How loosely is a topcoat supposed to fit?

[josepidal]

I saw a topcoat I'd like to buy online, but I've never had any sort of overcoat before. The vendor measured the coat's shoulder and armpit width, and these correspond to the measurements of my favorite jacket, something made of polyester I put on against rain and wind.

Would this be a good fit, or is a topcoat supposed to be even looser in the sense that it should be able to fit loosely over a jacket? I know a topcoat is supposed to fit nicely over a suit jacket, but I'm talking about a nylon/polyester type jacket for comparison where I've worn a denim jacket, a shirt, and an undershirt underneath.

Is shoulder width a good basis for a topcoat's measurements?

 

[Kai]

A topcoat should fit loosely, so that it is comfortable when worn over a suit jacket.

 

[Holdfast]

A topcoat or overcoat is one of the things I would just never buy online. The fit is so variable I just need to try it on to be sure it's right for me.

 

[LabelKing]

Ideally, it should fit just so over the suit. The best fits replicates the fit of the suit underneath with waist suppression, etc.

 

[kitonbrioni]

Try on a suit jacket and then try on an overcoat in that size. It should fit. If is doesn't then go up and down a size as necessary.