It is best to consult with the seamstress that will be sewing your dress to know how many yards to purchase. The yardage amount that you will need depends on the dress silhouette you have chosen. A sheath silhouette will only need 3-4 yards (if the gown won’t have a train, 3 yards could be sufficient, if there is a train,, you might need 4 + yards). An A-line gown could use 5-7 yards of fabric depending on how long the train will be and also the height and size of the bride. A ballroom gown could possibly need 8-10 yards of fabric. A trumpet or mermaid wedding gown could use 4-6 yards of fabric. A Tea length wedding gown could use around 3 yards of fabric.
These amounts of fabric are estimates. The exact amount needed depends on how tall the bride is, what size she is, if there is a train or not, how full the skirt will be.
Your seamstress will help you know how much fabric is needed.