What is a seam allowance?
To understand what the term seam allowance means, start with “What is a seam?” A seam in sewing means sewing two pieces of fabric together to 1) create a larger piece for a garment, project, bedding, etc. 2) create a more decorative arrangement using different fabrics and 3) create a more complicated piece that will help shape a garment – all of this with a needle and thread (and probably a sewing machine but not necessarily). Yes, sewing together two or more pieces of fabric makes sense, but what is a seam in tangible terms?
The thread that holds the fabric together is the location of the physical seam. The seam is used for both utilitarian reasons and for decorative reasons. If a larger piece of fabric is needed than what is currently available, sewing two or more pieces of the same fabric together would solve that problem. If you have a single type of fabric but want to do something fun and decorative with it, cut it up and sew it back together in different ways. Take a look at the following sections if you can’t picture it in your head. You know, ‘a picture is worth a thousand words’ 🤣
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A quick way to see both of these ideas in action is to look at a simple, large quilt. The back is likely made of a single fabric that was sewn together to make a piece large enough to cover the back of the quilt but the front is usually smaller fabric pieces sewn together to make something more decorative and interesting.
Seams: Fabric pieces right sides together
Technically, you can sew two pieces of fabric together in any way. You could sew two pieces down the centers of both and get them to stay connected. But I don’t know how you would use it if you were expecting to make a larger piece of fabric to work with. Where should/does a seam go? Let’s get more precise with our terminology and techniques, starting with the right and wrong sides of the fabric.
Right sides vs Wrong side (A quick lesson inside this post)
What is the right side of the fabric? What is the wrong side? It depends. Usually the right side is the brighter, smoother, more ‘attractive’ side of the fabric. 90%+ of the time we use the ‘right’ side, the side that the manufacturer/creator designed. It is the result of their vision for the fabric. Makes sense, right?
What is the wrong side of the fabric? It too depends but is usually the opposite side of the right side. It’s usually duller, lumpier (thick woven fabric) or just less vibrant than the right side.
**Don’t be afraid to use the /wrong’ side as your right side. I have had a few instances of liking the ‘wrong’ side more than the right side! And have used the wrong sides for curtains and quilting. Many times quilting cotton wrong sides are just a little different than the right side. This minor difference can be enough to create more interest in the project by using both the right and wrong sides. Especially if you do not have a lot of fabrics to choose from or are on a limited budget. You know it will coordinate seeing as it is just the back of the printed area.
Some examples of cotton woven fabric:
Solid fabric: Can’t tell which side is the right vs wrong?
Batik Fabric: really hard to tell which side is right or wrong.
Printed Cotton Quilting Fabric: Not immediately obvious which side is right or wrong, but after a closer inspection it should be obvious. I say should be because my eyes aren’t that great anymore ang this is an inclusive place for all.
Woven Upholstery Fabric
Simple woven upholstery weight fabric: no difference between sides
Textured Upholstery Fabric: Easy to see there are differences, but which is the right vs wrong side? In this scenario, the right side of the image is the ‘right side’ of the fabric. How do I know? It’s part of a fabric sample and the little paper giving the details is stapled on that side. I’ve also worked with enough upholstery fabric to get a feel for which way is up. Literally, the side that feels nicer is usually the ‘right side’.
Geometric woven upholstery fabric: Can use either side or both sides for a lot of contrast. Again, I know that the left side is supposed to be the ‘right side’ of the fabric because of where the information card was stapled.
Printed Upholstery Fabric: Printed on one side and the other side is plain, not much interest. I hope this example is obvious…
Right sides together
Normally you will be laying the fabric down with the right sides facing/right sides together and lined up along an edge where the seam will be sewn. *There is a technique called a French Seam that doesn’t follow this convention, but that’s a topic for another day.
*This is a place in a pattern where you can get in trouble if you aren’t paying attention to the small details like the way the print is facing on the fabric or the nap of the fabric.
Seams are created close to edges. This reduces the amount of bulk on the wrong side of the item and reduces waste. Some patterns call for a very specific amount of fabric and if you don’t use the fabric wisely or it isn’t cut perfectly, you may be ‘left hanging’. Literally, you will probably be hanging out of the item since you ran out of fabric and now it’s too small. Yikes.
Sewing on the edge
Now that the seam has been explained, we can dive into the seam allowance. That line of sewing connecting the pieces allows fabric to be made into different sizes, shapes, things, etc. It’s used to manipulate the fabric into something other than a flat, one dimensional design that comes straight from the fabric store. But, what about the physical means of sewing pieces together? If I take two pieces of fabric and butt them up against each other which is what a seam actually looks like if you think about it… the needle and thread cannot get any grip on the edge because there is just nothing there to grab.
Coming away from the edge: the Seam Allowance
Now, if the stitching is done a short distance away from the edge, there is something for the needle and thread to hold onto, some ‘meat’ to bite into. I know it’s a weird way to say it, but it makes sense. You can’t live on air alone, you need substance!
The distance between the fabric edges and the sewing line is called the seam allowance. Sure, it probably could have had a different or better name to describe it’s purpose, but that’s what we have to work with. It’s really a simple concept, it’s just that measurement between the edge and the sewing line.
How much of a seam allowance?
How much space to leave between the edge and the stitching depends on a few factors. Such as:
Thickness of fabric
The thicker the fabric, the thicker the seam. But do not automatically assume that it needs to be smaller to reduce the bulkiness. Does the fabric ravel easily? Then you’ll need either a wider seam allowance to compensate for how easily the edges come undone or another method to preserve the edges like using pinking shears or some sort of extra stitching on the edge.
Type of Fabric: stretch vs woven?
Really thin, woven, see through fabric calls for very thin seam allowances. No one wants to see the seam allowance on a sheath covering an elegant ball gown. Yuck.
Stretch fabrics require a stretchy seam, otherwise the seam would stay put while the rest of the garment moves. Not so easy to put a shirt over your big head if the seam doesn’t stretch with the stretchy fabric is it?
**Keep in mind that a stretchy fabric needs a stretchy seam, many of the examples and photos show a straight stitch which is best suited for woven, non-stretchy fabric. It’s easier to illustrate, I’m kinda lazy, remember?
Construction of Garment
Smaller tubular sections like straps or ties usually have a smaller seam allowance to reduce bulk.
Professional seamstress/tailor created garments may have larger seam allowances to accommodate future size adjustments.
How much wear and tear
Heavy duty fabric needs a heavy duty seam allowance due to all the wear and tear put on these types of garments. Especially in areas that get the most tension on the fabric. Think crotch and elbows.
Items that are worn infrequently do not need as thick of a seam allowance.
Cleaning method
Delicate garments that need to be hand washed usually have thinner seam allowances because the fabric isn’t going to be roughed up in a washing machine.
Will it need to be resized in the future?
The seam allowance is determined in patterns by the pattern creator. Patterns usually have the general kind of fabric recommended for use, or at lease what the designer envisioned and the right fabric for the project’s purpose.
Curves vs Straight fabric cuts
Curves look more professional and work better with less bulk in the seam allowance. Imagine trying to make your way through a small maze. You have a nice sweatshirt on but it’s really boxy. It’s getting in your way at every curve. Wearing a tighter fitting sweatshirt will be just as comfortable but allow you to exist without being too lumpy. No extra bulk getting in your way. That was the first analogy that came to mind, my apologies since it isn’t the greatest. I’d rather save my brain energy on teaching than perfecting analogies.
Hidden seams/facings, like curves, look better with a smaller seam allowance. There still needs to be some allowance for the garment to be stable but in a facing you do not want extra bulk. A hidden seam is just like what it sounds like, it is used in a place where it is hidden, even when looking at the garment inside out.
Sewing straight pieces of fabric together allows for more room and a larger seam allowance. It’s like walking down a straight, wide hall without worrying about bumping that large, boxy sweatshirt into anything.
Seam allowances in sewing patterns
Does the pattern you are going to make include the seam allowances on the printed pieces or do you need to add them yourself? Some have them, some don’t. Read instructions very carefully, because if you don’t include them, the garment is going to be too small!
How to add seam allowances to a pattern:
Seam allowances can be added to the pattern paper or to the fabric while marking cut lines. There needs to be more space around each pattern piece when marking the paper, or else you will cut into other pattern pieces.
What on earth does the word ‘Scant’ mean when discussing Seam Allowance?
Scant is a word tossed around when talking about seam allowances, but you may not know what it really means in sewing. Scant means just a little less. In this case, the word scant means to measure/cut/sew the fabric about 1/32″ (or so) less than the seam allowance you are shooting for. Sounds crazy right? But, if you think about it on the smaller scale of the seams, every little big of thread takes up space. The seam that was just sewn will take up a tiny amount of space when you iron the seam open. Cutting the fabric that little scant amount makes up for the space the seam allowance thread and folded bit takes up.
Seam Allowance Determined, Now What?
Once you have determined the size seam allowance and your fabric pieces are cut out (including seam allowance) it is time to sew. The seam allowance may or may not be marked directly on the fabric pieces, depends on the way the pattern was transferred or cut out. If there is a line marked for the seam, sew along that line. If the fabric was cut without a seam line marked, refer to the pattern’s stated seam allowance and keep it in mind as you start to sew. That seam allowance will be how far in from the edge you sew, we’ve been talking about it this whole time but here is where it means something tangible. Most sewing machines have seam allowance markings on the throat plate. (What is a throat plate? See post here.) You can also use any number of ways to mark the seam allowance.
Options:
Masking tape
Air Dry Marker
Tailors Chalk
Ruler
Seam guide – on throat plate of machine, adhered to sewing bed, can be metal, plastic, many different styles on the market
Laser beams – minus the sharks freaking heads… bonus points if you know my movie reference 😊 This is a newer contraption, so I do not have any personal experience. Nor should I have any lasers close to my eyes, I’m too clumsy to not hurt myself.
Use whatever option or gadget to sew your garment, keeping the seams accurate.
**As with all sewing projects, keep an eye on your bobbin thread. It has this horrible habit of running out without warning and after you have sewn a particularly pain in the butt seam. You would think that after all the technological advancements and innovation in the world, a sewing machine company could figure out a way to warn us about low bobbin thread. Let me know if you know of any machine that does this, I’ve not seen any… I’d buy that after market for sure.
What happens if I mess up the seam allowance?
Grab that seam ripper, gently undo all that hard work and do it all over again! Hopefully you can still see any markings on the seam allowance and the fabric is not fraying. Some fabrics will let you do this over and over again, other more delicate fabrics may not be so forgiving. Remark that seam allowance on the fabric if needed, it’s worth the time here to get it right.
How to resize a garment at the seam
Why would you resize a garment at the seam? The seams determine the size and shaping of a garment so resizing them using the seam allowance just makes sense.
Resizing Items to make them smaller:
Looking at the inside of the item/garment, you can see how the seams are what make the ‘outline’ of the item. The garment is already sewn with a seam and some way to stop the fabric from fraying. If you need that item to be slightly smaller, you can sew a line parallel to the existing seam to make it more snug/snugger (not sure the grammatically correct word here but snugger did not seem right, sounds more like the sound a dog would make when getting a belly rub). In theory, just by following the existing seam but stitching a little bit into the meat of the item will make it smaller. The result is a larger seam allowance than the original one. You can cut the original seam thread, but you don’t have to if the seam isn’t too big. Remember to always treat the seam edges to stop fraying.
Letting out: Cutting the thread that holds a seam together to let fabric be free or out. At least that sounds like the reason it’s called that. If there is a decent size seam allowance, letting out the seam and then resewing with a smaller seam allowance will automatically make the garment larger. Only slightly though, there is only so much that can be done with the original seam allowance.
Consistency is Key:
Keeping seam allowances the same size over the course of a project like a quilt is very important. Imagine having 14 seams to sew vertically across the front of a quilt.
If you want your quilt to measure a specific size, when cutting and sewing the fabric, precision and consistency matter. With 14 seams, even the smallest seam allowance error adds up!
If the pieces are cut to be sewn and measure exactly 10 1/2 inches across (1/4 inch Seam Allowance + 10 inch Final Size of Fabric + 1/4 inch Seam Allowance) you expect your quilt to be 150 inches wide (I like big blankets, I cannot lie. I also don’t ‘math’ so we are going with simple numbers).
If the seam allowance is off by +1/16 of an inch on all of the seams, you are left with 14 seams x 1/16 inch of a difference error. That’s almost a whole inch error at 14/16ths (I know you can reduce it to 7/8 if you insist, but using 14/16 helps with the concept) added to the width. Small mistakes get amplified if there are a lot of seams.
Now imagine being off in the horizontal direction too, the quilt would be almost an inch longer too… Doesn’t seem like much, but a large quilt with 14 seams across is not super detailed. A more intricate quilt with 3X that many seams would be WAY OFF. Not LOL.
Will the same seam allowance be used throughout a pattern?
Usually, patterns use the same size seam allowance throughout and tell you that early in the pattern directions. Sometimes seam allowances are adjust based on the amount of tension on the seam or if multiple pieces connect at the same intersection. Mostly for the reasons listed in the previous section called “How much of a seam allowance?”
When do I trim back a seam allowance?
Trimming back a seam allowance is mostly done when there will be too much bulk when the item is right side out and pressed nicely. Corners like the tips of collars typically get trimmed back, that’s a lot of fabric to hang out inside an acute angle. Too close for comfort! Use very sharp scissors so you have complete control, longer cuts with a medium length scissor is probably best. Little trimming scissors are more likely to leave a crooked cut edge and huge shears might not be easy enough to maneuver.
90° Corners and Obtuse Angled Corners
Acute Angle Corners
Patterns include all different types of written instructions when it comes to how to cut out the fabric and which way to lay the pattern on the fabric. But there are more details that need to be transferred to the fabric such as darts.
Prevent Fabric from Fraying
What about the absolute edges of the fabric? Won’t they fray? Most likely, YES. There are a few techniques that are helpful to stop the fray, but not practical for all skill levels. A beginner sewist will probably not have a serger, which is a special, separate machine used mainly for trimming and sewing edges at the same time. Remember I’m clumsy, so I haven’t explored this cutting and sewing at the same time experience, I like my fingers the way they are…
Options:
- Do nothing – if the fabric has an incredibly tight weave and the item will not be used much at all, don’t bother! An outfit for a doll that sits on a shelf or curtain topper that never moves don’t need anything extra.
- Pinking Shears – cuts a zigzag pattern on the edge of fabric which stops it from fraying, weird huh? Can be expensive to buy a decent pair of pinking shears but once you have them, no need to buy again (unless you have a scissor thief in your midst who uses them on paper)
- Fray Check – good for super small projects where using pinking shears or another row of stitching would be a pain, it is expensive & consumable – you have to keep buying it
- ZigZag stitch on sewing machine – right after sewing your seam, take the same seam allowance and run through the machine with a zigzag stitch at the very edge. Test beforehand on scrap fabric, like everything else, the tension, width, length of the zigzag stitch matters!
- Overlock stitch on sewing machine – Some more advanced sewing machines have an overlock stitch that stops the fray. It’s a glorified zigzag stitch that is a little more professional looking than a zigzag.
- Serger – cuts and sews seam at the same time, what the professionals use. Separate machine that costs bucks, has a big learning curve and has fast moving, cutting parts… A warning to those who are clumsy like me but wish to keep their digits intact.
DISCLOSURE: THIS POST/PAGE MAY CONTAIN AFFILIATE LINKS, MEANING I MAY GET A COMMISSION IF YOU DECIDE TO MAKE A PURCHASE THROUGH MY LINKS, AT NO COST TO YOU. PLEASE READ MY DISCLOSURE PAGE FOR MORE INFO.
Anything else you can think of that I missed about seam allowances? Feel free to tell me with a comment!
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