How to Add Seam Allowance to a Sewing Pattern
Sewing patterns don’t always include seam allowance — and that’s not a mistake.
In fact, leaving seam allowance off can give you more control over fit, adjustments, and finishing.
If you’ve ever wondered:
- What seam allowance should I use?
- How do I add it evenly?
- What tools do I need?
- What happens if I forget?
This guide will walk you through it calmly and clearly.
What Is Seam Allowance?
Seam allowance is the extra fabric between your stitching line and the raw edge of your fabric.
The stitching line is where your garment is designed to fit correctly.
The seam allowance is simply the space outside of that line that allows you to sew the pieces together.
Why Some Patterns Don’t Include Seam Allowance
There are a few reasons:
- It allows you to choose your preferred amount
- It makes pattern adjustments easier
- It’s common in professional drafting
- It gives you more flexibility when fitting
For my patterns:
- Beginner patterns include seam allowance
- Some advanced patterns require you to add your own
And this is always clearly stated in the guide.
What Seam Allowance Should You Use?
As a standard, I recommend:
- 1cm for most seams
- 2cm if you’d like extra fitting room
You may also choose different seam allowances depending on the seam type (for example, hems or French seams).
How to Add Seam Allowance (Step-by-Step)
What You’ll Need:
- A ruler
- Tailor's chalk or heat-erasable pen
- A set square or seam gauge (optional but helpful)
1. Pin the pattern onto your fabric
Position your pattern piece correctly on the fabric, following the grainline markings as instructed in your guide.
2. Measure outwards from the pattern edge
Using a ruler or seam gauge, measure outwards from the edge of the pattern piece by your chosen seam allowance.
As a reminder, I recommend:
- 1cm for most seams
- 2cm if you’d like a little extra wiggle room
3. Mark evenly all the way around
Using tailor’s chalk or a heat-erasable pen, make small, consistent marks around the entire edge of the pattern piece.
Keep your spacing even — accuracy here matters.
4. Connect the marks smoothly
Join your markings with a smooth, continuous line.
This outer line becomes your cutting line.
5. Cut along the outer line
Cut carefully along the new outer line — this is your seam allowance included.
Around Curves
Curves need a little extra care.
- Add small dots at regular intervals around curved edges.
- Blend the dots into a smooth, flowing line.
- Avoid creating sharp corners on necklines and armholes; keep everything soft and rounded.
Taking your time here will help your finished piece sit beautifully.
Notches & Markings
After adding seam allowance:
- Transfer notches and guide markings carefully.
- Keep grainlines exactly as drafted; these do not change.
Accuracy in marking will make assembly much easier later.
Common Mistakes to Avoid
- Printing at the wrong scale before adding seam allowance
- Forgetting to add seam allowance to one piece
- Adding seam allowance to fold edges
- Adding seam allowance twice
There’s no “right” seam allowance.
There’s only what works for you.
Adding your own seam allowance gives you flexibility and confidence — and once you’ve done it a few times, it becomes second nature.
