lum vs lux, What's the Difference?

Which to use

“lum” and “lux” are a confusable English pair: similar on the page, but distinct in meaning, check the gloss before you choose.

#47,736
“lum” frequency rank: #18,878
“lux” frequency rank: 66614
confusion score

Side-by-Side Comparison

Feature	lum	lux
Definition	A chimney; also, the top part of a chimney.	In the International System of Units, the derived unit of illuminance or illumination; one lumen per square metre. Symbol: lx

Where the eye mixes lum and lux

Muted = shared; highlighted = the visual trap. They share 2 letters in sequence.

3 ch

lum

3 ch

lux

Spelling & Dictionary Insight

A purely visual mix-up. lum (/lʌm/) and lux (/lʌks/) are said differently, so reading them aloud is the quickest way to catch the wrong one. On the page they stay close: they differ by a single letter - m in “lum” becomes x in “lux”. Our composite confusion score for this pair is 66614, derived from the frequency rank of both members and their visual similarity.

lum is recorded at frequency rank #47,736, classified as anoun, pronounced /lʌm/. lux is at rank #18,878, tagged as anoun, pronounced /lʌks/.

Glosses for this pair are partially populated in our dataset, but the full side-by-side definitions above should still guide you to the right choice.

With a confusion score of 66614, this pair ranks #67,180 of 530,003 scored English confusable pairs - among the most confusable pairs.

Orthographic DNA of lum vs lux

Shared letters: lu. Private to "lum": m. Private to "lux": x.

"lum" · 3 letters · shape CVC · "lux" · 3 letters · shape CVC

Frequently Asked Questions

Can "lum" and "lux" be used interchangeably?

No. They are said differently (/lʌm/ versus /lʌks/) and carry different meanings; reading the sentence aloud is usually enough to catch the wrong one.

Which is more common, "lum" or "lux"?

"lux" is the more common of the two: it sits at frequency rank #18,878 in our English list, against #47,736 for "lum". The two are close in frequency, so neither is a safe default; check the meaning rather than guessing.

What to do next

You've compared "lum" and "lux". Here's where to go from here.

Full dictionary entry for "lum" with definition, IPA, and generated misspellings. Word entry
Full dictionary entry for "lux" - compare definitions side by side with the headword page. Word entry
This pair ranks #67,180 of 530,003 scored English confusable pairs in this corpus vintage. Corpus statistics
"lum" and "lux" also get mixed up with nearby pairs, the highest-scored neighbour is "LSC vs lum". Nearby pair

PlainSpell is a reference corpus, not spelling advice. Verify unusual forms in context.