Lux vs Foot Candles: Calculator & Conversion

This calculator converts illuminance between lux and foot-candles, two units that measure the same thing: the amount of light falling on a surface. You enter a number in either lux or foot-candles, and the calculator instantly gives you the equivalent in the other unit, plus the inverse conversion for reference.

Formula

The conversion is a fixed ratio based on the definition of each unit:

• 1 foot-candle (fc) = 1 lumen per square foot (lm/ft²)
• 1 lux (lx) = 1 lumen per square meter (lm/m²)
• Since 1 m² = 10.7639 ft², 1 fc = 10.7639 lx

So the formulas are:

• fc = lux ÷ 10.7639
• lux = fc × 10.7639

Both units measure illuminance (symbol E), which is the luminous flux (lumens) striking a surface area. The unit you choose does not change the physical light; it only changes the number you write down.

Worked Example

Using the default values in the calculator above:

1. You enter 500 lux in the first input.
2. To convert to foot-candles: 500 ÷ 10.7639 = 46.45 fc (rounded to 2 decimals).
3. The calculator also shows the reverse: if you enter 46.45 fc, you get 46.45 × 10.7639 = 500.0 lux.

That matches the output shown in the calculator block.

Why Two Units Exist

Lux is the SI (metric) unit and is used almost everywhere in the world. Foot-candles are the customary unit in the United States, especially in architectural lighting, film production, and older building codes. The two units are not different kinds of measurement—they are the same physical quantity expressed in different area units. Think of it like inches versus centimeters: both measure length, but you use one or the other depending on your context.

In practice, you will see lux on European light meter apps, LED packaging, and international standards (like ISO for photography). Foot-candles appear on US lighting design plans, OSHA guidance (though they often give both), and in older theatrical lighting equipment.

How to Choose Which Unit to Use

Your choice depends on where you are and what you are comparing:

• If you are reading a lighting design spec from the US, it will likely be in foot-candles.
• If you are buying bulbs or sensors from Europe or Asia, the spec sheet will use lux.
• If you are using a smartphone light meter app, check the settings—many let you switch between lux and fc.
• For scientific work, always use lux (SI).

When you are comparing recommended light levels (e.g., 300 lux for office work), you must convert to the same unit before making a judgment. A common mistake is to think 500 lux is “brighter� than 50 fc, when in fact 50 fc ≈ 538 lux—nearly the same.

Common Mistakes and Pitfalls

• Mixing up lumens and lux: Lumens measure total light output of a source; lux and fc measure how much of that light lands on a surface. A 1000-lumen bulb can produce 100 lux at 3 meters but 1000 lux at 0.5 meters.
• Using the wrong conversion factor: Some sources round 10.7639 to 10.76 or even 10.8. For rough work that is fine, but for design or comparison, use the full factor. The calculator above uses 10.7639.
• Confusing foot-candles with foot-lamberts: Foot-lamberts measure luminance (light reflected off a surface), not illuminance. They are not directly convertible without knowing the surface reflectance.
• Forgetting that distance matters: Illuminance falls off with the square of the distance from a point source. A reading at 1 meter is not comparable to one at 2 meters unless you account for the inverse-square law.

Real-World Examples and Comparisons

To give you a sense of scale, here are typical illuminance levels you might encounter:

• Full daylight (not direct sun): 10,000–25,000 lux (about 930–2,320 fc)
• Overcast day: 1,000–2,000 lux (93–186 fc)
• Office desk lighting: 300–500 lux (28–46 fc)
• Supermarket or retail: 750–1,000 lux (70–93 fc)
• Home living room: 100–300 lux (9–28 fc)
• Street lighting at night: 10–50 lux (0.9–4.6 fc)

These are approximate and vary widely. But they show that a “lux� number and a “fc� number can look very different even when the light is identical. For example, 500 lux is about 46 fc—both are comfortable reading light.

How to Measure Illuminance Yourself

You do not need a dedicated light meter to get a rough reading:

1. Use a smartphone app that measures lux (most use the front-facing camera sensor).
2. Place the phone flat on the surface you want to measure, screen facing up.
3. Make sure no shadow falls on the sensor.
4. Read the value and note the unit (usually lux).
5. If the app gives fc, multiply by 10.7639 to get lux.

For professional work, a calibrated light meter is more accurate, but a phone app is fine for comparing rooms or checking if a desk lamp is too dim.

Frequently Asked Questions

Is 1 foot-candle equal to 10 lux?

No, 1 fc = 10.7639 lux. The number 10 is sometimes used as a rough approximation, but it introduces a 7.6% error. For serious measurements, use the full factor.

Which unit is brighter: 100 lux or 100 fc?

100 fc is much brighter because it equals 1,076 lux. A foot-candle is a larger unit than a lux, so the same number in fc represents more light.

Can I convert lux to foot-candles by dividing by 10?

You can for a quick estimate, but it is not accurate. Divide by 10.7639 for exact conversion. For example, 500 lux ÷ 10 = 50 fc, but the true value is 46.45 fc—a 7% difference.

Why do light meters show both units?

Because users in different countries and industries expect different units. A meter that shows both saves you from doing the math. The conversion is linear, so it is trivial to display both.

Does the conversion factor change with the type of light?

No. Lux and foot-candles are purely geometric units of illuminance; they do not depend on the light source (LED, fluorescent, sunlight). The same physical light will have the same lux and fc values regardless of its color or spectrum.

What is the difference between lux and lumens?

Lumens measure the total light output of a bulb (all directions). Lux measures how much of that light lands on a given area. A 1000-lumen bulb can give 100 lux on a table if it is close, but only 10 lux if it is far away.

Related

• DC Cable Size Calculator: How to Size Wires
• Custody x Change Calculator: How It Works
• Dash Score Calculator: How to Use It

in lux_val | Lux (lm/m²) | lx | 500 | 0
in fc_val | Foot-candles (lm/ft²) | fc | 46.45 | 0
out fc_result | Foot-candles | fc | lux_val / 10.7639 | 2
out lux_result | Lux | lx | fc_val * 10.7639 | 2