暗算

1. Origins, structure, and basic meaning

• Structure: 暗算, read あんざん, means calculating in one's head without writing out the steps or formulas.
• Basic usage: It is used for adding up a total at the register or doing complicated division mentally.
  • レジに並びながら合計金額を暗算した。

2. Common pitfalls and similar expressions

• Method and precision compared with 概算
  • 暗算 identifies the method of calculating mentally, while 概算 identifies an approximate rather than exact calculation.