Illuminance Converter
Convert lux and footcandles — the light arriving on a surface — with the exact 10.7639 factor, not the rough ‘multiply by ten’ that runs seven per cent low.
Illuminance Converter
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Illuminance describes light arriving, not how bright a surface will look — that is luminance.
- The Lambertian bridge assumes a perfectly diffuse surface; real materials are glossy, textured or directional.
- Light-level bands are orientation, not a citation of any particular code.
- The lumens-for-a-room figure ignores fitting efficiency, beam distribution and inter-reflection.
In short: Illuminance is light arriving on a surface, measured in lux (lumens per square metre) or footcandles (lumens per square foot). One footcandle is exactly 10.763910417 lux, because a square metre holds 1/0.3048² square feet. The common shortcut of multiplying footcandles by ten is 7.1% low. Do not confuse this with luminance, the light a surface sends back.
Formula
Each factor is the number of lux in one of that unit. One lux is one lumen per square metre; one footcandle is one lumen per square foot. The ratio is exactly the number of square feet in a square metre, 1 divided by 0.3048 squared.
Worked Example
- Identify the source factor. One lux is 1 lx.
- Convert to the SI unit. 500 × 1 = 500 lx.
- Identify the target factor. One footcandle is 10.763910417 lx.
- Divide. 500 ÷ 10.763910417 = 46.4515 fc.
- Reject the shortcut. Dividing by ten would have given 50 — wrong by 7.1%.
Where the factor comes from. A footcandle is one lumen spread over a square foot; a lux is one lumen spread over a square metre. A square metre contains 1/0.3048² = 10.7639 square feet, so the same lumen spread over the larger area gives a proportionally smaller illuminance. The number is purely geometric and exact, inherited entirely from the exact definition of the international foot.
Strengths & Limits Of This Model
Where this engine is strong
- Exact geometric factor rather than the lossy multiply-by-ten shortcut.
- Keeps illuminance distinct from the other three photometric quantities.
- Offers the diffuse-surface luminance bridge, clearly labelled as an estimate.
- Reports lux and footcandles together for cross-standard work.
Where it stops
- Cannot predict real luminance without knowing the surface's true behaviour.
- Not a lighting design tool — no fitting, geometry or inter-reflection modelling.
Practical Use Cases
Lighting design and compliance
American codes and IES guidance are written in footcandles while European and international standards use lux. A scheme checked against the wrong unit is out by an order of magnitude.
Photography and cinematography
Light meters may read either unit, and exposure calculations depend on using the one the meter is actually reporting.
Workplace and study environments
Most office guidance targets 300 to 500 lux at desk height; converting to footcandles makes American fixture data directly comparable.
Horticulture and display
Growing and retail lighting are quoted in both units depending on supplier origin — see the Luminance Converter when the specification concerns screen or surface brightness instead.
Methodology & Editorial Standards
All conversions route through the lux, defined as one lumen per square metre. The footcandle, one lumen per square foot, is exactly 10.763910416709722 lux, being the reciprocal of the square of the exact international foot; this is a geometric identity with no measurement uncertainty. The phot is the CGS unit at 10,000 lux. The engine deliberately keeps illuminance separate from luminance, luminous flux and luminous intensity, which are three different photometric quantities, and offers the Lambertian bridge L = ρE/π only as a clearly labelled estimate for a perfectly diffuse surface. The π in that relation arises from integrating a cosine-distributed emission over the hemisphere and is exact for an ideal diffuser, though no real surface is one. Light-level descriptions are orientation only, since published standards differ between jurisdictions. All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Illuminance Converter — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many lux are in a footcandle?
Exactly 10.763910416709722 lux. The factor is the number of square feet in a square metre, since a footcandle is one lumen per square foot and a lux is one lumen per square metre. Because the international foot is exactly 0.3048 metres, the conversion is exact and purely geometric.
Is multiplying footcandles by ten accurate enough?
For a rough mental estimate, yes; for design or compliance, no. The true factor is 10.7639, so the shortcut understates the lux figure by about 7.1 per cent. On a 50 footcandle target that is a 38 lux shortfall, which is enough to fail a specification written to a hard minimum.
What is the difference between lux and lumens?
Lumens measure the total light a source emits in all directions; lux measures how much of it lands on each square metre of surface. The same 1,000 lumen lamp gives 1,000 lux over one square metre or 100 lux spread over ten. Lumens describe the lamp, lux describes the place.
What is the difference between lux and nits?
Lux is illuminance, light arriving on a surface. Nits are luminance, light leaving a surface toward your eye. They are different quantities and cannot be converted without knowing the surface's reflectance or emission. A desk at 500 lux and a monitor at 300 nits are describing entirely different things.
How much light does an office need?
Most guidance targets 300 to 500 lux at desk height for general office work, with higher levels for detailed tasks and lower for circulation space. Requirements vary by jurisdiction and by task, so treat any single figure as a starting point and check the standard that applies to you.
How bright is direct sunlight in lux?
Roughly 100,000 lux at midday in summer, falling to perhaps 10,000 to 25,000 under overcast conditions. A full moon gives about 0.25 lux. The range the human eye handles comfortably spans six orders of magnitude, which is why interior lighting can feel adequate at levels a hundred times below daylight.
What is a phot?
The CGS unit of illuminance, equal to one lumen per square centimetre, or 10,000 lux. It is rarely used today but appears in older optical and photographic literature. The nox, at one millilux, was used for very low night-time levels.
Can I calculate lux from a bulb's lumen rating?
Only approximately, and only if you know the area being lit and how the light is distributed. Dividing lumens by area gives a theoretical average that ignores fitting efficiency, beam angle, mounting height and how much light the room surfaces absorb. Real designs need substantially more lumens than that arithmetic suggests.
Does illuminance fall off with the square of distance?
For a small source approximating a point, yes: doubling the distance quarters the illuminance. For large area sources such as a luminous ceiling the falloff is much gentler near the source, and the inverse-square law only becomes accurate at distances several times the source's largest dimension.
Why is there a π in the luminance conversion?
Because a perfectly diffuse surface emits light in a cosine distribution over a hemisphere, and integrating that distribution introduces a factor of π. So a Lambertian surface under one lux with full reflectance has a luminance of 1/π nits. The same π is why the foot-lambert is defined as one footcandle divided by π.