How to say "additional" in Japanese

• 副ふくCommon
  noun, prefixFor 'additional' in the sense of 'more', 副 is seldom the word you want: it is really a prefix-like element meaning 'vice-', 'sub-', 'associate', or 'assistant', marking a secondary position or role.
• 番外ばんがいCommon
  no-adjectiveCovers 'additional' as something outside the regular series or lineup — an extra performance or item that is not counted among the main numbered ones; think 'not part of the regular set'.
• エクストラCommon
  prenominal modifierA loanword for 'extra': use it when something is additional in the sense of surplus or excess — more than what is needed or beyond the set amount.
  ツインの部屋へやにエキストラベッドを入いれてください。↗

  I'd like a twin with an extra bed.

For 'additional' meaning 'more' or 'extra', choose 「エクストラ」 for surplus or 番外 for something outside a numbered series; 副 is really a 'vice-/sub-/associate' prefix and is seldom the word for 'additional'.

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

• 副ふくassistant
• 別枠べつわくadditional
• アディショナルadditional
• 追加的ついかてきadditional
• 番外ばんがいextra (e.g. performance, item, object, etc.)
• エクストラextra