Why the width matters more than the yardage
Fabric is sold by length off a bolt of fixed width, so a yard is not a fixed quantity of cloth. A yard of 44/45 inch quilting cotton is 1,620 square inches. A yard of 58/60 inch knit is 2,160. A yard of 108 inch backing goods is 3,888. Those are not small differences, and they are why a pattern envelope lists separate yardage for each width rather than one number.
What decides your yardage is not area, though. It is how many pieces fit side by side across the usable width, because the fabric only gets consumed in whole rows. Two 20 inch pieces need 40 inches of width. On 45 inch goods with an inch trimmed off the selvages that fits with 4 inches to spare, and both pieces come out of one 28 inch row. On 38 inch goods it does not fit, you get one piece per row, and the same job takes twice the length. The step between one piece across and two is a cliff, not a slope, and this is the single most common reason a yardage estimate comes out wrong.
Nap, one-way prints and why rotating is not always allowed
Rotating a piece ninety degrees is often the cheapest thing you can do to a layout, and often the thing you are not permitted to do. Three situations forbid it:
- Pile fabrics. Velvet, corduroy, velour and most fleece have a direction the fibres lie in. Cut two pieces facing opposite ways and they read as two different colors under the same light. This is nap in the strict sense.
- One-way prints. Anything with a top and a bottom. Trees, text, animals, a border. Rotating gives you a piece with the print running sideways.
- Directional weaves and knits. Twill runs a diagonal that reverses when turned. Knits stretch mostly across the width and barely along the length, so turning a piece changes how the finished thing behaves, not just how it looks.
Set the layout direction to with nap in any of those cases and take the extra yardage. The rule of thumb people repeat is that nap adds a quarter to a half yard on a garment, but that is an average of many patterns rather than a fact about yours. Run it both ways here and read the actual difference for your pieces.
Pattern repeat, and the rounding that eats the difference
A repeat is the distance down the fabric before the design starts over. It is printed on the bolt end for decorator goods and is usually measured for you on the label; on quilting cotton you often have to measure it yourself with a tape and two matching motifs.
When you need pieces to match each other, every row of cutting has to start at the same point in the repeat, so the length consumed by a row rounds up to a whole number of repeats. A 26 inch piece from a fabric with a 24 inch repeat consumes 48 inches. A 25 inch repeat on a 26 inch piece consumes 50. The penalty is worst when the piece length is just past a multiple, which is a good argument for measuring the repeat before you finalise a cut length rather than after.
This calculator applies the rounding to every row. If only some of your pieces need to match, calculate those separately from the ones that do not and add the two results.
Shrinkage and prewashing
Woven cotton commonly loses a few percent on the first hot wash and dry, and it usually loses more in the length than the width because of how it was tensioned during finishing. Linen can lose more. Wool felts rather than shrinks in a predictable way. Polyester barely moves. Rayon can surprise you badly.
Prewashing before cutting is how you find out on the whole yardage rather than on a finished garment. The consequence for buying is that you must buy the pre-shrink length, not the post-shrink length. The default here treats the allowance as a percentage lost off both dimensions: the usable width shrinks, which can change how many pieces fit across, and the purchase length is grossed up so that what survives the wash is what you needed. If you are not washing the finished item ever, set it to zero and buy the bare figure.
Where this estimate is honest and where it is not
The model is a rectangle-in-rows layout: every piece treated as its bounding rectangle, laid in tidy lanes. Real garment pieces are shaped, and a skilled cutter nests a sleeve into the curve of a bodice and beats this figure. Real quilt cutting is closer to the model because the pieces genuinely are rectangles. So treat the result as an upper bound for garment work and as a fair estimate for straight-cut work.
What the model does not know about: grainline requirements that pin a piece to one orientation regardless of nap, pieces cut on the fold that need double the width, matching across a seam which costs more than matching down a length, and directional shading in a solid that only shows up after it is sewn. If the fabric is expensive, buy for the calculated figure and then buy a quarter yard more. For rectangular cutting from sheet goods the same nesting question comes up in wood, and the cut list optimizer solves it more thoroughly than a row layout can. For window panels specifically, the curtain size calculator works out the finished sizes first, and those become the pieces you bring here.
Questions people ask
How many inches are in a yard of fabric?
36 inches of length, at whatever the bolt width happens to be. A half yard is 18 inches, a quarter yard is 9 inches. A fat quarter is a different thing: it is a quarter yard cut as an 18 by 22 inch rectangle rather than a 9 by 44 strip, which is the same area arranged more usefully for small pieces. If a shop cuts to the quarter yard, a piece that needs 39 inches of length forces you to buy 45 inches, so the eighths shown here are the mathematical answer and the shop counter will round it up again.
Does buying 60 inch fabric always cost less than 45 inch?
No, and the calculator will show you when it does not. Wider goods only help if the extra width lets another piece fit across. If your pieces are 20 inches wide, two fit across both 45 and 60 inch fabric and the wider bolt buys you nothing but scrap, at a higher price per yard. If your pieces are 28 inches wide, one fits across 45 and two fit across 60, and the wider bolt roughly halves the length you buy. Run both widths through and compare the yardage, then multiply by the actual prices.
How much extra should I buy for a pattern repeat?
Enough that every cutting row can start at the same point in the design, which is what this calculator computes rather than guesses. The old trade rule for drapery is to add one repeat per panel beyond the first, which is close to what the rounding here produces when the pieces are long. For short pieces the rounding is proportionally much worse, because a 30 inch repeat rounds an 8 inch piece up to 30 inches of cloth. If you are cutting small pieces from a large-repeat fabric, consider whether the pieces really do have to match at all.
Should I prewash before cutting?
If the finished item will ever be washed, yes, and this is the reason the shrinkage allowance exists. Washing after cutting shrinks the pieces unevenly relative to each other and the seams pull. Washing after sewing shrinks the fabric against the thread, which does not shrink, and puckers every seam. The exception is anything that will only ever be dry cleaned or never cleaned at all, such as fixed upholstery or wall panels, where prewashing removes the finish and can change the hand of the cloth for the worse. There is no universally right answer here; it depends on the fibre and how the finished thing will be used.
Why is my layout waste over 40 percent?
Usually because the piece width and the fabric width are close to but not quite a fit — a 23 inch piece on 45 inch goods wastes the same as a 44 inch piece, because in both cases only one fits across. Try the piece rotated if nap allows, try the next fabric width up and down, and check whether a small dimension change puts two pieces across. Some waste is unavoidable: rectangles do not tile a fixed width neatly unless the arithmetic happens to work out, and the offcuts are still fabric you can use for something smaller.