1Excessive, extra, or more than necessary — describes something superfluous or too much.
余計な心配をしても仕方がない。
There's no point in worrying unnecessarily.
そんな余計な一言で雰囲気を壊してしまった。
That unnecessary remark ruined the mood.
旅行には余計な荷物を持って行かない方がいい。
It's better not to bring extra luggage on a trip.
2Unnecessary or uncalled-for — describes meddling or an action that was unwanted and unasked-for.
余計なお世話だ。
Mind your own business.
彼に余計な口出しをしないでください。
Please don't interfere with him unnecessarily.
そんなことを言うのは余計だよ。
It's uncalled-for to say something like that.
3As an adverb, meaning "all the more" or "even more" — used to intensify a comparison or a resulting state.
風邪薬を飲んだら、余計に眠くなった。
After taking the cold medicine, I felt even sleepier.
彼が遅刻したので、余計に会議が長引いた。
Because he was late, the meeting ran even longer.
期待が大きかった分、失望も余計に大きかった。
Because expectations were high, the disappointment was all the greater.