How to say "smooth" in Japanese

• スムーズCommon
  na-adjectiveCovers the figurative sense: a process, negotiation, or transition that goes ahead without problems or hitches. This loanword is the natural choice when English 'smooth' means trouble-free rather than physically even.
  課長かちょうが我々われわれに対たいして、あれこれとうるさく言いうのを止とめた途端とたんに、何なんもかもがスムーズに行いき始はじめた。↗

  As soon as our manager got off our backs, everything started to go off without a hitch.
• 滑らかなめらかJLPT N1approx.Common
  na-adjectiveThe word for smoothness of a surface or texture — glassy, velvety, soft to the touch, like polished wood or silk. Choose this for tactile smoothness rather than for smooth progress.
  海うみは穏おだやかに、そしてなめらかに見みえる。↗

  The sea looks calm and smooth.
• つるつるCommon
  na-adjective, no-adjective, adverb, suru verbA mimetic word with a vivid, sensory feel: smooth, shiny, and slick, often with a hint of slipperiness. Suits things like a glossy, slippery floor or a smoothly shaven head.
  私の父ちちはビリヤードの玉たまのように頭あたまがつるつるだ。↗

  My father is as bald as a billiard ball.
• 円滑えんかつJLPT N1approx.Common
  na-adjectiveA Sino-Japanese word for smoothness as the absence of friction: dealings, communication, or operations that run undisturbed and harmoniously. Typical of business and official writing.
  彼かれは、中国語ちゅうごくごをしっていたので、私わたしたちは計画けいかくを円滑えんかつに実行じっこうできた。↗

  His knowledge of Chinese enabled us to carry out our plan smoothly.
• 平らたいらJLPT N3approx.Common
  na-adjectiveChiefly means flat or level: an even surface rather than a smooth one. Its 'smooth' gloss is marginal — for smoothness itself, 滑らか or つるつる is normally the word you want.
  あの山やまの頂上ちょうじょうは平たいらだ。↗

  The top of that mountain is flat.

Use 「スムーズ」 or 円滑 for smoothness in the sense of trouble-free progress, 「滑らか」 or 「つるつる」 for the smoothness of physical surfaces, and reserve 「平ら」 for flatness rather than smoothness.

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

• スムーズsmooth (i.e. without problems)
• 滑らかなめらかsmooth (of a surface)
• つるつるsmooth
• 円滑えんかつsmooth
• 平らたいらflat
• 円満えんまんharmonious
• 快調かいちょうgood (condition)
• かっこいいattractive
• すべすべsmooth (skin, stone, etc.)
• さらりsleek
• きめ細かいきめこまかいsmooth
• きめ細かきめこまかsmooth
• 円滑にするえんかつにするto smooth
• とりなすto mediate
• 円転滑脱えんてんかつだつsuave
• トントン拍子とんとんびょうしeasy
• ずんべら棒ずんべらぼうplain (e.g. face)
• ずべら坊ずべらぼうplain (e.g. face)
• よどみないfluent
• 平坦へいたんeven
• のっぺりfeatureless (face)
• 坦々たんたんlevel
• かっちょいいattractive
• 丸いまるいcurved
• 気持ちいいきもちいいsmooth (of function)
• 気持ちがいいきもちがいいsmooth (of function)
• なだらかsmooth (process)
• まろやかmellow (flavour, voice, etc.)
• ちゅるちゅるsmooth