Seam Allowance Calculator: Finished Size to Cut Size and Back

Enter a finished size to get the size to cut, or a cut size to see what it finishes at. The second part shows what happens to fit when you sew a pattern at a different seam allowance than it was drafted for.

Machines for garment seams

Consistent 1/2 and 5/8 inch seams come from even feeding and a clearly marked needle plate.

Intermediate

Juki

Juki HZL-G120 Computerized Sewing Machine

$819.00

Box feed for precise seams on slippery and fine garment fabrics.

Check price

Intermediate

Juki

Juki HZL-DX5 Computerized Sewing Machine

$1,099.00

Adjustable foot pressure and an automatic needle threader.

Check price

Intermediate

Juki

Juki Pearl Line MO-654DE Serger

$599.00

A serger that trims and finishes the allowance in one pass.

Check price

Prices change often, so confirm on Amazon. As an Amazon Associate we earn from qualifying purchases.

How do you calculate seam allowance?

Seam allowance is the strip of fabric between the cut edge and the stitching line. The finished size of a piece is measured at the stitching lines; the cut size includes the allowance on every edge that gets sewn. The whole calculation is one line:

Cut size = finished size + (number of seamed edges x seam allowance)

Most pieces are seamed on two opposite edges in each direction, so the calculator adds the allowance twice to width and twice to length. For the default example, a piece that should finish at 10 x 12 inches with 5/8 inch seams:

1. Width: 10 + 2 x 5/8 = 11 1/4 inches.
2. Length: 12 + 2 x 5/8 = 13 1/4 inches.
3. Going back: subtract instead. A piece cut at 11 1/4 x 13 1/4 inches finishes at 10 x 12 inches.

If an edge is hemmed, bound, placed on a fold or left raw, change the count for that edge. A pocket with a folded top and seams on three sides gets the allowance on three edges and a hem allowance on the fourth. Quilters use the same math with 1/4 inch: a 2 inch finished square is cut 2 1/2 inches, which is why so many quilt patterns call for 2 1/2 inch strips.

What are the standard seam allowances?

The allowance a pattern uses is a design choice, and the reference data below shows where each is common. The cut size column shows what a 10 inch finished square becomes with each allowance, so you can see how much the choice matters on a real piece.

For a printable version with more detail, see the seam allowance chart. When a pattern does not state its allowance, look for a line inside the cutting line on the pattern tissue, or check the instructions for "seam allowances included".

What happens when you change the seam allowance on a pattern?

Sewing a smaller allowance than the pattern was drafted with moves the stitching line toward the cut edge, which makes the finished piece bigger. Sewing a larger allowance makes it smaller. The change is small on one seam but it happens on both pieces at every seam, and garments have several seams around the body.

The math, using the default values: a 5/8 inch pattern sewn at 1/2 inch moves each stitching line out by 1/8 inch. Each seam joins two pieces, so each seam adds 1/4 inch to the width. With 4 vertical seams around the body, such as two side seams plus a center back and a center front seam, the garment grows by 1 inch in circumference. Going the other way, a 1/4 inch quilting-style allowance sewn at 1/2 inch across four seams shrinks a piece by 2 inches.

This is also a simple fitting tool. Letting side seams out from 5/8 to 3/8 inch buys you an inch of room across four seams, and taking them in to 7/8 inch removes an inch. Only vertical seams change circumference; shoulder and waist seams change length instead.

How much does a small seam allowance error change a quilt block?

Quilt piecing is where tiny errors become visible, because a block may have a dozen seams in a row. A seam that is 1/16 inch too wide costs 1/8 inch per seam, and a seam 1/8 inch too wide costs 1/4 inch per seam.

That is why a consistent seam matters more than a perfect one. A machine with a dedicated 1/4 inch foot and a straight-stitch needle plate, such as the Juki TL-2000Qi, makes it easier to repeat the same seam hundreds of times. Garment sewists get the same benefit from a well-marked needle plate and a machine that feeds evenly, like the Juki HZL-G120 with its box feed. If you finish seams with a serger, the Juki MO-654DE trims to its stitch width as it sews, so plan narrower allowances or guide the extra past the knife.

How do you add seam allowance to a pattern without one?

Many European magazine patterns, some vintage patterns and most self-drafted blocks are printed at the stitching line with no allowance included. Adding it is simple but worth doing carefully, because the allowance you choose becomes part of every later measurement.

1. Choose the allowance for each edge. Side and shoulder seams usually get the same allowance throughout, often 5/8 inch or 1.5 cm for wovens and 3/8 inch or 1 cm for knits. Necklines and armholes that will be faced are often given a narrower 1/4 or 3/8 inch so they turn smoothly. Hems get a separate, deeper allowance, commonly 1 to 2 inches.
2. Mark a parallel line. Trace the pattern onto paper and draw a second line outside each stitching line at your chosen distance. A clear ruler with 1/8 inch markings or a double tracing wheel keeps the distance even around curves.
3. Walk the seams. Lay matching seam lines against each other to be sure they are the same length before cutting fabric, and add notches at the same points on both pieces.
4. Write it on the pattern. Label each piece with the allowance you used, so the next time you use it you can trust the cut size from this calculator.

Changing to a 1/2 inch allowance on the default piece would make it 11 x 13 inches to cut, a small but useful saving on every piece of a big home decor project.