illvsit'llWhat's the difference?

Quick tell: ill is a adjective, it'll is a contraction, so they fill different roles in a sentence.

Which to use

“ill” is an adjective and “it'll” is a contraction - they look or sound alike but fill different roles in a sentence.

#1,946
“ill” frequency rank
#2,584
“it'll” frequency rank
4530
confusion score

Side-by-Side Comparison

Feature ill it'll
Definition Evil; wicked (of people). Contraction of it + shall.

Where the eye mixes ill and it'll

Muted = shared; highlighted = the visual trap. They share 3 letters in sequence.

3 ch
ill
5 ch
it'll

Spelling & Dictionary Insight

These two trip readers on the page rather than in the ear: ill is /ɪl/ while it'll is /ˈɪtl̩/. Spoken aloud the problem disappears. In writing it persists, because they differ by 2 letter(s) in length, and the parts of speech differ too (adjective vs contraction), which is usually the fastest check. Our composite confusion score for this pair is 4530, derived from the frequency rank of both members and their visual similarity.

ill is recorded at frequency rank #1,946, classified as anadj, pronounced /ɪl/. it'll is at rank #2,584, tagged as acontraction, pronounced /ˈɪtl̩/.

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 4530, this pair ranks #518,620 of 530,003 scored English confusable pairs - a relatively easy-to-tell-apart pair.

Orthographic DNA of ill vs it'll

Shared letters: il. Private to "ill": -. Private to "it'll": t.

"ill" · 3 letters · shape VCC  ·  "it'll" · 4 letters · shape VCCC

Frequently Asked Questions

Can "ill" and "it'll" be used interchangeably?
No - they are not even the same part of speech. "ill" is an adjective and "it'll" a contraction, so a sentence built for one is usually ungrammatical with the other.
Which is more common, "ill" or "it'll"?
"ill" is the more common of the two: it sits at frequency rank #1,946 in our English list, against #2,584 for "it'll". The two are close in frequency, so neither is a safe default; check the meaning rather than guessing.

Source: Wiktionary (via kaikki.org) Structured Wiktionary extract

Source: FrequencyWords open word-frequency list