WHAT IS IT?
Seam allowance is the margin of fabric added to pattern seam lines so that the pieces can be sewn together and pressed open. Turn a finished garment inside out to see all of the seam allowances.
Seam allowance indicated on modern pattern
Seam allowance on vintage pattern
WHEN DO YOU USE IT?
For every seam you sew, you must include a seam allowance. (The exception is abutted seams, page link.) On most sewing patterns, the standard seam allowance is 5⁄8″ (1.5cm.) Seam allowances are marked on the pattern pieces or listed in the instruction sheet. Make sure you know what the seam allowance is before you start sewing.
Some patterns are sold without any seam allowance added, so you will have to add it as you cut out the pieces. This is convenient if you want to change the seam allowance, but it’s an additional step to do before you can start sewing.
WHEN TO USE WIDE SEAM ALLOWANCES
If you have wider seam allowances on your fabric pieces, there is room to let out the seams. When making a muslin (page link), you might want to cut the muslin pieces with extra-wide seam allowances for fitting. Leaving wider seam allowances on your finished garments gives you the chance to let out the seams later on, if needed. However, they may show through the garment and add bulk. You might want to keep the seam allowances wider than normal in certain areas of a garment, for example, at center back.
WHEN TO USE NARROW SEAM ALLOWANCES
Often sewing patterns for knit fabrics will have ¼” (6mm) seam allowances for sewing with a serger. If you are very short on fabric and know you won’t have to make fitting changes, you could reduce the seam allowances on your fabric pieces. Also, on very tight curves, the seam allowance can be reduced to make it easier to sew. Sometimes strap or loop pieces will have narrow seam allowances since a wider seam allowance would have to be trimmed off. Narrow seam allowances don’t allow any room for alterations later on.
HOW TO CHANGE THE SEAM ALLOWANCES ON A PATTERN
Figure out where your seam lines are. If your pattern has 5⁄8″ (1.5cm) seam allowances, the seam line is located 5⁄8″ (1.5cm) in from the cutting line. Draw in the seam lines on the pattern piece.
Decide what your new seam allowance is going to be. If you are adding to the seam allowance, tape a piece of scrap paper along the seam line for the extension. Or, if you haven’t cut the pattern piece out completely, leave extra tissue paper where you plan to add. Add this amount to the seam line, measuring it off from the seam line at the beginning, end and several points in between.
Draw in your new cutting line. When you sew this seam, remember to take the new amount of seam allowance instead of the original amount! Make a note of this in the pattern sewing instructions so you don’t forget. Remember to adjust the seam allowances on both sides of the seam.
Source : The Sewtionary An A to Z Guide to 101 Sewing Techniques + Definitions
About the Author : Tasia ST. Germaine