1 \text{ lm/ft}^2 = \frac{1}{0.092903} \text{ lx} \approx 10.7639 \text{ lx} \)
\( 1 \text{ lx} = 0.09290 \text{ fc} \quad \Leftrightarrow \quad 1 \text{ fc} = 10.7639 \text{ lx} \)
\( 1 \text{ ph (phot)} = 1 \text{ lm/cm}^2 = 10{,}000 \text{ lx} \quad \Rightarrow \quad 1 \text{ lx} = 10^{-4} \text{ ph} \)
\( 1 \text{ nx (nox)} = 10^{-3} \text{ lx} = 1 \text{ millilux} \quad \Rightarrow \quad 1 \text{ lx} = 1000 \text{ nx} \)
\( \text{General: } E_B = E_A \times \frac{F_A}{F_B} \quad (F = \text{lux-equivalent factor}) \)
\( \Phi_v = P \times \eta \quad \text{(lumens from power and efficacy)} \)
\( P = \text{Power (watts)} \quad \eta = \text{Luminous efficacy (lm/W)} \)
\( E_v = \frac{\Phi_v}{A} = \frac{P \times \eta}{A} \quad \text{(lux, uniformly lit area)} \)
\( \text{Number of luminaires: } N = \frac{E_{\text{required}} \times A}{\Phi_{\text{per luminaire}} \times \text{LLF} \times \text{UF}} \)
\( \text{LLF} = \text{Light Loss Factor} \quad \text{UF} = \text{Utilisation Factor} \)
📊 Complete Illuminance Unit Conversion Reference
| From Unit | To lux (lm/m²) | Exact Factor | Notes / Use Case |
|---|---|---|---|
| 1 lux (lx) | 1 lx | 1 (exact) | SI base unit — international lighting standard |
| 1 kilolux (klx) | 1,000 lx | 1,000 | Outdoor / horticultural grow lighting |
| 1 millilux (mlx) | 0.001 lx | 0.001 | Starlight, nighttime sky glow |
| 1 footcandle (fc) | 10.7639 lx | 1/0.092903 | US/Canada lighting standard — IESNA specs |
| 1 phot (ph) | 10,000 lx | 10,000 | CGS unit; 1 lm/cm². Direct sunlight ≈10 phots |
| 1 milliphot (mph) | 10 lx | 10 | CGS sub-unit; 0.001 phot |
| 1 nox (nx) | 0.001 lx | 0.001 | Scotopic (dim) lighting; identical to millilux |
| 1 lm/m² | 1 lx | 1 (same unit) | Explicit flux-per-area form of lux |
| 1 lm/cm² | 10,000 lx | 10,000 | Same as phot |
| 1 lm/ft² | 10.7639 lx | 10.7639 | Same as footcandle |
| 1 lm/in² | 1,550.003 lx | 1,550.003 | High-intensity spot illuminance |
| 1 metercandle (mc) | 1 lx | 1 (exact) | Historical synonym for lux |
🏛️ ISO / EN / IESNA Recommended Illuminance Levels
| Work Area / Environment | Lux (lx) | Footcandles (fc) | Standard |
|---|---|---|---|
| ☀️ Direct sunlight | 100,000 | 9,290 | Natural |
| 🌤️ Full daylight (overcast) | 10,000–20,000 | 929–1,858 | Natural |
| 🌳 Shaded outdoor area | 1,000–5,000 | 93–465 | Natural |
| 🏥 Operating theatre | 10,000–40,000 | 929–3,716 | EN 12464-1 / HTM 08-03 |
| 💊 Pharmaceutical clean room | 500–2,000 | 46–186 | GMP Guidelines / ISO 14644 |
| 🖥️ Office — screen-based work | 300–500 | 28–46 | EN 12464-1, ISO 8995-1 |
| 📐 Drawing / technical work | 500–750 | 46–70 | EN 12464-1 / IESNA RP-1 |
| 🏫 Classroom / lecture theatre | 300–500 | 28–46 | EN 12464-1 |
| 📖 Library reading room | 300–500 | 28–46 | EN 12464-1, IESNA RP-4 |
| 🏪 Retail — general merchandise | 300–750 | 28–70 | IESNA RP-2, EN 12464-1 |
| 🍽️ Restaurant / hospitality | 100–300 | 9–28 | IESNA RP-16 |
| 🏠 Domestic — kitchen task | 300–500 | 28–46 | BS 8206-2 |
| 🏠 Domestic — living room | 50–300 | 5–28 | BS 8206-2 |
| 🛣️ Road — motorway lighting | 1–2 | 0.09–0.19 | EN 13201, CIE 115 |
| 🌱 Plant growth (low light) | 500–2,500 | 46–232 | Horticultural guidance |
| 🌱 Plant growth (high light) | 10,000–50,000 | 929–4,645 | Horticultural guidance |
| 🌙 Full moon (approx.) | 0.1–0.3 | 0.009–0.028 | Natural |
| ⭐ Starlight only | 0.0001–0.001 | 0.000009–0.0001 | Natural |
💡 Understanding Illuminance Units
Lux (lx) — The SI Standard
The SI unit of illuminance, defined as 1 lumen per square metre (lm/m²). Adopted by the International System of Units (SI) in 1948, published in CIE Standard 017.3. Used globally in lighting engineering, architectural standards (EN 12464, ISO 8995), photography, and agriculture. The lux enables internationally consistent lighting specifications — a requirement for multinational construction projects and medical facility standards that cross regulatory jurisdictions.
Footcandle (fc) — The Imperial Standard
Defined as 1 lumen per square foot (lm/ft²). 1 fc = 10.7639 lux. The name derives from the original definition: the illuminance produced by a standard candle at a distance of one foot. This unit remains dominant in the United States across all lighting sectors — IESNA publications (RP-1 through RP-31), architectural lighting contracts, building code compliance inspections, and theater/stage lighting specifications all use footcandles. The conversion factor 10.7639 = 1/(0.3048)² since 1 foot = 0.3048 m.
Phot (ph) — CGS Extreme Unit
1 phot = 1 lm/cm² = 10,000 lux. The CGS (centimetre-gram-second) unit of illuminance — like the stilb for luminance. Direct sunlight at solar noon is approximately 10 phots (100,000 lux). The milliphot (1 mph = 10 lux) was used in some European photometry literature before SI adoption. Today both units are deprecated in favour of lux, but they still appear in pre-1970 laboratory notebooks, optics research papers, and some government archives.
Nox (nx) — Night Illuminance
1 nox = 0.001 lux = 1 millilux. From Latin nox (night). Proposed for very low light levels in the scotopic range — below 0.01 lux where rod vision dominates. Full moonlight: ~0.25 lx = 250 nox. Starlit night without moon: ~0.001 lux = 1 nox. Overcast starless night: ~0.0001 lux. The nox was recognised by the IEC but never widely standardised — millilux is now preferred. Relevant to: night photography, astronomy (sky brightness measurement in magnitudes per arcsecond²), military night vision equipment, and wildlife research on nocturnal animal behaviour.
Lumens vs Lux — The Critical Distinction
Lumens (lm) measure total light output from a source — a property of the light fixture, independent of space. Lux (lx) measures how much of that light arrives at a surface per unit area — a property of the illuminated surface, dependent on distance and geometry. A 1,000-lumen bulb produces 1,000 lux on 1 m² (theoretical perfect sphere). In a 10 m² room it might produce only 60–80 lux (accounting for surface absorption, fixture efficiency, and room geometry). Lux is what matters for vision; lumens determine the source power required to achieve target lux levels.
Illuminance in Photography
Camera exposure relates to illuminance via the reflected-light equation: EV = log₂(L × S / K) where L is scene luminance (cd/m²). But incident light meters measure illuminance (lux), then derive exposure using the incident-light formula: EV = log₂(E × S / C) where C = 250 (Minolta standard) or 320 (Canon/Nikon). At ISO 100 and f/8 at 1/125s, the EV is approximately 13. Direct sunlight at solar noon yields ~EV 15 (100,000 lux). Deep shade: ~EV 10 (1,000 lux). Tungsten interior: ~EV 5–7 (100–500 lux). Understanding lux-to-EV relationships allows manual exposure calculation independent of camera metering systems.
1 fc ≈ 10.76 lux · 1 lux ≈ 0.093 fc · 1 phot = 10,000 lux · 1 nox = 0.001 lux · 1 klx = 1,000 lux. When designing for US clients, multiply lux by 0.0929 for footcandles. When converting IESNA footcandle specs to European EN 12464 format, multiply footcandles by 10.764.
📚 Complete Guide to Illuminance — Science, Standards, and Real-World Applications
Illuminance is the foundation of all lighting design — the single numerical quantity that describes how much light arrives at a surface, and the primary metric used in every professional lighting standard worldwide. Whether you are an architect specifying luminaire layouts for an open-plan office, a horticulturalist calculating grow light intensities for a vertical farm, a photographer selecting an exposure value, or a healthcare manager ensuring surgical theatre compliance with HTM 08-03, you are working with illuminance measured in lux (or fotcandles in North American practice). Understanding illuminance — its definition, measurement, real-world reference values, and relationship to other photometric quantities — is the entry point to all of applied photometry.
The formal definition of illuminance was codified by the International Commission on Illumination (Commission Internationale de l'Éclairage — CIE) in CIE Standard 017.3:2011, the International Lighting Vocabulary. The definition: illuminance (symbol E_v) is the quotient of the luminous flux (Φ_v, in lumens) incident on a surface element divided by the area of that element: E_v = dΦ_v/dA. The SI unit is the lux (lx), equal to one lumen per square metre (1 lx = 1 lm/m²). The lux was formally adopted at the 9th meeting of the Conférence Générale des Poids et Mesures (CGPM) in 1948, consolidating earlier French, German, and British illuminance standards into a single international unit. Before 1948, illuminance was measured in metercandles (France), lux (Germany), and foot-candles (UK/US), all numerically equivalent to what we now call lux or footcandles respectively.
The inverse square law is the governing physics for illuminance from point sources — and it is perhaps the most important relationship in practical lighting design. If a point source has luminous intensity I_v (measured in candela), the illuminance at distance d from the source is E_v = I_v / d². This follows directly from the geometry of spherical wave propagation: the same luminous flux spreads over the surface area of an expanding sphere (4πd²), so flux per unit area decreases as 1/d². Practical consequence: doubling the distance from a fixture to a work surface reduces the illuminance by a factor of four, not two. A 1,000 cd spotlight at 1 m produces 1,000 lux. At 2 m: 250 lux. At 3 m: 111 lux. At 10 m: 10 lux. This dramatic fall-off is why architectural lighting design positions luminaires much closer to work surfaces than casual intuition suggests — to achieve 500 lux on a desk from a single luminaire at 3 m height, you need a luminous intensity of 500 × 3² = 4,500 cd per steradian toward the work surface.
International illuminance standards are published by three primary organisations: (1) The CIE (International Commission on Illumination) publishes foundational photometric standards (CIE 115, CIE 232); (2) ISO and CEN (European Committee for Standardisation) publish binational standards — most significantly EN 12464-1:2021 "Light and Lighting — Lighting of Work Places", which specifies maintained average illuminance (Ēm), uniformity ratio (U₀ = E_min/Ēm ≥ 0.6 for most areas), and Unified Glare Rating (UGR ≤ 19 for offices); (3) the IESNA (Illuminating Engineering Society of North America) publishes the Lighting Handbook (10th edition, 2011) and Reference Publications (RP series) specifying footcandle-based targets for the North American market. For most office environments, EN 12464-1 and IESNA RP-1 converge on 300–500 lux / 28–46 fc as the Maintained Average Illuminance.
The natural illuminance range spans approximately 10 orders of magnitude — from 0.0001 lux (starlit night) to 100,000 lux (tropical direct sunlight). The human visual system adapts to this range through pupillary constriction (a factor of ~16× in retinal light flux) and neural sensitivity adaptation (another ~5 orders of magnitude through photoreceptor gain control). The switchover between rod-dominated (scotopic) and cone-dominated (photopic) vision occurs in the mesopic range: approximately 0.001–10 lux. Below 0.01 lux, only rods function, giving high sensitivity but no colour discrimination and poor acuity. Above 10 lux, cones dominate, enabling colour vision and high spatial resolution. This transition zone is critical for road lighting design — EN 13201 specifies road lighting classes from M1 (2.0 cd/m² for motorways) down to P6 (0.5 lux for cycle paths), carefully balancing energy efficiency with safe visibility in the mesopic range.
Horticultural lighting is one of the fastest-growing applications of illuminance measurement, driven by the rapid expansion of vertical farms, greenhouse supplemental lighting, and indoor cannabis cultivation. Plants respond to photosynthetically active radiation (PAR, 400–700 nm) — measured in μmol/m²/s (PPFD), not lux. However, lux remains useful for approximating plant light requirements when spectrally calibrated PAR meters are unavailable. General conversions for common light sources: sunlight ≈ 54 μmol/m²/s per 1,000 lux; LED grow lights ≈ 15–25 μmol/m²/s per 1,000 lux (spectrum-dependent). Low-light plants (ferns, pothos): 500–2,500 lux (27–135 μmol/m²/s). Medium-light plants (ficuses, orchids): 2,500–10,000 lux. High-light food crops (lettuce, herbs): 10,000–30,000 lux. Fruiting crops (tomatoes, peppers): 30,000–80,000 lux — approaching full sunlight intensity. Modern ISO/DIS 21461 provides guidance on measuring plant-relevant light metrics including PPFD, DLI (Daily Light Integral), and spectral distribution.
Photography and cinematography use illuminance extensively for exposure planning and lighting consistency. A standard cinematography incident light meter (such as the Sekonic L-858D-U) measures illuminance at the subject position in lux or footcandles, then translates to EV (Exposure Value). The incident-light exposure formula: EV = log₂(E_v × S / C_i) where E_v is illuminance (lux), S is ISO, and C_i is the incident-light calibration constant (typically 330 for modern lux-calibrated meters, giving the key-light value that drives the exposure decision). Photography studio lighting: typically 500–2,000 lux at subject position — corresponding to EV 8–11 at ISO 100. High-speed sports photography may require 5,000–10,000 lux minimum. Motion picture production historically specified lighting levels in "book values" — a standard scene lit for ISO 400 film at T2.8 requires approximately 1,100 lux (100 fc). Modern digital cinematography at ISO 6,400, T2.8 can achieve acceptable exposure at 70 lux — enabling documentary-style "available light" shooting that was impossible on film.
❓ Frequently Asked Questions — Illuminance & Light Level Conversion
What is illuminance and how does it differ from brightness?
How do I convert lux to footcandles?
Formula: fc = lux × 0.09290 = lux / 10.7639
Examples: 500 lux = 46.45 fc · 1,000 lux = 92.90 fc · 100,000 lux = 9,290 fc
The conversion factor 10.7639 = (1/0.3048)² because 1 foot = 0.3048 m, so 1 m² = (1/0.3048)² ft² = 10.7639 ft². Therefore 1 lm/m² = 10.7639 lm/ft², i.e., 1 fc = 10.7639 lux.
How do I convert footcandles to lux?
Formula: lux = fc × 10.7639
Examples: 50 fc = 538 lux · 100 fc = 1,076 lux · 9,290 fc = 100,000 lux (sunlight)
This is the exact reverse of the lux-to-fc formula. The IESNA Lighting Handbook uses FC throughout, while EN 12464-1 uses lux — engineers working across US/European projects must convert regularly.
What is the difference between lux and lumens?
What recommended illuminance do offices need?
IESNA RP-1 (North America): 30–50 footcandles (323–538 lux) for general office, 50–75 fc (538–807 lux) for detailed tasks.
Task breakdown: Corridors: 100 lux. Reception: 200 lux. General office: 300–500 lux. Technical drawing: 500–750 lux. Colour matching/inspection: 1,000–2,000 lux. Use our calculator to convert any specification between lux and footcandles instantly.
How much lux does direct sunlight provide?
What is the inverse square law for light?
How many lux do plants need to grow?
Low light (500–2,500 lux): Ferns, pothos, snake plants, ZZ plants — suitable for rooms far from windows.
Medium (2,500–10,000 lux): Fiddle leaf figs, orchids, African violets — bright indirect light near windows.
High (10,000–30,000 lux): Herbs, lettuce, seedlings — require supplemental grow lights in most homes.
Very high (30,000-80,000 lux): Tomatoes, peppers, fruiting crops — near-full-sun conditions.
Note: for precise plant science, use PPFD (μmol/m²/s) not lux — lux weights wavelengths for human vision, not plant photosynthesis.
How do I calculate lumens needed to achieve a target lux level?
For a simple approximation: Lumens total = Target lux × Room area (m²).
Example: Need 500 lux in a 6 × 4 m = 24 m² office: 500 × 24 = 12,000 lumens total (theoretical). With typical UF = 0.60, LLF = 0.80: actual lumens needed = 12,000 / (0.60 × 0.80) = 25,000 lumens. Five luminaires of 5,000 lm each would achieve this. At 100 lm/W (LED): 25,000 / 100 = 250W total installed load, giving 250/24 = 10.4 W/m² lighting power density.