必要 and 必要性 usages


What's the difference between 必要 and 必要性 and in what situation should I use
which




First of all, 必要 is primarily a **na-adjective** rather than a noun. It means
"necessary".

  * ✅必要な量 ❌必要性な量  
necessary amount

  * ✅それは必要だ。 ❌それは必要性だ。  
That is necessary.

  * ✅必要になった。 ❌必要性になった。  
It became necessary.

If you have _really_ used 必要 only as a pure noun, this is the most important
difference you have to remember. (And I think this is why your question looked
confusing and got downvotes. Your question almost looks to me like "What's the
difference between _necessary_ and _necessity_?", which is too elementary.)

* * *

That being said, there _are_ some cases where 必要 is used as a noun. In some
cases, 必要 and 必要性 are interchangeable, but in some cases, they are not:

  * ✅読む必要がある ✅読む必要性がある 
  * ✅読む必要がない ✅読む必要性がない 
  * ✅必要に応じて読む ❓必要性に応じて読む 
  * ✅読む必要を感じる ✅読む必要性を感じる 
  * ✅必要に駆られる ✅必要性に駆られる 
  * ❌必要を検討する ✅必要性を検討する
  * ❌読む必要が高い ✅読む必要性が高い
  * ❌読む必要が乏しい ✅読む必要性が乏しい
  * ❌ワクチンの必要について ✅ワクチンの必要性について

It's hard to generalize and I may be wrong, but I feel:

  * When the focus is the **degree** of necessity (高い/低い/etc), 必要性 must be used, whereas 必要 and 必要性 are usually interchangeable when the focus is the **existence** of necessity (ある/ない/感じる/etc).
  * 必要 follows a verb, but it tends not to be linked to a noun using の.
  * 必要性 sounds a little more formal or academic.




First of all, 必要 is primarily a **na-adjective** rather than a noun. It means
"necessary".

  * ✅必要な量 ❌必要性な量  
necessary amount

  * ✅それは必要だ。 ❌それは必要性だ。  
That is necessary.

  * ✅必要になった。 ❌必要性になった。  
It became necessary.

If you have _really_ used 必要 only as a pure noun, this is the most important
difference you have to remember. (And I think this is why your question looked
confusing and got downvotes. Your question almost looks to me like "What's the
difference between _necessary_ and _necessity_?", which is too elementary.)

* * *

That being said, there _are_ some cases where 必要 is used as a noun. In some
cases, 必要 and 必要性 are interchangeable, but in some cases, they are not:

  * ✅読む必要がある ✅読む必要性がある 
  * ✅読む必要がない ✅読む必要性がない 
  * ✅必要に応じて読む ❓必要性に応じて読む 
  * ✅読む必要を感じる ✅読む必要性を感じる 
  * ✅必要に駆られる ✅必要性に駆られる 
  * ❌必要を検討する ✅必要性を検討する
  * ❌読む必要が高い ✅読む必要性が高い
  * ❌読む必要が乏しい ✅読む必要性が乏しい
  * ❌ワクチンの必要について ✅ワクチンの必要性について

It's hard to generalize and I may be wrong, but I feel:

  * When the focus is the **degree** of necessity (高い/低い/etc), 必要性 must be used, whereas 必要 and 必要性 are usually interchangeable when the focus is the **existence** of necessity (ある/ない/感じる/etc).
  * 必要 follows a verb, but it tends not to be linked to a noun using の.
  * 必要性 sounds a little more formal or academic.

