Illuminance: converting a metric lux target to footcandles for a US lighting plan
I was reviewing a lighting fixture spec from a European manufacturer rated in lux, but the electrical plan for the US office space specified footcandles — the two numbers looked close enough at first glance to seem inter
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
I was reviewing a lighting fixture spec from a European manufacturer rated in lux, but the electrical plan for the US office space specified footcandles — the two numbers looked close enough at first glance to seem interchangeable, but they're off by a factor of ten.
I still keep a screenshot of this relationship because memory is where the wrong cousin of a unit sneaks in.
The factor that settles arguments
This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.
1 fc = 10.7639 lux
Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.
The mistake that keeps showing up
Comparing a lux value directly against a footcandle target without converting, since a lux figure numerically resembles a footcandle figure roughly ten times smaller for the same actual brightness.
I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.
- Write down the known value in lux.
- Confirm the target unit is really footcandles, not a similar-looking one.
- Apply the factor: 1 fc = 10.7639 lux.
- Round only at the end, to the precision your tool or spec supports.
Worked example
A fixture is rated for 500 lux output, and the lighting plan specifies a target in footcandles.
500 lux × 0.0929 = 46.45 footcandles
That's 46.45 footcandles — round only to the precision your tool or spec actually needs.
I usually convert once, write the answer with units, then convert back. That two-step habit has saved more weekends than any fancy app.
Quick reference table
| lux | Calculation | footcandles |
|---|---|---|
| 1 | 1 × 0.0929 | 0.0929 |
| 2 | 2 × 0.0929 | 0.1858 |
| 5 | 5 × 0.0929 | 0.4645 |
| 10 | 10 × 0.0929 | 0.929 |
Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.
Why the exact number matters
Under- or over-lighting a workspace affects both energy use and code compliance for task lighting — mixing up lux and footcandles by a factor of ten is an easy way to badly miscalculate fixture count.
Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.
Pocket recap
Divide lux by 10.764 to get footcandles — 500 lux is about 46.5 fc, a typical office task-lighting target.
Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.
Try the converters mentioned in this article
FAQ
What is the exact lux to footcandles factor here?+
1 fc = 10.7639 lux
Should I round partway through the calculation?+
No — carry full precision through the multiplication and round only the final answer, to the precision your tool, gauge, or spec actually supports.
What's the single biggest mistake people make with this conversion?+
Comparing a lux value directly against a footcandle target without converting, since a lux figure numerically resembles a footcandle figure roughly ten times smaller for the same actual brightness.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-25 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.