What to know before altering a garment

Before taking in or letting out a garment, check three things: how much fabric the seams allow you to work with, what the fabric and construction can tolerate, and how the garment fits now. Many alterations are straightforward when the garment has enough material and simple construction. Others are limited by the fabric, the lining, the placement of darts and seams, or the shape of the shoulders, and those limits are easier to see before cutting or unpicking anything.

Check the seam allowances first

The seam allowance is the strip of fabric between the stitching line and the raw edge. Commercially made garments often have allowances of roughly 1 to 1.5 centimeters, which is about 3/8 to 5/8 inch, although the width varies by brand and by seam. Letting a garment out depends on having enough allowance to move the seam line. Taking it in is usually easier, because the excess can be stitched further in, but that also reduces the room for later changes.

To measure the allowance, turn the garment inside out and measure from the stitching line to the cut edge. There is no need to unpick anything. Unpicking a seam just to measure it risks damaging the fabric and leaving needle holes behind. While the garment is inside out, look at how each seam was finished. Serged seams in ready-to-wear are usually trimmed to the width of the serger, so there is little or nothing to let out. Other seams are pressed open or simply trimmed. Trouser and skirt center back seams often have extra allowance for letting out, so they are a good place to look first. Note the allowance on each seam you plan to change, and check whether there are darts, princess seams or pleats that will also affect the shape of the garment. A seam with only a few millimeters of spare fabric may still be possible to adjust, but it leaves little margin for error.

Consider the fabric and its limits

Some fabrics are easier to alter than others. Patterned fabrics such as plaids, stripes and large prints need the seams to be matched, and altering one seam can throw off the match across the whole garment. Stripes or checks can also shift when a side seam is moved, which is visible at once. Fabrics with a nap, such as velvet or corduroy, need careful handling, because the nap can flatten or show lines where the seam was.

Old stitching and pressing lines can remain visible after a seam is moved. Needle holes from the original seam may show, particularly in leather, suede and some tightly woven fabrics. Synthetic blends can hold a crease, and heavy wool can stay stiff at the fold. Delicate fabrics such as chiffon and some silks may pull or snag under a machine, which often means hand stitching is the better option. When the fabric is uncertain, test on a scrap or an inside allowance before committing to the garment.

Assess how the garment fits now

Try the garment on with the undergarments and shoes you would normally wear, and look at it in a mirror with good light. Pull gently at the chest, waist and hips to see how much ease the garment has and where it is tight. Ease is the room built into a garment beyond the body's measurements, and it determines how the garment moves and feels. Altering the waist or side seams changes the balance of the whole garment, so look at how the change will affect the pockets, buttons and the line of the hem.

Pay special attention to the shoulders and sleeves. Shoulder seams are difficult to move without reshaping the entire upper garment, and sleeve length is often easier to change at the hem than at the seam. Mark the planned changes with pins or a fabric marker while the garment is on, then take it off and check the marks against each other. Make the changes in small steps, and check the fit after each one. Taking in too much in one move is difficult to undo, and it can leave the garment looking pinched.

Plan the work and keep the original measurements

Write down the original measurements and the planned changes before you begin. Remove stitches carefully, using a seam ripper and keeping the blade pointed away from the fabric to avoid cuts. Press the seams after each change, because pressing sets the new shape and shows the true fit more clearly than the garment lying flat.

Keep the original seam allowance intact where possible, so that a future alteration remains an option. Trim an allowance only where bulk makes it necessary, such as at a thick corner or where several layers meet, and never on a seam that may need letting out later. When trimming is needed, do it only after the final fitting, since the fabric may need more room than expected. Finish raw edges with a zigzag, a serger or a narrow hem to prevent fraying, and check that the finished seam lies flat without twisting. If the work is taking longer than expected or the result is uncertain, stop and assess again before cutting further.

When a tailor is the better choice

A tailor is usually the better choice for structured jackets, lined coats, garments with set-in sleeves, and anything that requires reshaping the shoulders or the lapels. The same is true for leather, heavily embellished garments, and fabrics that are difficult to mark or press. A tailor has the tools and experience to work with the internal construction of these garments, which is often hidden and easy to damage with a simple alteration. For significant changes, such as a large size adjustment or a major change in the garment's shape, professional work is often more reliable than a home attempt.

Choosing a tailor for these jobs is a sensible way to keep a valuable garment in good condition for later wear. For simpler changes, such as shortening trousers, taking in a side seam on a straightforward shirt or adjusting a hem, careful home work is often perfectly suitable. The most useful habit is to check the garment before starting, so the choice is made on the basis of the fabric and construction rather than guesswork.