Illuminance Converter

Introduction

Illuminance is the photometric quantity that tells you how much light from a source is actually landing on a real surface. It is measured in lumens per unit area, which is exactly what the SI unit the lux (lx) is: one lux equals one lumen falling on one square metre. The same physical quantity can be expressed in foot-candles (lumens per square foot, the US customary unit), in phot (lumens per square centimetre, a CGS-era unit), in nox (a thousandth of a lux, used for very low light levels), in lumens per square metre or square foot written out longhand, or in any of the SI prefix-chain multiples such as the kilolux. Every one of these is the same physical quantity. The conversion between them is exact, no measurement uncertainty enters the arithmetic, so picking the right unit is purely a matter of convention and audience.

This Illuminance Converter exists so that no matter which of those units you start with, you can see the equivalent value in every other unit at the same time, rounded to a number of significant figures you choose. The conversion arithmetic is built on a single canonical base, the lux, and every other unit is defined as an exact rational multiple or exact SI-prefix multiple of that base. Because the lux is dimensionless in SI (it is a ratio of a lumen to a square metre, and the lumen is itself a derived unit of solid-angle-weighted luminous flux), the conversion arithmetic never accumulates drift.

The reference panel below the main controls exposes a selected set of well-documented lighting levels, full sunlight, overcast sky, an operating theatre, a classroom, a candle at one metre, a full moon, starlight, so you can sanity-check an instrument reading against the published reference values without leaving the tool.

How to Use

1. Type a number into the Value field. You can use decimal notation, and you can paste values like 500, 10.7639, 0.1, or 100000.
2. Pick the source unit from the From unit dropdown. The dropdown lists every unit the converter understands: lux, foot-candles, phot, milliphot, nox, kilolux, lumen per square metre, and lumen per square foot.
3. Choose the Significant figures you want. The converter offers 4, 6, or 8 significant digits. 4 is plenty for sketching a fixture layout; 6 is the default for lighting specifications; 8 covers laboratory and calibration work.
4. Read the live results table below. Every other unit appears in its own row, formatted to the chosen significant-figure precision. The input unit is marked with a small "(source)" tag and shaded.
5. Click any reference-level button to load a typical real-world illuminance. The presets include full sunlight, overcast daylight, an operating theatre, an office desk, a classroom, a residential living room, a hallway, a well-lit street, a candle at one metre, full-moon night, quarter-moon night, and starlight only.
6. Use the canonical-lux readout at the top of the results area as a single-number summary when you need to quote the value to a colleague or paste it into a specification.

The Formulas

The conversion arithmetic is built on a single canonical base, the lux. The full set of exact factors the converter uses is:

• 1 lux (lx) = 1 lumen per square metre (lm/m²), the SI definition, by construction.
• 1 foot-candle (fc) = 1 lumen per square foot (lm/ft²) = 10.763910416709722 lux, exactly. The numerical factor is the ratio of 1 m² to 1 ft², namely 1 / 0.09290304 m²/ft².
• 1 phot (ph) = 1 lumen per square centimetre (lm/cm²) = 10 000 lux, by SI prefix-chain scaling.
• 1 milliphot (mph) = 0.001 phot = 10 lux, by SI prefix-chain scaling.
• 1 nox = 0.001 lux, by SI prefix-chain scaling.
• 1 kilolux (klx) = 1000 lux, by SI prefix-chain scaling.

For every conversion between two units, the math is exactly:

lux_canonical = value_in_source_unit * factor[source_unit]
value_in_target_unit = lux_canonical / factor[target_unit]

where factor[unit] is the number of lux per single unit. Because every factor is an exact definition (or, in the case of the foot-candle, an exact ratio of two exact SI lengths), every conversion between two non-prefix units is exact to floating-point precision, and conversions that involve only SI-prefix multiples (lux ↔ kilolux ↔ milliphot ↔ phot) round-trip with zero drift.

Worked Examples

One foot-candle to lux. A typical incident-light meter for US photographers reads in foot-candles. One foot-candle = 1 lumen per square foot. Because 1 square foot is 0.09290304 square metres exactly, 1 fc = 1 / 0.09290304 = 10.7639104167 lux. Round to six significant figures and you get 10.7639 lux. This is the single most-used conversion in the tool, because most published US lighting standards still specify in foot-candles while most published international standards specify in lux.

500 lux to foot-candles. 500 lux / 10.7639104167 lux per fc = 46.4515 foot-candles. This is the EN 12464-1 general-office task-illuminance target re-expressed for a US specification. The reverse direction would be 46.4515 fc × 10.7639104167 lx/fc = 500 lux exactly, confirming the round trip.

One phot to lux. One phot is defined as one lumen per square centimetre, which is 10 000 lux by the SI prefix chain (1 cm² = 1/10 000 m², so 1 lm/cm² = 10 000 lm/m² = 10 000 lux). Phot is a CGS-era unit that survives mainly in older photometry textbooks and in a few European lighting specifications, and the converter handles it for backward compatibility.

One lux to nox. One nox is one-thousandth of a lux, so 1 lx = 1000 nox. The nox unit is occasionally seen in astronomy and in civil-twilight measurements where the actual lux value would be a small fraction. Going the other way, 1 nox = 0.001 lx exactly.

2.5 kilolux to lux. One kilolux is 1000 lux by the SI prefix chain, so 2.5 klx = 2.5 × 1000 = 2500 lux. This is a typical overcast-daylight value expressed as a prefix multiple, and it is useful when scanning daylight sensor datasheets that quote output in kilolux.

A camera light meter at f/8, ISO 100, 1/60 s. The exposure value at those settings corresponds roughly to an incident-light illuminance of about 1000 foot-candles on the subject. Converted through the converter: 1000 fc × 10.7639104167 lx/fc = 10 763.9 lux, or about 10.76 klx. This is the kind of cross-check a studio photographer does when comparing a handheld meter reading (fc) against a published spec (lux).

Where It Shows Up

Architectural and interior lighting design. The European standard EN 12464-1 specifies task illuminance in lux: 500 lux for office work, 300 lux for classrooms, 100 lux for corridor circulation, 1000 lux for operating-theatre task areas. US specifications written by firms accustomed to the imperial system quote the same targets in foot-candles. Converting between the two systems is the bread-and-butter task for any lighting designer who works across both jurisdictions.

Photography and cinematography. Incident-light meters in the United States commonly read in foot-candles; studio specifications in the rest of the world read in lux. A cinematographer who quotes a key light at "2000 fc" is talking about the same physical quantity as a European gaffer who quotes "21 528 lux", the converter lets you see the two numbers side by side.

Astronomy and night-sky brightness. The natural sky brightness at night ranges from about 1 millilux (full moon, clear sky) down to about 1 microlux (moonless, remote site). The nox unit (0.001 lx) is occasionally used in this range because it expresses a typical night-sky value as a one- or two-digit number. The kilolux prefix doesn't appear in this domain.

Horticulture and plant-growth lighting. Horticultural lighting is increasingly specified in photosynthetically active radiation (PAR), measured in micromoles per square metre per second (µmol·m⁻²·s⁻¹). The broadband illuminance in lux is a separate but related quantity and is still useful as a quick sanity-check against an instrument reading or a manufacturer datasheet. Greenhouse daylight in summer at midday typically reaches 50 000 to 100 000 lux, well into the "full sunlight" range of the reference panel.

Industrial safety and emergency lighting. EN 1838 and equivalent standards specify minimum illuminance for escape routes and safety signage in lux. The converter lets a safety engineer convert a published lux target into foot-candles for a US-style specification, or scale an existing specification up or down by a factor for a non-standard installation.

Common Mistakes

Confusing illuminance with luminance. Illuminance (lux, foot-candles) is the light falling on a surface. Luminance (candelas per square metre, nits, foot-lamberts) is the light leaving a surface in a particular direction. They have different units, different measurement instruments (lux meter vs. luminance meter), and different physical meanings. The converter handles illuminance only, luminance values need a different tool.

Forgetting the area in lumens per square metre. A bare "lumen" without an area attached is a measure of luminous flux, not illuminance. A bare "candela" is a measure of luminous intensity, not illuminance. A common error is to read a lamp's lumen rating off its packaging and treat the number as an illuminance. To get illuminance, divide by the area the lumens land on. The lux value 500 lux means 500 lumens landing on each square metre of the work plane.

Using the wrong "foot-candle." The foot-candle (fc) is a unit of illuminance equal to one lumen per square foot. It is not the same as the foot-lambert (fL), which is a unit of luminance equal to 1/π candelas per square foot. The two share the word "foot" and the photometric context, but they measure different physical quantities.

Treating phot as equal to lux. One phot is 10 000 lux, not one lux. This is the most common mistake when reading older photometry textbooks that use the phot as their primary unit, because in those texts a value like "10 ph" sounds like "10 lux" but is actually 100 000 lux. The converter catches this immediately if you remember to set the source unit correctly.

Mixing photometric and radiometric quantities. Lux is a photometric unit weighted by the CIE photopic luminosity function V(λ), which models the human eye's sensitivity under daylight conditions. Radiometric quantities like watts per square metre are not converted to lux by a single constant, you need to integrate over the spectrum against V(λ). The converter is for photometric illuminance only.

Rounding too early. Because the conversion factors are exact, you should keep at least six significant figures in your intermediate value before dividing, or your final result will pick up rounding error. The converter's sig-fig control handles this for you by holding full double-precision internally and rounding only for display.

Frequently Asked Questions

How many lux is one foot-candle?

One foot-candle equals exactly 10.7639104167 lux, by the exact ratio of 1 square foot (0.09290304 square metres) to 1 square metre. Going the other way, 1 lux = 1 / 10.7639104167 = 0.09290304 foot-candles. The factor is exact because 1 foot is defined as exactly 0.3048 metres by international agreement, so 1 ft² is exactly 0.09290304 m².

What is the difference between lux and lumens?

A lumen is a unit of luminous flux, the total amount of visible light a source emits in all directions. A lux is a unit of illuminance, the amount of luminous flux landing on one square metre of a surface. The two are related by area: 1 lux = 1 lumen per square metre. A bare lumen number (say, 800 lm from a bulb) does not tell you the lux value until you know the area over which the lumens are spread, the distance from the source, and the beam pattern. The converter handles illuminance, not flux, so the input is always a lux-equivalent value.

What is a foot-candle used for?

Foot-candles are the imperial-customary unit of illuminance used primarily in the United States for architectural lighting specification, photography, cinematography, and horticulture. Most published US lighting standards (the IES Lighting Handbook, ASHRAE 90.1, the lighting sections of NFPA 101) quote task illuminance in foot-candles. Most international standards (EN 12464-1, ISO 8995, the CIE S 020 emergency-lighting standard) quote the same targets in lux. Converting between the two systems is the standard use case for this tool.

What is a phot and when is it used?

A phot is one lumen per square centimetre, equal to 10 000 lux. It is a CGS-era unit of illuminance that survives mainly in older photometry textbooks, in some European lighting standards published before 1970, and in the older scientific literature. Modern practice has standardised on the lux for SI work and the foot-candle for US work, so the phot is largely of historical interest. The converter includes it for backward compatibility with the older literature.

What is a nox?

A nox is one-thousandth of a lux, equal to 0.001 lx. It is used for very low light levels, civil twilight, deep night-sky brightness, the lower end of camera sensitivity. The nox is not as widely encountered as the lux or the foot-candle, but it shows up in astronomy and in night-vision engineering where the bare lux value would carry too many leading zeros to be readable.

Is illuminance the same as brightness?

No. Illuminance is the light falling on a surface, measured in lux or foot-candles. Brightness, in the strict photometric sense, is luminance, the light leaving a surface in a particular direction, measured in candelas per square metre, nits, or foot-lamberts. A white wall under bright daylight can have an illuminance of 50 000 lux and a luminance of a few thousand candelas per square metre at the same time; the two numbers describe two different physical aspects of the same scene. The converter handles illuminance only.

Can I use this converter offline?

Yes. The conversion logic is a few lines of arithmetic (multiply by the source factor, divide by the target factor), and the page is a static bundle after the first load. Once the tool is cached by your browser it works without any further network requests. Bookmark it or save it as a desktop shortcut and it functions like a local utility.

What illuminance level do I need for reading?

EN 12464-1 specifies 300 lux as the minimum maintained illuminance for task areas such as reading, writing, and classroom work. A typical reading lamp at the page produces 300 to 500 lux. Higher-end tasks (detailed technical drawing, sewing, surgical work) require 500 to 1000 lux or more. The reference panel of this converter includes the 300-lux classroom reading-room preset so you can compare your own lighting against the published standard.

References

• BIPM, The International System of Units (SI), 9th edition (2019). For the Illuminance Converter, Bureau International des Poids et Mesures, Sèvres. Chapter 2 discusses the lumen and the lux as SI-derived units of photometry, and lists the photometric quantities and their units.
• CIE S 017:2020, ILV: International Lighting Vocabulary, 2nd edition. International Commission on Illumination. Defines illuminance, the lux, the foot-candle, the phot, the nox, and the relations between them, including the area-based identities 1 lux = 1 lm/m² and 1 fc = 1 lm/ft².
• Illuminating Engineering Society, IES Lighting Handbook, 10th edition (2011). Reference standard for lighting practice in the United States. Contains the standard reference illuminance levels used in the reference panel (office task, classroom, operating theatre, corridor, street lighting) and the standard conversion factors for foot-candles.
• EN 12464-1:2021, Light and lighting, Lighting of work places, Part 1: Indoor work places. European Committee for Standardization. Specifies maintained illuminance levels in lux for indoor task areas, including the 500-lux office, 300-lux classroom, and 100-lux corridor figures cited in the reference panel and the worked examples.
• EN 1838:2013, Lighting applications, Emergency lighting. European Committee for Standardization. Specifies minimum illuminance in lux for escape routes and safety signage, the typical values cited in the "industrial safety and emergency lighting" section above.