How much extra fabric for a one-way print or nap?

Velvet, corduroy, panne and any print with an obvious up and down force every pattern piece to point the same way. The standard advice is to add 15 to 25 percent. That range is a guess, and openly so. The real cost of a one-way layout depends on the shapes of your pieces, and it runs from zero to well past 25 percent.

What makes a fabric one-way

Two mechanisms. Nap: velvet, panne, corduroy and other pile fabrics catch light differently along and against the pile, so two pieces cut in opposite directions sew up looking like two different colours. Print direction: any motif with a visible up and down turns upside-down when a piece is rotated. Either way the layout rule is the same: all pieces point one direction, and the head-to-tail flipping that tight layouts rely on is forbidden. Pattern envelopes call this a "with nap" layout, and many print a separate, larger yardage for it.

The numbers you will see quoted

These are buying allowances, meant to be applied in the shop before anyone has seen a layout. As insurance they are reasonable. As a statement about your pattern they are noise, and the next section is why.

Why the real penalty is not a percentage

A normal layout saves fabric mainly by flipping alternate pieces head to tail so that their shapes interlock. A one-way rule forbids exactly that. So the extra fabric you need equals whatever the flipping was worth for your particular shapes, and that varies wildly.

Rectangles lose nothing. A cuff, a waistband or a straight skirt panel looks identical rotated 180 degrees, so its one-way layout is the same layout, and the penalty is zero. Wedges lose a lot. Flared skirt panels, A-line pieces and circle-skirt quarter panels interlock only head to tail, wide end against narrow end. In our circle skirt guide, the rotatable quarter-panel layout takes about 2.4 m and the one-way version about 3 m: a 25 percent jump. A pencil skirt from the same bolt can be a zero percent jump. One rule of thumb, opposite outcomes, which is why the quoted percentages never agree: they average garments that pay nothing with garments that pay heavily.

Plaids and big repeats are a second, separate tax

Direction is one constraint. Matching is another. If a plaid has to run continuously across a side seam, the pieces are pinned to specific positions on the repeat, not just to a direction, and the waste grows with the repeat height. That is why plaid estimates run from at least 25 percent to a published 50, and why no percentage rule can be right about your specific repeat. Worth saying plainly: a nesting tool computes the direction penalty, not repeat placement. Matching plaids across seams stays handwork at the cutting table.

The safe fallback, and the exact answer

If your envelope prints a with-nap yardage, buy it. It is the pattern company's own worst-case figure for direction, and it exists precisely because the percentage rules are unreliable. If you want the actual number for your pieces on your width, run the layout both ways and compare. The difference is not an estimate; it is your answer.