sei vs sein, What's the Difference?

Which to use

“sei” is a num and “sein” is a verb - they look or sound alike but fill different roles in a sentence.

#48,072
“sei” frequency rank: #45,601
“sein” frequency rank: 93673
confusion score

Side-by-Side Comparison

Feature	sei	sein
Definition	Seis.	Ser, estar.

Where the spellings diverge

Shared letters are muted; the letters that actually set sei and sein apart are highlighted. They share 3 letters in sequence, which is exactly why the eye skips the difference.

3 ch

sei

4 ch

sein

Spelling & Dictionary Insight

These two trip readers on the page rather than in the ear: sei is /s̺ej/ while sein is [ˈzaɪ̯n]. Spoken aloud the problem disappears. In writing it persists, because they differ by 1 extra letter(s) - “sei” sits inside “sein”, and the parts of speech differ too (num vs verb), which is usually the fastest check. Our composite confusion score for this pair is 93673, derived from the frequency rank of both members and their visual similarity.

sei is recorded at frequency rank #48,072, classified as anum, pronounced /s̺ej/. sein is at rank #45,601, tagged as averb, pronounced [ˈzaɪ̯n].

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 93673, this pair ranks #1,048 of 323,831 scored Spanish confusable pairs - among the most confusable pairs.

Orthographic DNA of sei vs sein

Shared letters: eis. Private to "sei": -. Private to "sein": n.

"sei" · 3 letters · shape CVV · "sein" · 4 letters · shape CVVC

Known mistypes of this pair

sein ← esin · seinn · seni · ssein

Frequency comparison

sei#48,072

sein#45,601

Source: FrequencyWords open word-frequency list

Frequently Asked Questions

Can "sei" and "sein" be used interchangeably?

No - they are not even the same part of speech. "sei" is a num and "sein" a verb, so a sentence built for one is usually ungrammatical with the other.

Which is more common, "sei" or "sein"?

"sein" is the more common of the two: it sits at frequency rank #45,601 in our Spanish list, against #48,072 for "sei". The two are close in frequency, so neither is a safe default; check the meaning rather than guessing.

Remembering sei vs sein

The fastest way to pick the right one every time.

Check the role first: if you need a num, it's “sei”; for a verb, it's “sein”.
See each word in full, definition, IPA, etymology and its other confusables. Full “sei” entry
Browse more pairs most likely to be confused. Most confusable