If you purchased a pattern for the whole quilt, it should contain information on fabric requirements. If it's only a block, there are some on-line sites where you can build a quilt by selecting a block and colors then you can purchase a pattern with instructions...that site is quiltivate.com...there are also many programs you can purchase to use on your home computer, or, if you enjoy math, you can calculate your own fabric requirements.
Which situation do you have?