Lux to Foot-Candle Calculator

About the Lux to Foot-Candle Calculator

Lux and foot-candles are both units of illuminance — the amount of light falling on a surface — but they belong to different measurement systems. Lux (SI) measures lumens per square meter, while foot-candles (Imperial) measure lumens per square foot. The conversion factor is 1 fc = 10.764 lux. Despite the metric system's dominance in science, foot-candles remain standard in US building codes, IESNA lighting recommendations, and real estate specifications.

Architects, lighting designers, and facility managers constantly convert between these units when specifying lighting levels, evaluating compliance with codes, or comparing international standards. Office lighting standards typically call for 300-500 lux (28-46 fc), while surgical suites may require 1,500+ lux (140+ fc). Photographers use the conversion for exposure calculations across equipment calibrated in different systems.

This calculator performs bidirectional lux/foot-candle conversion, computes total luminous flux (lumens) needed for a given area, estimates reflected light (exitance) based on surface reflectance, and provides comprehensive reference tables for common illuminance levels. The illuminance scale bar provides visual context from moonlight to direct sunlight.

When This Page Helps

Use this calculator when a lighting spec, code reference, or fixture sheet switches between SI and US illuminance units.

It is useful for room-lighting design, facilities work, photography, and quick lumen estimates where the target light level matters more than the lamp wattage. It also keeps the area-based lumen estimate in the same place as the illuminance conversion, which makes it easier to compare a room target with a fixture specification.

How to Use the Inputs

1. Select conversion direction: lux → foot-candles or foot-candles → lux.
2. Enter the illuminance value.
3. Optionally enter the surface area and select the unit for total lumens calculation.
4. Set surface reflectance to compute reflected light (exitance).
5. Review all unit conversions and compare against the environment reference table.
6. Use the illuminance scale to see where your value falls.

Example Calculation

Tips & Best Practices

• Use lux for metric plans and foot-candles for US code or legacy specifications, but keep the target level tied to the same task.
• Multiply target lux by floor area only as a first-pass lumen estimate; fixture spacing and utilization still matter in a real design.
• Reflectance changes how bright a room feels, so dark finishes often require more delivered light than the raw floor-area math suggests.
• If you are comparing fixtures, check beam angle and distribution because equal lumens do not guarantee equal illuminance on the work plane.

Practical Guidance

Lux and foot-candles describe the same physical idea, so the important question is not which unit is better but which target level suits the space. Offices, classrooms, retail counters, and inspection benches all need different illuminance ranges, and that target should drive the conversion.

Common Pitfalls

The biggest mistake is converting the number correctly but applying it to the wrong reference plane or task. Floor area, desk height, beam spread, and surface reflectance all affect the real lighting outcome. Treat the total-lumens estimate as a planning tool, not as a substitute for a full fixture layout.

Frequently Asked Questions

• Why are foot-candles still used?

  US building codes (IES, ASHRAE), real estate, and many American industries use foot-candles. OSHA workplace lighting standards are also in foot-candles.

• How many lux do I need for an office?

  General office: 300-500 lux (28-46 fc). Detailed work: 500-1000 lux. Computer screens: 300 lux is usually sufficient to avoid glare.

• What is luminous exitance?

  It is the amount of light leaving (reflected from) a surface. Exitance = illuminance × reflectance. A white wall (ρ≈0.8) reflects much more light than a dark floor (ρ≈0.1).

• What is the exact conversion factor?

  1 foot-candle = 10.76391 lux, exactly. This comes from 1 lumen/ft² = 1 lumen/(0.3048² m²) = 10.7639... lumens/m².

• How does altitude affect daylight illuminance?

  At higher altitudes, there's less atmosphere to scatter light. Illuminance on a clear day can reach 120,000+ lux at 3000m elevation versus ~100,000 lux at sea level.

• Can I measure lux with my phone?

  Yes, most smartphones have ambient light sensors. Apps like "Lux Meter" provide reasonable readings, though professional lux meters are more accurate and calibrated.