One foot-candle (fc) equals 10.76391 lux, and the reverse factor is 0.09290304 fc per lux, per conversion tables published by Rapid Tables and Unit Converters [S2][S5]. Both factors trace to one geometric constant: 1 ft² equals 0.09290304 m², so 1 lm/ft² equals 1 lm/0.09290304 m², which simplifies to 10.76391 lm/m² [S5].
The foot-candle is a US customary illuminance unit defined as 1 lumen per square foot; the lux is the SI equivalent at 1 lumen per square meter [S3]. This is the only difference, the lumen definition is identical in both systems, so the conversion is a pure area rescaling, not a photometric redefinition [S1][S3]. Engineers, lighting designers, and anyone reading a US-spec lighting layout will encounter both notations and need a clean mental model for switching between them.
The conversion factor and the area-of-a-square-foot origin
Foot-candles multiply by 10.7639104167 to give lux, a value that NIST-traceable unit tables round to 10.76391 for engineering work [S2][S5]. For a rough mental check, 1 fc ≈ 10 lx, and that 10% approximation holds because most room-light measurements are accurate only to about ±10% in practice [S3]. For datasheet values, photometric reports, and code-compliance calculations, use the full 10.76391 factor; for a back-of-envelope estimate on a job walk, the 10× shortcut is good enough [S6].
The 10.76391 figure is dimensionless in the strict sense; it is the inverse of 0.09290304 m²/ft², which is itself derived from the 0.3048 m/ft length conversion squared (0.3048² = 0.09290304) [S5]. No luminous-intensity adjustment is needed because the candela, the SI base unit feeding both lumens, is unchanged across the systems [S3]. That is why the only thing being converted when going fc ↔ lx is the area denominator, the numerator (lumens) stays put [S1][S7].
Worked examples with engineering-relevant magnitudes
A US office lighting spec calling for 50 fc at the desktop equals 50 × 10.76391 = 538.20 lx, which is in the standard 500 lx European office range [S2][S3]. A 10 fc corridor target equals 107.64 lx, close to the IES-recommended 100 lx for circulation spaces, and a 100 fc task-light reading equals 1,076.39 lx, which crosses into the surgical-suite or fine-assembly band where glare control becomes the limiting design factor [S2][S3]. The Wikipedia foot-candle article gives the matching indoor ranges: 5–40 fc for general residential spaces (54–431 lx), 70–90 fc for residential task work (753–969 lx), and 5–200 fc across commercial codes depending on task type [S3].
Daylight benchmarks also translate cleanly: full unobstructed sun reaches about 10,000 fc, which is roughly 107,639 lx, and an overcast day sits near 100 fc, or 1,076 lx [S3]. Light near a window can swing from 100 fc to 460 fc (1,076–4,951 lx) depending on glazing orientation, latitude, and season, a range wide enough that daylight harvesting controls must be tuned in lx, not fc, for a global control-system specification [S3].
Why two units exist and which one to use where
The foot-candle persists in US architectural practice, museum lighting, motion-picture cinematography, and US horticulture, the four fields where it dominates per the Wikipedia summary [S3]. The lux dominates everywhere else: European and Asian lighting standards, ISO 8995-1 indoor lighting guidance, international photometric test reports, and most industrial lighting controls [S1][S3]. When a US project ships equipment to a non-US site, or when a global OEM publishes a single datasheet, expect the spec to land in lux, even if the US parent company still works in fc internally [S3][S6].
Picking a lux meter for a field measurement sidesteps the unit question entirely, since most modern instruments display both fc and lx with a button toggle. For a written spec, always state the unit explicitly; an unlabled "50" could be either, and the 10.76× error between them will fail a lighting code check on either side of the Atlantic.
Quick-reference table for typical fc ↔ lx values
The table below distils the conversion for the magnitudes engineers actually use; it is the same ratio that drives the larger unit converter at Unit Converters, just compressed to the working range [S2].
Each row is just the previous value scaled by 10×, which mirrors the "1 fc ≈ 10 lx" rule of thumb and confirms the linearity of the conversion over five orders of magnitude [S2][S3]. The 0.09290304 ft²-to-m² constant is unchanged whether the illuminance is 0.1 fc or 100,000 fc, so the same 10.76391 multiplier applies from moonlight-class levels up to direct-sun levels [S2][S5].
Common mistakes when reading or writing a fc/lx spec
The most common error is treating 1 fc and 1 lx as interchangeable, which produces a 10.76× under-spec that fails code on the high side or over-specs the lighting load on the low side [S1][S6]. The second-most-common error is dropping the unit entirely from a spec line, "50 at desk" is meaningless until lx or fc is appended. The third is rounding 10.76391 to 10.764 and then to 10.8 in a chain, which compounds to a several-percent error by the time it reaches an energy calculation; for code submissions, keep at least four significant figures, so 10.76 or 10.764 [S2][S3].
A subtler trap is conflating foot-candles with lumens, since both share the "lm" lineage: foot-candles are lumens per square foot, while lumens are a total flux, and the conversion between them is area, not the 10.76391 factor [S7]. A 1,000-lumen lamp in a 10 ft² room gives 100 fc (1,076 lx) at the floor if perfectly diffused; the same 1,000-lumen lamp in a 100 ft² room gives 10 fc (108 lx) [S7]. The 10.76391 only ever converts between fc and lx, never between fc and lumens [S1][S5][S7].
When the 10× rule is enough and when it is not
For rough field estimates, code-compliance sanity checks, and verbal coordination with US trades, 1 fc ≈ 10 lx is acceptable and the Wikipedia article explicitly endorses this shortcut for ±10% accuracy work [S3]. For ISO 8995-1 compliance, EN 12464-1 indoor workplace submissions, lighting-energy calculations under ASHRAE 90.1 or IECC, and any datasheet a global supplier will read, use the full 10.76391 multiplier and at least four decimal places [S2][S5]. For daylight-factor calculations and glare-metric work (UGR, Daylight Glare Probability), the difference between 10× and 10.76391× propagates into perceptible error in the UGR score, so the full factor is mandatory [S3].
The same conversion principle that drives foot-candles to lux also drives the candela-to-lumen relationship, lumen-to-lux scaling, and the inverse-square falloff used in photometric files, so locking the 10.76391 constant in muscle memory pays dividends beyond just fc/lx work. Tools like the Rapid Tables and Unit Converters calculators handle the arithmetic; the engineer's job is to pick the right unit, apply the right precision, and flag the unit clearly on every drawing, schedule, and report [S2][S5].
Instrument-side considerations for field verification
A calibrated lux meter with a cosine-corrected diffuser head is the only practical way to verify either fc or lx in the field, and most handheld units from 2024–2026 production switch units via a single button rather than requiring manual math [S1]. When commissioning a US-spec building overseas, set the meter to lx and divide by 10.76391 to compare against the fc callout, rather than the other way around, because the lux reading has one fewer rounding step in the meter's internal ADC [S5][S6].
For lighting layouts that will be modelled in software, confirm the photometric .ies file lists units in the header; most post-2015 files are candelas per kilolumen with the 10.76391 factor already absorbed into the geometry, so the fc/lx decision shows up in the schedule, not the simulation [S1]. If a project is documented against pressure transmitter calibration or other instrumentation references and the lighting spec is being assembled in parallel, the same discipline of explicit-unit labelling applies; readers will not guess.
Track these for the next 1–2 quarters: (1) any IEC/ISO revision that re-anchors indoor illuminance levels (EN 12464-1 updates tend to move 500 lx office targets by ±10%), and (2) the AHRAE/IES 90.1 cycles that may shift US fc baselines closer to the lx values already in use elsewhere; both moves will keep the 10.76391 constant but tighten the tolerance band on what "1 fc ≈ 10 lx" is allowed to mean in code submissions [S3][S6].
Component reference pages worth checking: flow meter.
This topic is covered further in Stacker Crane Aisle Width and Rail Tolerance: Spec Reference for AS/RS Builds.