Finished, pattern and cut are three different sizes
Sewing has three sizes for the same piece and confusing any two of them produces the classic result: something that fits the pattern and not the person, or fits the person and not the space it has to go in.
- Finished size is what is left when the thing is sewn. The dimension you measured off the window, the chair, the bed, the table.
- Pattern size is the finished size plus ease — the extra room the item needs beyond the object it covers, which can also be negative when the fabric is meant to stretch onto it.
- Cut size is the pattern size plus every allowance the construction consumes: seam allowances on the seamed edges, twice the allowance for each internal join, hem allowance at the bottom, and a shrinkage allowance if you are cutting unwashed goods.
The word that causes the most trouble is ease, because it means two things that get used interchangeably. Wearing ease is the room needed for the body to move and breathe. Design ease is the extra beyond that which makes something loose or oversized on purpose. On a cushion or a cover the equivalent is fitting ease, and it is often negative: a cover cut slightly smaller than the foam is what stops it wrinkling.
Which seam allowance, and why they differ
| Allowance | Where it is normal | Why |
|---|---|---|
| 1/4" | Quilt piecing, small bags, doll clothes | Small pieces, many seams; a wider allowance would swamp the block |
| 3/8" | Knits, serged seams | Matches the common serger cutting width |
| 1/2" | Home dec, bags, upholstery | Heavier fabric, seams under load, room for welt cord |
| 5/8" | Commercial garment patterns | Leaves room to fit, trim, clip curves and grade the layers |
| 1 cm | Metric and many independent patterns | Slightly under 3/8"; do not treat it as interchangeable with 3/8 on matched seams |
None of these is correct in the abstract. What matters is that you use one allowance consistently on any seam that has to match another, and that your machine actually sews the allowance you think it does. A seam guide off by a sixteenth is invisible on one seam and a half inch error across eight of them.
Counting the seams, not the edges
The commonest arithmetic mistake is adding one seam allowance where two get consumed. An edge that is sewn to something else contributes one allowance to this piece — the other allowance belongs to the other piece. But an internal seam inside the same finished panel, where you have pieced two parts together to make one wider piece, consumes two allowances out of your total, one from each part.
So: a 40 inch panel made from two pieces at a half inch allowance needs the two parts to add to 41 inches before sewing, not 40.5. And an edge on a fold, or a selvage edge you are using as a finished edge, contributes nothing — drop the count for that edge to zero. The fields above are counts rather than checkboxes precisely so you can say two, one or none for each direction.
Grading a size run
Grading is scaling a pattern from one size to the next. Real garment grading is not a uniform scale: the increment is distributed around the pattern at specific points, more at the bust and hip than at the shoulder, and the armhole and neckline change by their own rules. That is a skill, and a table of numbers will not replace it.
What a linear grade step does handle correctly is the class of pieces that really are rectangles: cushion covers for a set of chairs that differ slightly, a run of bags in three sizes, table linens for a 60, 72 and 84 inch table, shelf or drawer liners. For those, a constant step per size is exactly right, and the table above gives you the cut size at each step without recomputing the allowances each time. Note that the allowances do not grade — the seam allowance is the same half inch whether the piece is 18 inches or 28.
Once you have the cut sizes, the yardage question is next: the fabric yardage calculator takes a list of cut pieces and lays them out across a fabric width. If the pieces are cushion parts, the upholstery foam and batting calculator builds the cut list for you from the cushion dimensions instead.
Questions people ask
What seam allowance should I use?
Whatever the pattern specifies, and if there is no pattern, whatever suits the weight and the job. Quarter inch for quilt piecing, three eighths for serged knits, half inch for home decor and upholstery, five eighths for commercial garment patterns. The choice matters far less than the consistency: two pieces sewn at different allowances will not match, and a seam sewn at an allowance different from the one the pattern assumed changes the finished size by twice the difference on every seam.
Why do commercial patterns use 5/8 inch?
It is a fitting allowance as much as a sewing one. Five eighths gives you something to let out during a fitting, enough width to trim and grade the layers so a seam lies flat, and enough to clip into on a curve without cutting through. It is a garment convention that came from an industry where garments were fitted on a person after basting. For a straight seam in a stable woven that you are not going to fit, it is more than you need, and plenty of independent patterns use three eighths or a centimetre instead.
How much ease should I add?
This calculator will not answer that for you, because ease is a design decision rather than an arithmetic one. It depends on the fabric, the fit you want and what the item goes over. A fitted woven bodice, a loose shirt and a boxy jacket cut from the same body measurement have very different ease. For non-garment work the question is easier to answer empirically: for a cover over a solid object, ease is often zero or slightly negative, because a cover cut to the exact object size and then stuffed sits smooth. Make one in cheap fabric first.
Is a 1 cm seam allowance the same as 3/8 inch?
Close but not identical. One centimetre is 0.3937 inches; three eighths is 0.375. The difference is under two hundredths of an inch per allowance, which is invisible on a single seam and accumulates to about a sixth of an inch across ten seams. That is usually tolerable in a garment and not tolerable in quilt piecing, where the blocks have to meet at points. Pick one system for a project and stay in it rather than converting seam by seam.
Should I add shrinkage to the cut size or prewash?
Prewash if you can. Adding a shrinkage percentage to the cut size assumes the fabric shrinks evenly in both directions and by the amount you guessed, and neither is reliably true — most wovens shrink more in the length than the width, and the amount depends on the wash temperature. The shrinkage field here is for the cases where prewashing is not an option, such as fabric with a finish you need to keep. If you use it, cut a test square, wash it exactly as the finished item will be washed, and measure the actual loss rather than using a nominal figure.