adverb, noun which may take the genitive case particle 'no'
that much; to that extent; only that; that is all
Expresses degree, quantity, or limitation. Can mean 'to that extent' or 'only that much/no more than that'. Also used at the end of a statement to mean 'that's all I have to say'.
That's all you need to do, so don't worry.
That much is enough.
彼の成功はそれだけの努力の結果だ。
His success is the result of that much effort.
以上です。それだけです。
That's all. That's all I have to say.
He has ability to match that.
That alone isn't enough.
He's a hard worker; precisely because of that, this failure must be especially frustrating for him.
だけ is a bare particle meaning 'only/just' attached to any noun, while それだけ is the fixed phrase それ＋だけ meaning specifically 'only that much'.
Kanji form using 其れ (rare) and 丈 (ateji-like); rarely used in modern writing.
Mixed kanji-kana form; very rare and mostly historical.
Compound of それ (that) and だけ (only, extent). The derivation is transparent.