Whether you have received a dress from a friend or relative or simply experienced a slight change in body shape, altering a prom dress is a useful way of ensuring a perfect fit for your own body. The two basic alterations of a prom dress are letting out and taking in, or enlarging or decreasing the dress size. To let out a prom dress by altering the bust to make the dress larger, you first need to ensure that the dress has enough spare fabric to be let out. This is the fabric that is usually intentionally left on the inside of the seam for the purpose of letting the dress out.
Turn your prom dress inside out and find the seam. The seam is where the two raw edges of fabric are sewn together. Use your tape measure to find out how much extra fabric was left over after the seam had been sewn. This is known as seam allowance. If the seam allowance is more than ¼ inch you will be able to let the dress out.
Unpick the seam around the bust region so that the extra fabric is loose. Only unpick the seam where you need to let the dress out and be sure not to cut any other seams that your seam may cross. Do this gently to avoid tearing any holes in the fabric.
Measure around your bust region to see how much you will need to let the dress out. Divide this extra amount, in inches, by four. Mark this length on the loose fabric with a piece of chalk. This is where you will sew your new seam.
Pin the edges of the dress fabric together at the chalk mark. Check that the seam is straight and in the right place before you stitch the seam. Use a simple stitch along the chalk marks to create a new seam. After stitching, remove the pins and turn the dress right side out again to hide the raw edges of the seam.
After you stitch the dress, try on the prom dress to ensure it is a snug fit but not too tight. If it is still too tight, unpick the seam, let out a little more fabric and then restitch.