Changing Seam Allowances

The ideal width depends on the shape and the location.

Choose the seam allowance width that works best for the shape, location, and finish of each seam. At bottom right on the facing page, a narrow width works well when overcast and topstitched or on covered curves (top). At center, a moderate width works for a straight seam that’s pressed open and overcast. A wide seam allowance (left) can be turned under and whip-stitched to the under-lining.

Many commercial pat terns give sewers the impression that a 5/8” seam allowance will produce the best results in every part of a garment. But as a pattern-maker and frequent sewer, I’ve found many instances where the 5/8” seam allowance is simply not the best choice: A narrow allowance works better for some seams, and an extra-wide one gives superior results for others.

A simple definition of seam allowance is the amount added to the finished out line of each pattern piece so that the fabric pieces can be sewn together. Or, said differently, it’s the space between the stitching line and the cutting line.

To sew with maximum efficiency, I al ways cut the fabric with the seam allowances adjusted, which I’ll describe how to do. Of course, for narrow seam allowances, you could simply cut the pat tern as usual and trim the excess after stitching, but that creates an extra step, and certain fabrics are difficult to trim neatly. Imagine trimming exactly % in. from each edge of a silk charmeuse garment section—messy. In some cases, however, a wide seam allowance makes an accurate fitting impossible to do, such as at a jewel neckline. If you want wider seam allowances, you must correct the pattern in advance. For these reasons, I always adjust my pattern before cutting the fabric.

Many patterns, many choices

Patterns today range from one-size pat terns with seam allowances to multi-size patterns without allowances, plus various combinations in between. Without a clear understanding of seam allowances and their function, this variety of pattern types can cause confusion and lead to construction problems.

Of all the pattern types, multisize pat terns have the greatest chance of being misunderstood and used incorrectly, because either they indicate cutting lines for several sizes on each pattern piece and don't include the stitching lines, or they indicate multiple stitching lines with no seam allowances. To avoid confusion, always draw stitching lines onto any pattern pieces that don’t have these lines printed on the paper.

On patterns that don’t include seam allowances, the marked line is the stitching line. You’ll need to add a seam allowance to each edge of the pattern with a clear plastic ruler and a pencil. Be careful, though; flimsy tissue patterns tear easily and can be difficult to draw on. If you plan to sew the pattern more than once, I suggest using spray adhesive (preferably outdoors) to attach the pattern tissue to a more stable paper, such as white banner paper (sold by the roll in the wrapping-paper section of large office-supply stores such as Staples). For a pattern you plan to sew only once, you can use a rotary cutter with a built-in measuring device to automatically add the seam allowance as you cut the pattern piece. Whether you add or alter the seam allowances, write the amount of allowance for each seam on the paper pattern piece and refer to it while sewing.

Altering seam allowance widths for greater efficiency: You can vary the width of the seam allowances in a garment to reduce trimming and speed the construction process. Seams with concave curves require narrow allowances that won’t restrict fit or require clipping, while the convex curve of a puffy sleeve can have a wide seam allowance to support. The sleeve cap. Faced edges need only a narrow allowance (a wider amount would just be trimmed away). Straight seams can have wider allowances, depending on the seams’ location and the method used to finish the raw edges. Extra seam allowance supports the sleeve cap of a set-in sleeve. Fold seam allowance and shape end like the cut edge underneath. Extra width on the straight section will ease zipper insertion. Faced edges need only a ¼-in, seam allowance, since they will be protected by an extra layer.

What’s enough seam allowance?

For ease of sewing, make the seam allowances that will be sewn together the same width. It’s much easier to match edges exactly than to align seamlines with uneven edges.

Straight seamlines—It’s relatively easy to determine the width of seam allowances on straight lines such as side seams, shoulder seams, and center seams. Straight seamlines can have allowances of any width, depending on how you want to finish the raw edges. For example, on an unlined jacket you can finish the seams with one of several techniques: If you want to bind the edges with tape or a fabric strip, or turn them under and sew, a 1- or 1 1/4-in, seam allowance will be easier to finish neatly than a 3/8-in. seam allowance. However, if you decide to straight-stitch a seam, overcast the seam allowances together, then topstitch them to one side, you can accomplish this finish neatly with as little as a 3/8-in. seam allowance.

Curved seamlines—A pattern edge can either curve into the pattern piece (concave) or outward (convex). Curved pat tern edges need special attention because the cutting line will not be the same length as the stitching line. This can affect the fit of the garment and way the seamline lies.

You’ll find concave curves at necklines, armholes, and crotch seams. A concave curved seam can't be let out if the garment is too small, because the let seam will be smaller and tighter. And on this type of seamline, too much seam allowance will interfere with fit. The cut ting line is shorter than the seamline, so if you have a wide seam allowance, it will be necessary to clip it so the seam lies flat without binding or giving a false fit. How ever, concave curves are usually located at stress points on the garment, and clip ping the seam allowance significantly weakens the seam. I recommend using a narrow 1/4 to 3/8-in. allowance to eliminate the need for clipping.

Convex curves, such as at the cap of a sleeve and the hip section of a fitted skirt’s side seam, have cutting lines that are longer than the stitching lines. These edges don't present the same fitting problems as concave curves, but they may still require clipping to lie flat, if the seam allowance is wide. To avoid clipping, I recommend a 3/8” allowance for a convex curve. The cap of a puffy set-in sleeve is one exception, where an unclipped, wide seam allowance will help to support the shape of the sleeve (above).

Combination seamlines—For a seamline with both straight and curved sections, such as the crotch seam on a pair of pants, or one that contains both concave and convex curves, such as a sewn sleeve, you can use two different amounts of seam allowance for different sections of one seam, as shown above.

Extensions of seam allowances—The shape of the ends, or extensions, of wide seam allowances is particularly important, especially in curved areas such as a crotch seam or the underarm seam of a fitted sleeve (see the drawing at right above). When the seam is stitched and the seam allowances are pressed open or to one side, the extensions need to have the same shape as the areas they overlap. Patterns that are printed with seam allowances already include shaped extensions mirror the shape the ends of seam allowances you’ve had to add, fold the paper pattern along the stitching line and cut the end of the seam allowance to match the cut edge of the layer underneath. When it’s opened, the seam allowance extension mirrors the shape of the garment cutting line.

Facings—Most faced edges require only a 1/4-in, seam allowance on both the garment and the facing. A wider allowance would have to be trimmed away before the facing could be turned and pressed, which would waste fabric and time. Edges that will be faced can include straight lines such as center fronts and pocket flaps, concave lines such as necklines and sleeveless armholes, and convex lines such as the outer edge of a collar.

Even with a tiny 1/4-in, seam allowance on facings, you may occasionally need to clip or layer the edges so that the seam lies flat. But certainly this will happen much less frequently than if you used a wider seam allowance.

As you can see, using a variety of useful seam allowance widths can help to make your sewing more efficient. Don’t be limited by the instruction guide sheet pro vided with your pattern; feel free to alter the suggested widths to suit each seam location, its shape, and your sewing needs. A clear understanding of seam allowances helps to put you in charge of your sewing, making you a more efficient and knowledgeable sewer.

Marking seam allowances

I’ve noticed that the sewers in my classes dutifully draw and adapt each seam allowance to the most efficient width, and then cut out the garment pieces as instructed. But when it comes time to assemble the garment, they frequently forget about the altered seam allowance width and begin to stitch the seams with the usual old ½-in., seam allowance (see how well trained we are?), which results in incorrect stitching lines and garments that don’t fit.

To help sewers remember how much seam allowance they’ve planned for each seam of a garment, I developed a simple color marking system to prompt sewers’ memories. We place 3/4-in, color-coding label dots (available in a box of assorted colors at stationery stores) at each seam on the paper pattern piece. For example, a red dot indicates a 5/8-in, seam allowance, a blue dot is for ¼-in., and green means a 1-in, allowance. I use a yellow dot for hems, writing the amount of the allowance right on the dot. An orange dot indicates something unusual that will need special attention, and I write the necessary information on the dot. Besides being easily recognizable by color, the dots don’t tear when I write on them, which often happens with pattern tissue.

During garment assembly, each sewer keeps the marked pattern pieces handy for quick reference. This system helps prevent mistakes and unmatched stitching lines.