Translation guide
The English phrase "what for" is used to ask about purpose or reason, often in a direct or informal way. In Japanese, there is no single direct equivalent; the best choice depends on formality, context, and whether you are asking about purpose, reason, or expressing surprise/disapproval.
To ask why someone is doing something or what the purpose of something is, in a casual or direct manner.
Literally 'for what purpose'. This is the most direct equivalent and can be used in both casual and slightly formal contexts. It is neutral and widely understood.
何のためにそんなことをしたの？
What did you do that for?
A very common casual way to ask 'why' or 'what for'. It can be used in many situations, but it's informal. Be careful: it can also mean 'by what means' (e.g., transportation), but context usually makes it clear.
何でそんなこと聞くの？
What are you asking that for?
Another common way to ask 'why' or 'what for'. Slightly more polite than 何で, but still casual. It can also express surprise or disbelief.
どうしてそんなことするの？
What are you doing that for?
To ask politely or formally about the purpose or reason for something.
This phrase works in formal contexts as well. You can add ですか to make it polite.
何のためにこの資料が必要ですか。
What do you need this document for?
A very formal way to ask 'for what purpose'. Used in business or official settings.
どのような目的でご利用になりますか。
What will you be using it for?
To express shock, disapproval, or to question the necessity of an action, similar to 'Why on earth...?' or 'What's the point?'.
When said with a certain intonation, this can convey 'What's the point?' or 'Why bother?'.
何のためにこんなことしてるんだろう。
What am I even doing this for?
Expresses surprise or bewilderment: 'Why on earth...?' or 'What for, again?'.
何でまたそんなことするの？
What on earth are you doing that for?
To ask what an object is used for.
Literally 'What do you use this for?'. Natural and common.
このボタンは何に使うの？
What's this button for?
Literally 'This is a thing for what?'. Slightly more explicit about purpose.
この道具は何のためのものですか。
What is this tool for?
Do not translate 'what for' word-for-word as 何のために in every case. In casual questions, 何で or どうして are often more natural. Also, 何のために can sound overly literal or stiff in some informal contexts.
何で (nande) is most casual; どうして (doushite) is slightly more polite and can express surprise; なぜ (naze) is formal/literary and less common in daily speech. For 'what for', 何のために is more purpose-focused than these general 'why' words.