Quick answer
For an A-line dress on 44-inch fabric: roughly 2.5 yards for knee length, 3.25 for midi, 4 for floor length. On 58–60-inch fabric, take about a third off: 1.75, 2.25 and 2.75 respectively. Add half a yard for long sleeves.
These are for a solid fabric with no nap. Add 15% for stripes or plaids you want matched, and 15% for velvet or any one-way print, because every piece has to be cut in the same direction.
How the number is actually reached
Yardage is length multiplied by a fullness factor, divided by how many pieces fit across the bolt. An A-line skirt has a fullness factor around 1.3 — moderately flared, so the hem is wider than the hip but the pieces still nest reasonably. The calculator above works from your nape-to-waist plus your desired hem length rather than a standard height, which is why its answer will differ from a pattern envelope.
Why pattern envelopes over- and under-estimate
The printed figure assumes a standard height, a standard size, and one specific cutting layout. If you are 5'2" it is generous. If you are 5'11" or need a wider hip, it can leave you short — and running out mid-project usually means a different dye lot, which is visible.
Treat every number here as a starting point
Yardage depends on the cutting layout, the specific pattern pieces and the wearer's measurements. The figures on this page and in the calculator are estimates to help you budget and start the conversation — your dressmaker determines the final quantity. Send them the spec sheet and confirm before you order.
Buy this much extra
Add a quarter yard as insurance on anything simple, half a yard on anything you have not sewn before. Leftover fabric is worth having: it lets you make a matching belt, test your press settings, and repair the dress years later in the exact same cloth.
Don't forget lining
A fully lined A-line needs roughly 85% of the shell yardage in lining. A bodice-only lining needs about a yard. Lining is usually 45 or 58 inches wide and much cheaper than the fashion fabric, so this is not the place to economise.