Fabric comes in many different widths, so unless you know beforehand exactly how wide the fabric you intend to buy is, you will need to do a little bit of figuring out in the store.
When you go to the store, take with you all the information you’ll need, like all the pattern piece measurements, the zip lengths, elastic lengths etc. Think about these things too, will you need lining and how much? Will you need iron-on interfacing to stiffen collars or use on facing? Do you need matching thread? Bring pens and paper too! If you have this information, I’m sure most fabric store assistants will be happy to help you figure out how much you will need to buy.