The most common widths for fabric bolts are 45, 54, and 60 inches. Other common widths are 36, 38, 42, 44/45, 48, 54, 58, 59, 60, 72, and 108 inches. A label is often placed at the end of a flat fabric bolt to show the width.
How much fabric you need for a project usually depends on how wide the fabric is, which is usually written on commercial patterns. In general, less fabric is needed the wider the fabric is. Make sure you know how much fabric you need before a store clerk cuts it. Some patterns call for one long piece instead of several shorter ones.