How To Add Seam Allowance To Sewing Patterns {VIDEO TUTORIAL}

I love sewing, and I love the ease of having seam allowance included on patterns – it just makes sewing something new less of a faff. However, not all sewing pattern companies include an allowance, and sometimes, the amount added is less or more than what you want to use.

Which makes knowing how to add seam allowance to sewing patterns a rather important skill.

In this step by step tutorial – the video is towards the bottom of the page! – I’ll show you how to add on your chosen seam allowance amount using a simple pattern making ruler.

The first thing you need to decide is how much seam allowance you need to add. I personally prefer 1 cm but there are a few seams that require more allowance – the flat felled seam is a great example. And then there are the different garment sections.

I would never add more than 0.5 cm to the top edge of a collar stand or the corners of a collar, because it adds too much bulk to these seams, preventing a nice crisp finish.

You’ll also need to consider the type of fabric you’ll be sewing with, as this has an impact on your chosen seam allowance amount too!

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How To Add Seam Allowance To Sewing Patterns

Whether you are working with commercial sewing patterns or making your own patterns, seam allowance is something that will always come up, as it is an integral part of sewing most seams in sewing.

Here are some answers to your common questions about seam allowances, and a tutorial on adding seam allowance to sewing patterns yourself!

What Is Seam Allowance?

Seam allowance is the amount of fabric outside of the intended stitch line on sewing pattern pieces. It would be a struggle to sew a seam on woven or knitted fabrics without a seam allowance, as that extra fabric ‘allowance’ helps to prevent the seam from coming undone.

Do Seam Allowance Amounts Change?

Absolutely. There is a standard seam allowance in the sewing pattern industry of 1.5cm / ⅝ inch. But, seam allowance amounts can vary a lot depending on the country of origin for the sewing pattern company and even the sewing pattern piece itself. I’ve seen seam allowance amounts from as little as 3mm to as much as 2.5cm!

How Do I Know How Much Seam Allowance To Use?

If you’re creating your own sewing patterns, you’re free to decide how much seam allowance you would like to add. I personally use 1cm on regular plain seams, 1.5-2cm on flat felled seams and between 6-10mm on French seams. These amounts also change depending on the pattern piece I’m adding seam allowance to.

Can I Use A Small Seam Allowance Amount?

Smaller amounts of seam allowance can make it harder for anyone new to sewing to get a neat finish. That said, small seam allowance amounts are perfect for those areas that need less bulk – such as the points of collars!

Another consideration is that if your seam allowance is too small, you won’t be able to ‘let out’ your garment at a later date. This is another reason sewing pattern companies tend to add 1.5cm seam allowance to their main pattern pieces.

How Do I Stick To A Seam Allowance Amount When Sewing?

If you’re worried about going ‘off’ when sewing, you can use either:

Sewing machine seam allowance guide
Magnet guide placed on sewing machine

Both of these will help you to sew a consistent seam allowance amount, and this a more in-depth post on using a magnetic sewing guide.

How Do I Add Seam Allowance To A Pattern?

Adding seam allowance to a pattern is a quick and easy job, and there are several reasons for wanting to:

You’re more able to ‘let out’ your hand sewn garment later on
If the original pattern didn’t have seam allowance added in (Style Arc patterns often don’t!)
You want to make the pattern a little bigger in one area but have no idea how to grade it
You want to use a different seam allowance amount than what has been allocated by the sewing pattern company

Whichever your reason for wanting to add seam allowance to your sewing pattern, the following directions will help!

Tools Needed To Add Seam Allowance

Shoben fashion curve - pattern making tools!

To add seam allowance, you will need some specific tools for the job.

It is essential that this task is done on a flat surface – a carpeted floor will not work, as the pencil is likely to go through the paper when marking in your seam allowance amount.

Adding Seam Allowance To A Pattern

Step 1: Trace off the original pattern piece

I always trace off my pattern pieces when making changes. It helps to preserve the original, in case mistakes are made or you change your mind.
Make sure the paper you are tracing onto is bigger than the pattern piece, and trace along the actual stitching lines, not the pattern piece edge (which often has seam allowance added already).

Step 2: Decide your seam allowance amount

Consider the pattern piece in question. If it is a bodice section, then a larger seam allowance amount would be beneficial. If it is a collar pattern piece, then a smaller amount is suitable. You also need to consider if there are construction techniques which require larger seam allowances.

Some examples are:

Flat felled seams
Lapped seams
Zippers (dependant on the zipper tape width)

I’m going to go with 1 cm myself on this bodice pattern.

Step 3: Marking in the seam allowance

Using your chosen seam allowance ruler, mark in the seam allowance all the way around the key lines. If you have a dart in the pattern piece, ignore that for now, I have instructions in the next step on adding seam allowance to darts!

Don’t forget that you may need varying amounts of seam allowance within the one pattern piece.

Step 4: Adding Seam Allowance to Darts

This is the fun part, and the reason we added pins and a tracing wheel to your list of tools! Decide which direction your finished dart will sit. Here’s a guide:

Vertical darts on the front of of the body: towards the centre front
Vertical darts on the back of the body: towards the centre back
Horizontal darts anywhere on the body: down towards the floor

Now that you know which direction your dart will need to face, create a fold line along both dart legs, and bring them together to create a third fold line.

Make sure the excess paper is laying in the right direction, and pin into place to secure. The original seam allowance line up to the dart start and finish should now align.

Take your cutting matt or a piece of protective card, and place the pattern piece on top. It will not lay flat because we have created the dart, and that is ok.

Try to invert the dart, so that the dart opening is flat on the cutting mat.

Next, take the tracing wheel and trace over the seam allowance line you pencilled in during step 3. Make sure the markings transfer to the different layers of paper.

Remove the pins from the dart, and smooth the paper flat. You should now see a clear ‘design’ to your seam allowance which allows your dart to be sewn and lunges perfectly! Pencil this in, and make a note of the direction that the dart it to be sewn.

Step 5: True And Cut Out The Pattern Pieces

Now that the seam allowance is complete on the pattern piece, you’ll want to ‘true it’ – which means to check it against its matching pattern piece.

And if it matches (sometimes there will be ease added and that is normal) you’re ready to cut out your pattern pieces with their new seam allowance!

Seam Allowance questions

A few other questions that pop up when it comes to seam allowances.

You Forgot To Add Seam Allowance To Cut Fabric

If you were sewing without a pattern or forgot to add seam allowance to your pattern and have already cut the pattern in fabric, you’re a little stuck. Your best option is to go ahead and sew the item, using less than the recommended allowance. The item will still come out smaller than the size you intended but it won’t be as small as if you were to see with the fill allowance amount.

Your other option is to find away to add more fabric into the garment – you’ll want to work out how many seams around the bods there are that do not have allowance added, so that you know how much smaller the pattern will be. Then, find a way to add that amount back in.

You could potentially use:

Godets
Gussets

Should I Neaten My Seam Allowance?

Ways to neaten your seam allowance include:

Overlocking / serging the raw edges before sewing any seams
Zig zag stitching along the edge of the fabric allowance before sewing your seam
Pinking the fabric edge ones a seam has been sewn
Grading the seam allowance to reduce bulk once the seam has been sewn

And some seams are enclosed, so are neat anyway:

The way you finish your fabric edge is up to you!

Video Tutorial On How To Add Seam Allowance To A Pattern

How do you feel about adding seam allowance to patterns now? It’s less overwhelming than you thought right? Remember, you can always bookmark this page and ask me in the comments if you get stuck!