It is best to consult the seamstress who will sew your gown to know how many yards of wedding dress fabric to purchase. The total yardage depends on the silhouette you want.
A gown with a sheath silhouette and train will need at least 4 yards of fabric. You can trim it to 3 yards if you do not want a train. An A-line gown typically requires 5-7 yards of fabric, but it will depend on the length of the train and your height and build.
Ballroom gowns need 8-10 yards, while trumpet or mermaid gowns could use 4-6 yards. Lastly, a tea-length wedding gown needs around 3 yards.
Please note that these amounts of wedding dress material are estimates. The exact yardage depends on your height and dress size, the fullness of your skirt, and the presence or absence of a train. Talk to your seamstress to know how much fabric you actually need.