Mandatory and compulsory are close in meaning, but they are not always used in exactly the same way.
Mandatory usually points to something required by a rule, law, policy, or authority. It often appears in legal, workplace, school, and government contexts. If an action is mandatory, someone in charge has formally said it must happen.
Example: Wearing a seat belt is mandatory in many places.
Compulsory also means required, but it often emphasizes that there is no choice, either because of a rule or because the situation makes it necessary. In some varieties of English, it is especially common in education and formal writing.
Example: Attendance is compulsory for first year students.
- Use mandatory when you want to stress an official rule or policy.
- Use compulsory when you want to stress obligation or lack of choice.
- In many contexts, both words are correct, but the emphasis can differ slightly.
A simple way to choose is this: if you are thinking about a formal requirement, mandatory is often the more natural choice. If you are stressing that something must be done, compulsory often fits well.