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What Is Seam Allowance? (And How Is It Used?)

When we sew fabric, whether as clothing or items for the home, we create a line of stitching which is known as the ‘stitch line’. This stitching cannot be sewn right on the raw edge of the fabric because the stitches would slip off.

Instead, we sew a little way in from the raw edge of the fabric, and the difference created between the raw edge and the stitched line is then known as the seam allowance.

What is seam allowance?

Do You Sew On The Seam Allowance?

No. The seam allowance is used as an excess. If you needed to let something out, then you can definitely unpick the original seam line and then resew within the seam allowance amount, but typically, the seam allowance is just there to protect the fabric stitched line from a fraying raw edge.

How Much Seam Allowance Should You Use?

If using a commercial sewing pattern, then you should always use the amount of seam allowance recommended by your sewing pattern instructions. This is because the individual pattern pieces have had seam allowance added, but the amounts could vary, and if you decide to use a fixed allowance that you personally prefer, your final garment will not come out as intended.

I personally prefer to use 1cm as my ‘standard’ seam allowance amount, but there are few pattern companies that use this smaller amount. The standard in the home sewing industry is 5/8″ / 1.5cm for most seams, so I inevitably end up trimming seams down after reading through the instructions of course!

Differing Seam Allowance Amounts

While it is important to go with what the sewing pattern recommends, you also need to consider:

  • The type of fabric being used
  • The type of garment being created
  • How the garment is being constructed

Fabric Type

The type of fabric that you are using will impact on the amount of seam allowance that should be used. Sheer chiffon or tulle fabric types and heavy weight wool or canvas fabrics will require different seam amounts. Always consider the fabric type and weight before deciding on your seam allowance.

Garment Type

Another consideration is the garment type. Garments that may need altering, like dresses and pants, should have a little more seam allowance available to allow for letting out at the waist while shirts and coats will need less as these are unlikely to need extra space being quite loose fitting garments.

Construction Method

The final consideration is the construction method that will be used. Different seams will require different seam allowance amounts – a plain seam requires less than a felled or French seam!

Different Seam Allowance Amounts

While I do for the most part follow the guidance of purchased patterns, when I am drafting patterns, whether for myself or other people, I use 1cm on the majority of woven seams, 5-7mm on facing seams (which helps to prevents bulk), 7mm on knit seams (standard serge / overlocker amount) and then a variety of amounts for specific seam finishings such as lapped, felled and French and garment section (collars, cuffs etc).

Why Such Variations?

The reason for such variations is simple

  1. My standard 1cm keeps things nice and simple – consistency is key for me to be an efficient sewer.
  2. 5-7mm on facing seams helps me to keep the finish clean. Especially if it is a curved seam. A smaller allowance helps it to lie flat.
  3. The amount of seam allowance for knit fabrics is industry standard, as that is the ‘built-in’ allowance on industrial overlockers. The blade being positioned to cut away anything greater. There is also the baby lock machine which can finish a seam with just 3mm allowance!
  4. Garment closures. Things like zippers require a varying amount of seam allowance and it is the width of the zipper tape often decides this for you!
  5. The variations required for specific seams will vary also greatly, depending on the look you are going for. A felled seam could be anything from a very narrow 5mm finished to multiple centimetres wide when finished!

Hem Allowance Vs Seam Allowance

Some people will refer to the allowance at the bottom of a pattern piece as a seam allowance, even though it is quite clearly a hem. So what’s the difference between a hem allowance and a seam allowance?

While seam allowance is the difference between the raw fabric edge and the stitched seam line – seam being the important word here! – the hem allowance is the difference between the raw edge of the fabric and the hem.

Consider a tailored jacket. The hem of the jacket and the sleeves are created as folds of fabric, which is pressed for a neat finish. The fabric on the inside of the jacket and the sleeve is known as the hem allowance, and the standard amount is 1 5/8″ / 4cm. This hem allowance is then finished with:

  • Binding
  • Serged / overlocked
  • Attached to a lining

The reason the hem allowance is a greater amount than a seam allowance is to allow lining to be attached neatly and not be seen from the right side.

If a hem allowance was just the standard 1.5cm used in home sewing patterns, by the time the lining is attached, it will often hang down below the folded edge and spoil the garment.

How Much Seam Allowance Should I Add To My Patterns?

If you are creating your own patterns, you have free reign over how much seam allowance you should add, though there are some ‘standards’ to bare in mind!

Collars

When it comes to collars, less is more, though you don’t want to go smaller than 3/16″ / 5mm unless it’s trimming away excess fabric at the collar point.

I do stick to 3/8″ / 1cm for the collar edge which joins to the bodice neckline.

Sleeves

For the most part, 3/8″ / 1cm will be plenty of allowance! If you were using a heavier weight fabric and setting in a sleeve, you could even use 5/8″ / 1.5cm to get two lines of stitching in position for gathering, and then trim away afterwards if desired.

Shoulder, Side, Waist, CF, CB and Inseams

For all of the above seams, I use a standard 3/8″ / 1cm unless the seam finishing or fabric choice calls for a different seam allowance amount.

Seam Allowance Examples

As mentioned before, the way you plan to finish your garment will also affect the way you create your pattern. To help you understand the important of the seam allowance amount, I’ve pulled some examples!

Isabel Marant Tailored Jacket

Isabel Marant jacket

A tailored jacket typically has a 4cm hem allowance on the hem of the jacket and on the sleeves. This hem allowance will usually need to be interfaced to help keep that line crisp and offers longer garment life which means you also need to create interfacing patterns to use when cutting out your interfacings.

Interfacing patterns in ready to wear garments are different from those used in home sewing patterns!

Then there is the front facing so that when the jacket is partially open, you see the facing fabric rather than any lining. This front facing needs a separate pattern piece too, and will have seam allowance, not hem allowance, included.

To hide the inner construction of the jacket, there will need to be lining to keep it all hidden. Now you need to create a lining pattern.

From this little break down we know that once our block had been developed into a jacket pattern, we need to create facing, lining and interfacing patterns and that the way we seam these elements together impacts on the seam allowances we will use.

Karl Lagerfeld Jacket

Lagerfeld denim jacket

This denim and tweed jacket uses a raw hem for the neckline, rather than a finished edge.

Felled seams mean differing seam allowances on each of the felled sections. The shoulder seam and front yoke fells look to be wider than the armhole fell, so these will have different seam allowance amounts.

The tweed section will probably have been interfaced to give it more structure – it doesn’t appear to drape from the seam with the denim, rather it co-exists with the structure of the denim.

The denim button stand will need to have been interfaced to support the poppers, the seam allowance would have needed to be rather narrow so that there wasn’t added bulk in the two edged of the stand, and the seam allowances of both the button stand and the boucle will have been graded to keep the stand non-bulky!

Final Seam Allowance Thoughts

As you’ve learned, the amount of seam allowance you choose to use with existing sewing patterns or those patterns you are creating yourself will differ a lot. Make sure to consider the fabric type you’re planning to sew and the seam construction method before adding seam allowance to patterns, but most of all, enjoy the process!

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