Quick answer
A sheath dress needs roughly 2–2.5 yards on 44-inch fabric for knee length, or 1.5–1.75 yards on 58-inch. Midi adds about half a yard. Long sleeves add another half.
This is the cheapest dress to buy fabric for and the most expensive to get wrong. Every fitting error shows.
Why it uses so little
A sheath follows the body from shoulder to hem with almost no added fullness — the fullness factor is close to 1.0. The pieces are narrow and nest tightly on the bolt, so very little is wasted.
As always, this is an estimate. Confirm the final yardage with your dressmaker once they have your measurements.
The trade-off
All of that saved fabric is spent on labour instead. A close-fitting dress needs accurate measurements, well-placed darts or princess seams, and usually a toile — a test garment in cheap fabric — before the real cloth is cut. Ask your dressmaker to make one. It costs an extra yard of calico and it is the difference between a sheath that looks bought and one that looks homemade.
Fabric choices that behave
Crepe, ponte and stable wovens with a little stretch are ideal. Avoid anything very drapey and avoid anything very stiff — the first clings to every contour, the second wrinkles across the lap when you sit. If in doubt, hold a length against yourself in the shop and sit down in it.
Add a vent
Anything below the knee in a sheath needs a back vent or a side slit or you will not be able to walk properly. Say this before cutting; adding it afterwards is fiddly.