Both are things that are commonly done. A convention is the way something is normally done in regard to a procedure. eg. The convention at a restaurant is that you eat your food, and then you pay. A custom is a common way of doing something in regard to a society or culture. eg. In the United States, the custom is to tip at least 15% of the total bill at a restaurant. Does that clear things up?

convention have two meanings ,first it usually used to describe conference of professonal of members or political party ,and second it usually used to describ the agreement about people should behave in certain circumstance. generally accepted and long-established way of behaving or doing things custom mean generally accepted and loong established way of behaving or doing things.

Convention is usually used as a way of describing a meeting of people. Custom is usually a habit practiced by people or a single person.