How to say "efficiency" in Japanese

• 効率こうりつJLPT N1approx.Common
  nounCovers 'efficiency' in its core sense — the ratio of useful output to input, or getting results with minimal waste of time, effort, or energy. This is usually the word you want.
  彼かれは事務じむ効率こうりつをよくする案あんを出だした。↗

  He put forward a plan for improving office efficiency.
• 性能せいのうJLPT N2approx.Common
  nounChiefly the performance or capability of a machine, device, or system; its 'efficiency' sense is how efficiently equipment runs, not a person's working efficiency.
  この車くるまはあの車くるまより性能せいのうがよい。↗

  This car has a better performance than that one.
• 実効じっこうCommon
  nounIts core senses are practical effect and efficacy — how effective something actually is. Read as 'efficiency', it means actual effectiveness, not the ratio of output to input.
• 能率のうりつJLPT N2approx.Common
  nounLargely interchangeable with 効率, with a lean toward the productivity of work — how much gets done per unit of time or effort.
  そうすれば能率のうりつが上あがりますよ。↗

  That makes for efficiency.
• 効こうCommon
  nounReally means effect, efficacy, or benefit — the good something produces. As a translation of 'efficiency' it is marginal: seldom the word you want when you mean output relative to input.

効率 is the default for efficiency as output versus input; 能率 is a near-synonym for work productivity; 性能 covers machine performance; 実効 stresses practical effectiveness; 効 is really about effect or benefit, seldom efficiency.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 効率こうりつefficiency
• 性能せいのうability
• 実効じっこうpractical effect
• 能率のうりつefficiency
• 効こうefficacy
• 実力じつりょく(real) ability
• 効率性こうりつせいefficiency
• エフィシェンシーefficiency