expression
on second thought; come to think of it; actually
Used when reconsidering or realizing something after a moment of thought, often introducing a changed opinion or a new insight.
よく考えたら、彼の言う通りかもしれない。
Come to think of it, he might be right.
よく考えたら、今日は休みだった。
On second thought, today was a day off.
Similar meaning, but よく考えたら emphasizes a more deliberate reconsideration, while 考えてみれば is a lighter 'if you think about it'.
Phrase composed of よく (well, carefully) + 考えたら (conditional form of 考える, 'to think'). Literally 'if one thinks carefully'.