The fabric weight can affect many things other than the feel of your final textile item. Can the pouch stand upright or is it likely to slump? Will the case turn inside out through its opening? How much volume will there be in the corners? How easy will it be to sew multiple layers under the sewing machine foot?
Your initial project is best served by fabric that is easy to work with rather than fabric selected simply for its pattern or surface. Woven cotton or cotton-blend fabrics are often easier to mark, cut, pin, and sew than fabrics that are very stretchy, slippery, or heavy.
Examine the fabric to determine how it behaves before you begin cutting. Place a portion of fabric in half and gently rub it between your fingers, allowing it to hang from one corner. Lightweight fabrics may drape and show little structure, whereas heavier fabrics may maintain the fold and be difficult to bend. Neither is necessarily better. A drawstring bag may perform well with a lighter fabric, but an organizer or structured case may require a fabric with more body. Ask yourself if the fabric will help you achieve the shape you want, or will it simply add bulk to your seams and corners.
The thickness of the fabric is important when working with linings, interfacing, zippers, boxed corners, or turned edges. A single layer of medium-weight fabric might be easy to work with, but four or six layers meeting at a single seam can become difficult to sew and turn. Therefore, judging the thickness based on a single layer can be misleading.
To estimate, take a scrap of fabric and fold it several times to simulate a series of seams. Is the resulting stack stiff and bulky? How would that change if you added a lining or reinforcement? Consider using a lighter interfacing, a thinner lining, or a lighter outer fabric.
To evaluate the fabric, try two sample seams before you cut your pattern pieces. Cut two squares of fabric of equal size. Place the squares right sides together and sew a straight seam using the seam allowance specified for your project. Press the seam, and turn the square over to see how the edge behaves. Make another sample using the interfacing you intend to use. Which seam pucks up more? Which is stiffer? Is either more difficult to press flat? Does either seam feel too floppy? These simple tests provide more insight than guessing from the fabric before it has been worked.
The fabric weight also influences the accuracy of your cuts. A very soft fabric can slide around as you trace the cutting line, particularly if you are cutting two layers of fabric. A very heavy fabric can be difficult to cut, but it will not move as easily, though cutting through multiple layers can cause them to separate from each other. Pinning, fabric clips, rulers, and rotary cutters can all help, depending on your fabric and tools.
Be mindful of the fabric’s grain line. A fabric that is the proper weight can still act strangely if pattern pieces are cut out of alignment with the grain, resulting in a crooked or unevenly shaped item.
A light interfacing can add body to a fabric that is a little too soft. Try the interfacing on a fabric scrap. If you add too much reinforcement, you will find it difficult to turn your item’s openings, and corners will be stiff and bulky. If you do not add enough, the finished item may be less structured than you want.
Use the correct iron temperature, press the fabric and interfacing, and then fold, bend, and sew. The point is not to make the fabric as rigid as possible; instead, add enough support for the item to maintain its shape, but also to facilitate the clean assembly of seams, curves, and corners.
The right fabric makes itself apparent during the construction process. Pattern pieces are easier to match up, seam allowances are more consistent, pressing is easier, and the finished item is appropriate for its function. When selecting fabrics, note how they behave throughout the entire process of cutting, stitching, pressing, turning, and shaping. This information is more valuable than simply knowing that the fabric is “lightweight” or “medium.”