noun
mental calculation; calculating in one's head
Refers to making a rough estimate or plan in one's mind without writing anything down. Often implies a preliminary or private reckoning.
彼は胸積もりで予算を立てた。
He drew up a budget by mental calculation.
胸積もりではうまくいきそうだったが、実際は違った。
In my head it seemed like it would work, but in reality it was different.
Compound of 胸 (muna, 'chest, heart, mind') and 積もり (tsumori, 'intention, plan, estimate'), with rendaku voicing. The exact historical derivation is uncertain, but the word literally suggests 'piling up in one's chest', i.e., building a plan in one's mind.