Have you ever cut two pieces of fabric exactly to the right size and then sewn them together and found the resulting item to be smaller than expected? What happened?
The answer is seam allowance, and for the beginner it can be one of the most difficult concepts to grasp. The area of the fabric between the stitching line and the raw edge is known as the seam allowance, and it is the amount of fabric that will be lost in a seam. When you join together two pieces of fabric a certain amount of fabric from the cut width of each piece will be taken up in the seam. For example, if you cut two pieces of fabric that are 20cms wide, you will not get a pouch that measures 20cms when finished. The amount of fabric lost in the seam means that it will be smaller than 20cms. If you are unsure of this take two small squares of fabric, mark 1cm from the edge and sew along the marked line. Press the seam open. The total width will be less than the two squares combined.
Many items have several seams, for instance a cover or organiser may have side seams, a seam at the bottom, a hem, and a lining to consider. Failing to allow for seam allowance on even just one side will mean that the item will be distorted or the corners may not meet, the opening may be too small, the lining may be too tight and so on.
Even the most accurate cutting will not help if the piece you are working on is not cut from the correct measurement. Always make sure you know if the measurement is a finished measurement (i.e. what you want the measurement to be when you have finished), or if it is a cutting measurement that has allowed for any seams, joins, hems and turnings.
Equally if the cutting is correct, but the stitching is inaccurate, you can end up with a lopsided, distorted piece. If you stitch one side at 1cm and the other at 1.5cms the item will come out wrong. It’s not the fault of your piece, check your stitching first! This can be difficult to do as you learn how to use your sewing machine, so look for ways to help you, use a line marked on the machine or presser foot as a guide, or even make a mark on the fabric so that you always stitch along the same place. Although it can be difficult to do this at first, with practice you can learn to gauge where to stitch. Remember that millimetres can make a big difference when there are several joins.