A gusset is a small shaped panel set into a seam to create useful room where fabric is under strain. At the underarm, it allows the sleeve and side of a garment to open as the arm lifts, rather than pulling the whole bodice upward.
It is especially useful in shirts, smocks, blouses, work dresses, and garments with cut-on, kimono, or closely fitted sleeves. A gusset can be included while making a new garment, or added later to relieve a tight underarm seam. The work is precise but not difficult: the important part is to mark the sewing lines clearly and stop each seam exactly at its end point.
A one-piece square gusset is the usual starting point. Turned on the diagonal, it becomes a diamond: one point sits toward the sleeve, one toward the body, and the remaining two points lie at the front and back underarm seams.
Materials and tools
Gather the following before opening any seam:
- Matching woven fabric, or a sturdy contrasting cloth if the gusset is intended as a visible repair
- Matching thread
- A hand-sewing needle, sewing machine, or both
- Tailor’s chalk, washable marker, or a fine pencil suitable for fabric
- Small sharp scissors and a seam ripper
- Pins or hand-basting thread
- An iron and pressing cloth if needed
- A ruler or small square
Choose a gusset fabric with a similar weight, drape, and care requirement to the garment. A crisp cotton gusset in a soft rayon blouse may pull and behave differently over time; equally, a very light patch in a hard-wearing work shirt may wear through quickly. For a repair, cloth cut from an inside hem facing, pocket, or leftover seam allowance can sometimes make a discreet match, provided removing it will not weaken another part of the garment.
Photo by tian dayong on Unsplash.
Decide on the gusset size
The finished size of a gusset determines how much extra movement it gives. For a slight restriction, begin with a finished square about 4 to 5 cm (1 1/2 to 2 inches) across. For a garment that is notably tight when the arms are raised, a finished square of about 6 to 8 cm (2 1/2 to 3 inches) is often more useful.
A larger gusset gives more reach, but it can create a visible fold under the arm if the garment is otherwise very close fitting. It is wise to start conservatively, particularly in a fine blouse. You can test the size with a paper square or a scrap of fabric pinned into an opened seam before cutting the final piece.
For a standard 1.5 cm (5/8 inch) seam allowance, cut a square that is the desired finished dimension plus 3 cm (1 1/4 inches) overall. Thus, for a finished 5 cm square, cut an 8 cm square. If your garment uses narrower seam allowances, add twice that allowance instead.
Mark the wrong side of the cut square with its seam lines. Draw a line parallel to each edge at the seam allowance distance. Where these lines meet are the four crucial corner points. Mark them clearly. You will sew only from point to point, not into the seam allowance beyond them.
Grain direction
For most woven garments, place the square with its edges on the straight grain, then turn it diagonally when it is sewn into the garment. This leaves the bias running across the areas that must flex, which gives a little natural ease. If the garment has a strong stripe, check pattern direction before cutting; a deliberately aligned or contrasting gusset often looks better than a nearly matched but crooked one.
Prepare an existing garment
If adding a gusset to a finished garment, first identify the underarm junction: the place where the sleeve seam meets the side seam. Work from the inside so that the seam construction is visible.
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Try on the garment and mark the strained area. Raise and reach forward with your arm. Use pins or chalk to mark where the seam begins to pull. Usually the opening extends an equal distance down the sleeve seam and down the side seam from the underarm junction.
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Measure the opening needed. For a square gusset, each side of the opening must match one side of the gusset’s finished sewing line. A 5 cm finished square needs four seam sections, each 5 cm long.
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Unpick the seam carefully. Remove stitches along the sleeve seam and side seam, working out from the underarm in all four directions. You are creating a cross-shaped opening, with the original underarm point at its centre. Preserve as much sound fabric as possible.
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Remove or open seam finishing only where necessary. If the garment has overlocked edges, flat-felled seams, or a binding, unpick only enough to expose the new seam allowance. Do not cut away seam allowance before confirming that the gusset fits.
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Square the opening. On the wrong side, mark the four seam end points at equal distances from the original underarm junction. The fabric between them should form four short seam edges around a diamond-shaped gap once opened.
For a new garment, leave the sleeve seam and side seam unsewn for the gusset length. Mark the same four end points before joining the surrounding seams.
Photo by Rita K on Unsplash.
Sew the four sides of the gusset
The insertion is easiest when treated as four separate short seams. Do not attempt to stitch continuously around the entire diamond. At every corner, end precisely at the marked seam intersection, lift the work, and begin the next side afresh.
1. Match the first point
Place the garment wrong side up. With right sides together, align one marked corner of the gusset with one end point of the opened seam. Pin at the point, then align the two marked sewing lines along that edge.
It helps to assign each gusset point a direction before beginning: sleeve, body, front, and back. Mark these lightly on the wrong side of the gusset if there is any chance of confusion.
2. Sew the first seam
Stitch from one marked point to the next, following the drawn seam line. Backstitch very lightly at the beginning and end if using a machine. Do not sew past the point.
For hand sewing, use a small backstitch for strength. A running backstitch, with occasional backstitches, also works well in a stable woven fabric. Keep the stitches even and close enough to support a seam that will be pulled during wear.
3. Attach the opposite side
Lay the garment flat again and bring the opposite free edge of the gusset to its corresponding opening edge, right sides together. Pin both end points first. This prevents the gusset from twisting.
Sew from point to point. Check that this seam finishes exactly at the same marked junctions as the first. If it runs beyond either point, the next seams will not meet cleanly.
4. Sew the remaining two sides
Attach each remaining gusset edge in the same way. Pin the two end points, smooth the small section of fabric between them, and stitch only along the marked line.
The garment may look awkward while the final side is pinned. This is normal. The fabric must bend around a small three-dimensional space beneath the arm. Use several pins or a line of hand basting if the layers creep.
5. Inspect the centre points
Turn the work right side out and look closely at all four joins. The seams should meet at tidy points without a hole, a pucker, or a stitched-over corner. If there is a small gap at one junction, turn the garment wrong side out and reinforce only that spot with a few hand stitches.
Photo by Joéma Friperie on Unsplash.
Trim, finish, and press the seam allowances
Before trimming, try the garment on carefully. Raise both arms, reach forward, and bend at the elbow. The gusset should open comfortably without pulling at a single point. If it is too tight, it is better to replace it with a larger piece now than to force the garment into service.
Once the fit is satisfactory, finish the raw seam allowances in a manner suited to the fabric and the garment’s original construction.
- Plain woven cloth: Press the allowances open where possible, then zigzag, overcast by hand, or use a narrow overlock stitch along the raw edges.
- Fine or fraying fabric: Use a narrow French seam only if the gusset was planned from the beginning and adequate allowance is available. For a repair, a closely worked hand overcast or narrow machine zigzag is usually less bulky.
- Hard-wearing shirts and smocks: Trim the allowances evenly and finish them with overlocking, bias binding, or a neat hand overcast.
- Handmade or historical-style garments: A flat-felled finish may be used where the fabric and seam layout allow it, though the tight corners of a small gusset can become bulky. A pressed-open seam allowance with hand overcasting is often quieter and more manageable.
Clip the seam allowance only when the fabric requires it to lie flat, and make very small clips that stop short of the stitching. Do not notch into the seam itself. Press from the wrong side first, using the point of the iron around the gusset corners. A tailor’s ham or a rolled towel can support the curved underarm area without flattening the rest of the sleeve.
Common mistakes to avoid
Cutting the opening before marking the seam ends
The four seam sections must be equal to the gusset’s finished sides. Mark first, then unpick or cut. An uneven opening produces a twisted gusset and makes one seam difficult to ease.
Sewing over a corner point
A gusset works because its seams pivot at four exact points. If a stitching line continues beyond a marked point, the next seam cannot meet it neatly. Stop with the needle down at the point, secure the thread, and begin the next side separately.
Using a gusset that is too flimsy
The underarm is subject to rubbing, sweat, and repeated pulling. A delicate patch may be suitable for a light occasional garment, but everyday clothing benefits from a closely matched, sound woven fabric.
Pulling the fabric to make it fit
If the gusset edge and garment opening are not the same length, do not stretch one to force the match. Check the markings and seam allowances. A small discrepancy can usually be corrected by resewing; a strained seam will pucker and wear quickly.
Forgetting to reinforce a damaged area
If the original underarm seam has torn or the fabric is thinning, extend the repair beyond the damaged threads. Trim away only fabric that has truly failed, and use a gusset large enough to anchor into firm cloth on all four sides.
A useful variation: the two-piece gusset
A two-piece gusset is made from two matching triangles joined down the centre before insertion. It can be helpful in a very fitted garment because the centre seam lets the gusset take a more shaped form. It may also be easier to handle when a single diamond feels difficult to align.
To make one, cut two triangles with seam allowance on every edge, sew them together along one long edge, press the centre seam open, and treat the assembled shape as the gusset. The same rule remains: mark the four points and sew each side from point to point.
For most repairs and ordinary shirts, the one-piece square is simpler, strong, and entirely sufficient. It is a small alteration, but it changes how a garment behaves in the body: the arm may lift freely, the side seam stays where it belongs, and a much-used piece of clothing can remain in service.