Quick answer
A floor-length ball gown needs roughly 7–9 yards on 44-inch fabric, or 5–6.5 yards on 58-inch. Add 2–4 more yards if you want a petticoat or an underskirt, and 15% on top of everything for velvet or any directional print.
This is the silhouette where fabric cost dominates the project. At $30 a yard you are looking at $200–270 in shell fabric alone before lining, boning or notions.
Why so much
A ball gown's skirt is gathered or pleated to the waist at a ratio of roughly two to three times the waist measurement, then falls to the floor. That means each panel is nearly full-length and there are a lot of panels. Nothing nests efficiently. The fullness factor is around 2.3 against about 1.0 for a sheath.
These are estimates. A gown is the silhouette where layout matters most, so ask your maker to confirm the quantity from the actual pattern before you order — the difference between a good and a poor layout on a full skirt can be a yard or more.
Where you can economise, and where you cannot
- Do not skimp on the shell. Running out is catastrophic on a gown — dye lots will not match across that much visible surface.
- Do economise on the underskirt. Nobody sees it. Cheap netting or a bought petticoat works perfectly.
- Consider a wider bolt. Moving from 44 to 58-inch fabric saves 2–3 yards, which on an expensive cloth pays for itself immediately.
- Buy the whole lot at once, from one bolt, in one order.
The other costs a gown carries
Boning for a strapless bodice, an internal waist stay, horsehair braid or a petticoat to hold the hem shape, and a long invisible zip. Budget $40–80 in notions and expect labour at the top of the range — a structured gown is the most demanding thing on this list to make well.