How to say "diameter" in Japanese

• 直径ちょっけいJLPT N2approx.Common
  nounThe usual word for the diameter of a circle, sphere, or round object; whatever the context, this is the choice to reach for first when you mean 'diameter'.
  その穴あなは直径ちょっけい二にメートルです。↗

  The hole is two meters across.
• 径けい
  nounCovers the same geometric sense as 直径 but is less common as a standalone word; unless you have a particular reason, 直径 is the safer choice in most contexts.
• ダイアミター
  nounA katakana transliteration of the English word itself. It is less common and seldom the word you want for 'diameter'; say 直径 instead.
• 差し渡しさしわたし
  nounStresses the measured distance across something — how far it is from one side to the other — rather than the geometric property; less common, and suited to describing an actual object's span.

Choose 直径 for the diameter of a circle or round object in nearly any context; 径 is a less common standalone equivalent, 「ダイアミター」 a katakana borrowing you will seldom need, and 「差し渡し」 stresses the measured distance across an object.

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

• 直径ちょっけいdiameter
• 径けいdiameter
• ダイアミターdiameter
• 差し渡しさしわたしdistance across