Pattern Making Fundamentals: Seam Allowance

When taken back to it’s most basic level, sewing is the means by which we have clothed ourselves for thousands of years. According to wikipedia the act of sewing dates back to the stone age, where even before there was woven fabric humans would sew together fur and skin using handmade needles and threads.

The join or stitching line where one fabric meets the other is called a seam, and although seams can now be sewn by hand or sewing machine and finished in different ways, the basic principle remains the same – two pieces of fabric are laid on top of each other and a needle and thread goes back and forth along a line, joining the two together. When pattern making it is always important to remember to allow space for the seam, as you can’t sew right on the edge of the cut fabric or else it will fray. This is what is known as the seam allowance

On patterns the seam allowance is most often marked by a small notch which indicates how far from the edge the stitching line is to be sewn. On patterns for home use the seam allowances are sometimes unfortunately not marked on the pattern, leaving the sewer to rely on the make instructions.

In reference to the picture above:

  • If the centre front seam has seam allowance of 1cm, then a notch is marked 1cm from the centre front edge.
  • The notch placement will be marked first in pen or pencil and then cut out using a special tool called a pattern notcher.
  • You only need to notch into the pattern about 0.5cm, using the pattern notch tool.
  • You should mark stitching lines for all seams and hem etc. but each corner should only be notched once. If you notch a corner from both sides then it will weaken the paper or cardboard and will eventually rip out altogether.
  • When choosing which seam to notch, notch the seam which will be sewn first.
  • Notches are an aid for sewing the garment correctly later so when in doubt think where the notch will make the most sense to you when it is marked onto the fabric.

Seam allowances can vary from a small amount (6mm) up to a larger amount (1.5cm) where room may have been left to make adjustments. In haute couture sewing some reference books will explain that the seam allowances are much larger then 1.5cm to allow space for fittings, and to allow for alterations later on and the same is true for some areas of tailored suits. For more commercial garments the seam allowance will be at the minimum amount required in order to get the most out of the fabric.

Here are some general rules for seam allowances:

  • If a seam allowance is too large when used around a curved area then it will become too cumbersome to manage and may cause seams to be hard to ease together.
  • Larger seam allowances can be trimmed or slightly cut into in order to better fit around curved seams.
  • Seams which need to curve around areas of the body such as collars and armholes are normally smaller, about 6mm.
  • Seams which require a special finish are normally larger (1.2 – 1.5cm). This would be for seams that require a french seam, a flat felled shirt seam, a bias binding etc.
  • You will need to allow more seam allowance where you have a closure such as a button placket or a zip. You may need to leave about 2cm seam allowance for a zip.

Pattern images by The Cutting Class»

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