How to Convert Illuminance Online: Lux, Foot-Candles and Phot Explained

You can convert illuminance online for free in seconds: open the Illuminance Converter, enter the value, and choose the units on both sides. The two factors that run the whole topic are these — 1 foot-candle equals exactly 10.7639 lux, and 1 phot equals exactly 10,000 lux. With those, a typical office at about 500 lux is about 46.5 foot-candles, and every other conversion in this guide follows the same arithmetic.

Illuminance is the measure of how brightly a surface is actually lit, and it is the number lighting design, building standards and workplace guidance are written in. It is also a number surrounded by confusion, because everyday language uses brightness for three different things: how much light a lamp produces, how much lands on the desk, and how bright the desk looks to the person sitting at it. Illuminance is only the middle one — light arriving at a surface, measured per unit area. The converter handles the three units that carry it: lux, the metric standard used across Europe and most of the world; foot-candles, the unit still standard in United States practice; and the phot, an older metric unit that survives in scientific literature. This guide explains what each unit is built from, works the office conversion in full, and places the figures against typical lighting levels so a converted number can be judged, not just produced.

How Do You Convert Illuminance Online, Step by Step?

1. Open the converter. Go to the Illuminance Converter with the lighting plan, standard or meter reading that gave you the figure.
2. Confirm the figure really is an illuminance. It should be in lux, foot-candles or phot — units of light per area. A figure in lumens is a lamp output, not an illuminance, and converting it needs the area and distance as well.
3. Enter the value and choose its unit. Lux for metric sources and most light meters, foot-candles for United States plans and standards, phot for older scientific sources.
4. Choose the unit your reader or project needs. The converted value appears using the exact factors in the table below.
5. Judge the result against a typical anchor. Ordinary indoor working light sits in the hundreds of lux. A conversion that turns office lighting into 5 lux or 50,000 lux has almost certainly multiplied where it should have divided.

Lux, Foot-Candles and Phot: What Each Unit Is Built From

All three units measure the same physical thing — luminous flux landing on an area — and they differ only in the area unit they spread it over. The lux is one lumen per square metre. The foot-candle is one lumen per square foot. The phot is one lumen per square centimetre. That single fact generates every factor in the converter. A square metre contains about 10.7639 square feet, so the same light spread over one square metre is diluted 10.7639 times more than the same light spread over one square foot — which is why 1 foot-candle equals 10.7639 lux, and why foot-candle figures always look smaller than lux figures for identical lighting. A square metre contains 10,000 square centimetres, so 1 phot, concentrating a lumen onto a single square centimetre, equals 10,000 lux. The phot is a large unit for very concentrated light, which is why everyday lighting in phot produces tiny decimals: a typical 500 lux office is 0.05 phot.

The practical consequence is that illuminance conversions are area conversions in disguise. Anyone who has converted square metres and square feet already owns the intuition: the factor 10.7639 is the square-foot-to-square-metre factor, appearing here because the area sits underneath the light. It also explains the most common error direction. Because a foot-candle packs its lumen into a smaller area, it is the stronger unit, so there are fewer of them: converting lux into foot-candles must make the number smaller, by dividing by 10.7639. If a conversion makes the number bigger, the factor has been applied backwards — a mistake the converter exists to prevent, and the anchor check in the steps above exists to catch.

Conversion	Factor	Worked check
Foot-candles to lux	Multiply by 10.7639	1 fc = 10.7639 lux
Lux to foot-candles	Divide by 10.7639	500 lux ≈ 46.45 fc
Phot to lux	Multiply by 10,000	1 phot = 10,000 lux
Lux to phot	Divide by 10,000	500 lux = 0.05 phot
Phot to foot-candles	Multiply by about 929.03	1 phot ≈ 929.03 fc

Worked Examples: An Office, a US Specification and Overcast Daylight

Example 1 — the typical office, converted both ways. Office lighting is commonly planned at a typical level of about 500 lux. In foot-candles that is 500 ÷ 10.7639 = about 46.45 foot-candles, usually quoted as about 46.5. In phot it is 500 ÷ 10,000 = 0.05 phot. The three figures describe identical lighting; the American reader sees a number in the forties, the metric reader a number in the hundreds, and anyone reading the old literature a small decimal — which is precisely why specifications must always travel with their unit attached.

Example 2 — a United States specification read in metric. A US document calls for 100 foot-candles at a work surface. In lux that is 100 × 10.7639 = 1,076.39 lux. In phot it is 1,076.39 ÷ 10,000 = about 0.1076 phot. Set against the office example, the comparison becomes clear: 100 foot-candles is a little more than double a typical 500 lux office, which marks it as a level intended for demanding visual work rather than general desks — a judgement you can only make once both figures share a unit.

Example 3 — overcast daylight, labelled as the range it is. Overcast daylight is typically about 1,000 to 10,000 lux. Taking a representative value in the middle of that range, 5,000 lux, gives 5,000 ÷ 10.7639 = about 464.52 foot-candles, and 5,000 ÷ 10,000 = 0.5 phot. Direct sunlight is far higher than the overcast range, which is why a lux meter that reads happily indoors can be overwhelmed outdoors, and why daylight figures should always be quoted as ranges with the sky condition named.

The chart places four typical indoor levels on the lux scale, and even this modest set spans nearly a sevenfold range from a living room to bright retail. That spread explains why a single phrase like well lit is useless in a specification: the living room at a typical 150 lux feels restful precisely because it is dimmer than the office desk at a typical 500 lux, and the retail space at a typical 1,000 lux feels vivid because product lighting is deliberately doubled again. It also calibrates the daylight examples above — even the low end of overcast daylight matches the brightest bar on this chart, and the top of the overcast range is ten times beyond it. Indoor lighting lives in the hundreds; daylight lives in the thousands and far beyond, and any conversion that swaps those worlds has gone wrong.

Which Unit Will You Meet Where?

Situation	Unit you will usually see	Watch for
Lighting design outside the USA	Lux	Standards quote levels at the work surface, not at the lamp
United States plans and standards	Foot-candles	Figures look about eleven times smaller than the lux equivalent
Older scientific and photographic sources	Phot	A very large unit: 1 phot is 10,000 lux, so values are small decimals
Light meters and phone apps	Usually lux, sometimes foot-candles	Check the meter setting before recording; the digits alone do not say
Daylight and outdoor work	Lux, as a range	Overcast is typically 1,000–10,000 lux; direct sun is far higher

Five Mistakes That Spoil Illuminance Conversions

• Applying the 10.7639 factor backwards. Foot-candles are the stronger unit, so lux to foot-candles divides and the number shrinks. A converted office that grows from 500 to over 5,000 has the factor the wrong way round.
• Confusing lux with lumens. Lumens describe the lamp; lux describes the surface. No converter can turn one into the other without the area, the distance and the fitting, so a lumens figure fed into an illuminance converter produces a confident answer to the wrong question.
• Treating typical levels as legal requirements. The 500 lux office in this guide is a widely used typical value, labelled as such. Actual requirements live in your local code and standards, vary by task, and are the authority a project must be checked against.
• Using 10 as the foot-candle factor in precise work. The rough factor is about 7 percent low. Across a whole lighting plan that error is systematic, not random — every level lands understated by the same margin.
• Quoting a lux figure with no position or condition. Illuminance changes with distance from the source and, outdoors, with the sky. A number with no surface, no height and no condition attached cannot be compared with anything.

Conclusion: Small Units, One Factor, Honest Anchors

Illuminance has only three units in the converter and only two factors worth memorising: 10.7639 between lux and foot-candles, and 10,000 between lux and phot. Everything else is judgement — knowing that indoor working light lives in the hundreds of lux, that overcast daylight lives in the thousands, that typical values are anchors rather than rules, and that a converted figure should always be read against those anchors before it is believed. Keep the unit attached to every number, state the surface a level applies to, and let the Illuminance Converter handle the division. The units underneath are areas, so the length converter and the area converter are the natural companions when a plan mixes feet and metres. For background reading, our guide to converting area units online explains the square metre and square foot relationship that generates the 10.7639 factor, and our guide to converting length units online covers the feet and metres those areas are built from.

Frequently Asked Questions

What is the difference between lux and lumens?

Lumens measure the total light a source produces. Lux measures how much of that light actually lands on a surface — lumens spread over an area. The same lamp produces a high lux reading close up, where its light is concentrated, and a low reading across a large room, where the identical lumens are spread thin.

How do I convert lux to foot-candles?

Divide by 10.7639. One foot-candle equals 10.7639 lux, so an office lit to a typical 500 lux is lit to about 46.45 foot-candles. To convert foot-candles back to lux, multiply by 10.7639 instead.

How many lux is bright daylight?

It depends on the sky, which is why daylight figures are always ranges. Overcast daylight is typically about 1,000 to 10,000 lux, and direct sunlight is far higher again. Any single lux figure for daylight should be read as a representative value inside a wide range, not a fixed property of the outdoors.

What is a phot, and will I ever meet one?

A phot is an older metric unit of illuminance equal to 10,000 lux, or one lumen per square centimetre. It survives mainly in older scientific and photographic literature. Modern lighting design works in lux or foot-candles, so the phot appears in this converter for completeness when an old source uses it.

What lux level does an office need?

A typical office lighting level is about 500 lux at the desk, which is roughly 46.5 foot-candles. Treat that as a widely used typical value for general office work rather than a rule for every task: detailed drawing, inspection and older workforces are commonly served by higher levels, and circulation areas by lower ones. Your local building code or lighting standard is the authority for a specific project.

Why does the same lamp give different lux readings in different places?

Because lux measures light arriving at a surface, and that amount falls as the light spreads over a larger area and as the distance from the source grows. Moving a meter closer to a lamp raises the reading without the lamp changing at all. Always note the measurement position alongside any lux figure you record.

Is a foot-candle exactly 10 lux?

No — that is a common rough approximation, and it is about 7 percent low. The exact factor is 10.7639 lux per foot-candle. The approximation is acceptable for a quick estimate, but specifications, comparisons and compliance figures should use the full factor, as the converter does.