Seam allowance & How to get the seam line right every time

What is a Seam Allowance?

Seam allowance is the distance between the seam stitching line and the cut edge of the fabric. The most common seam allowance used when joining fabric pieces together are 1/4 inch, 1/2 inch and 5/8 inch.

In every sewing tutorial/book, there is mention of Seam allowance, with emphasis on its importance in sewing. But a beginner sewist may wonder at the fuss. Is seam allowance that important? Why don’t you just leave some fabric from the stitching line, and leave it at that? No, you cannot do that & Yes, it is very important.

Seam allowance is there for a reason.

• You want enough space between the cut edge and the stitching line to take into account fraying of cloth;
• You want enough fabric when you have to press the seam open;
• You need enough fabric on the edge when fabric pieces with seam allowances are joined together so that the front of the fabric look seamless after you have pressed the seam open;
• You also need enough seam allowance to finish the edges of the cut fabric.

In flat felled seam, french seams and other attractive seam finishes, you need to have enough seam allowance to lie inside the fabric folds. You need a consistent seam allowance so that your seam line does not look wonky and ill-fitting.

Related posts :  How to sew french seam

When you are sewing garments or quilts following readymade store-bought patterns it becomes very important that you follow the seam allowance mentioned in the pattern; otherwise, you may end up with seamlines that do not match.

You want all pattern pieces to match, don’t you?. When you are sewing on your own, you can adjust seam allowance the way you want, but it is a good idea to keep it consistent through out the project. 

What are the usually used seam allowances ?

The way you are stitching the seam is one important decisor in the seam allowance you use. Checkout the different methods of sewing seams – there are about 17 ways.

Some seams are better for particular projects than other. Choose the right one for you and this will decide on the correct seam allowance you should be using .

For eg. a french seam will call for a different seam allowance than a butt seam.

In garments, I usually leave a seam allowance of 1/2 inch for straight seams and 1/4 inch for curves, like armholes and collars. In commercial patterns, the given seam allowance is usually 5/8 inch. This is one mark more than 1/2 inch on the tape measure.

Checkout the post on reading tape measure if you do not know how to.

In quilting or piecing fabric pieces for patchwork, a seam allowance of 1/4 inch is common; at times even a 1/8 inch is used. When making home decor accessories a wider seam allowance is used like 3/4 ” or 1″.

Sometimes the seam allowance is deliberately left more than on the pattern to account for weight / size increase, say for a baby frock. A large seam allowance is deliberately used especially when sewing rather tight clothes – you may need to let out some or to account for the fabric shrinkage. When sewing with easily fraying fabric it is necessary to leave extra in case your seam allowance gives away .

It is essential that the edges of the seam allowance are finished; this is one of the basic principles when you want your sewing to look professional. You can use a serger to finish the edges or the ordinary zig zag stitches on your sewing machines to finish the edges.

Checkout the other fabric edge finishes here. 

How to get the seam line straight and accurate

The seam allowance and its accuracy is especially important when you are matching seam points and sewing – like you do when making a quilt or joining pieces in making garments.

In quilting it is very important that the seams match so that the pattern pieces line up.

On most sewing machines, the throat plate under the needle usually has markings for the seam allowance. You only need to keep the fabric edge on the necessary marking say 1/2 inch and your machine will sew the seam line with the 1/2 inch seam allowance. This is a good way to keep your stitching line straight.

You will have to ensure that the measurement is correct on your machine, though (on some machines it is not). 

A 1/4 inch foot ( also called quarter inch foot or patchwork foot) is a great help in getting the seam allowance right. It has a special edge which makes sure that you sew with 1/4 inch seam allowance, at all times.

Learn more about the 1/4 inch seam presser foot here

Another presser foot which is great for keeping accurate seam allowance consistently is an edge stitching foot.

The walking foot has a guide bar to sew. Learn more about a walking foot here.

Then there are the screw-down or snap on seam guides, which ensures your seam line do not stray. Some clever souls even use rubberbands circling their machine bed to make the marker where they want.

Another DIY idea -Measure 1/4 inch to the left of your sewing machine needle. (or any other seam allowance -If you are always sewing with a particular seam allowance, use that). Take a thick tape/a stack of post-it notes and keeps it on that point. It will serve as an abutting line to keep your fabric aligned, at all times. You can align the edge of your fabric along with this guide you have attached and can sew with the exact seam allowance you want continuously.

I do this when I sew patchwork pieces, where seam allowance accuracy is important. 

This is especially useful in quilting because you allow a scant 1/4 inch for seam allowances. The scant 1/4 inch seam allowance is used because it is just inside of the 1/4 inch seam line – one or two threads outside of the 1/4 inch mark ( because if you sew exactly on 1/4 inch line the seam allowance will be slightly more because of the thread of stitching and fold of cloth)

What I personally use when sewing a 1/4 inch seam is a simple trick. Keep the zig-zag pressure foot on the edge of the fabric as in the picture below. I get a 1/4 inch seam allowance all the time (because it is 1/2 inch wide). To tell you the truth I have not tried this with any other pressure foot than mine. Why don’t you try it yourself?.

Do you use a zig-zag needle plate and the zig-zag pressure foot all the time because it is convenient ( I do) – you are right  you can sew zig zag and other decorative stitches as well as straight on this plate without changing the plates.

But it is not the best needle plate for a straight seam. After all there is the straight needle plate and straight sewing pressure foot for a reason. You get a straight seam line more accurately with even seam allowance if you use that. 

Ofcourse do not use the straight stitch plate and pressure foot when you have to make a zig zag seamline like you sometimes have to especially for sewing knit fabric. 

Then when you have done your sewing, you can use your tape measure or ruler to measure the accuracy of your seams. Simple. There are sophisticated seam guides for that but I like simple. If the seam allowance is less, stitch the correct one. If it is more you may have to take out your seam ripper. 

Trimming the seam allowance

Sometimes the seam allowance is reduced / trimmed after the seam is done. Like when you sew corners, curves, sheer clothes. Making snips (small portions of the seam allowance is cut out) or deep cuts till the stitching line are made along the seam allowance) may be necessary for the seam allowance to behave, especially when sewing steep curves.

Grading, which is cutting one seam allowance less wide than the other one may be necessary at times, like when sewing facings. This is done when the seam lies very close to the body.

In grading, you trim one seam allowance first and then the other seam allowance is trimmed about 1/8″ even more than the first one. This shorter one is usually the one closest to the body.

How to do Grading on your seam allowance

Grading means trimming the standard seam allowance into layers so that there is less bulk. This will give a smoother finish from the right side. It gets rid of the bulky seams and gives you crisp and flat seams.

You should grade seams of thick fabrics, seams that will be pressed together and kept like that instead of pressed open. You can also grade seam allowances when seams need to be kept in between other layers of fabrics and then stitched.

You can trim the seam allowance on top close to the seam stitching (about 1/8 inch close to the stitching line) The other layer of seam allowance can also be trimmed – trim this a little less (about 1/4 inches or so)

When sewing facing another manipulation called understitching is done so that the facing rolls smoothly to the back of the fabric. Understitching refers to stitching the seam allowance to the facing close to the seam line.

Pressing the seam allowance

This is a very important step in making your seam look good and professionally stitched. Most home made clothes look homemade because sewists forget to press as they sew (guilty )

Most of the time in making garments the seam allowance is pressed open. Do not iron over the seam allowance. Ironing with a hot iron in a back-and-forth motion will stretch the seam line out of shape. The correct way is to gently lower and take up the iron from the seam allowance. Infact it is a good idea to press the seam allowance  flat to one side of the seam line before sewing the seam allowance open. This will set the thread and your seam line will become strong and look better.

Always remember to press the seam allowance open for every single seam line, rather than wait for the end when all the pieces are joined and then press. Press as you sew is the general (intelligent) idea. 

When you are sewing two fabric pieces together and one is more revealing(sheer) than the other remember to press the seam allowance to the side of less sheer fabric. Otherwise the seam allowance will show through the sheer fabric; Catch the seam allowance in this pressed position for any side seams or seam joinings or bindings.

Another place you press to the side is when you have to stitch in the ditch. When you have to stitch in the ditch, the seam allowances pressed open can cause weakening of the seam line. In this case press the seam allowances to the side.

For quilting the seam allowances are pressed open and then pressed to one side rather than just pressed open. 

If you find yourself frustrated at making a straight seam line and getting wonky seam allowance you may need more practice in cutting fabric and sewing a straight line. 

If you are a beginner sewist, it is a good idea to practice stitching on straight lines on a fabric scrap. Drawing the stitching lines beforehand is important – then you know how ‘off’ you are from your line when you sew. Then practice practice and practice more.

With practice, you can eyeball the seam allowance and get the straight line – everything right every single time,  without any external aids.

Related posts :100+ sewing tips for learning to sew ;  Different types of seams ; How to sew a strong rip proof seam