All About Seam Allowances: Life on the Edge

  • By: joleenllorence
  • Date: October 1, 2022
  • Time to read: 7 min.

You probably have a favorite seam allowance. (Yes, that’s the kind of blog this is – favorite seam allowances. Life on the edge, indeed).

What it is depends on many things, most likely the type of sewing you first learnt, or the type you do most often. And if you switch to a different type of project you may find yourself wanting to stick to what you know or, worse, having to confront (gasp!) change.

I started off sewing clothing, with its generous 16mm (5/8″) seam allowances and got a bit of a shock when I tackled my first quilting project – 6mm (1/4”) seam? Are they mad? How can anyone work with so little?

So, are they mad? Let’s think about what kind of seam allowances people use, why and what that tells us about the best choice.

When can you change the seam allowances?

It took me quite a long time to realize this, but the truth is – there’s no seam allowance police waiting to arrest you if you dare to change the seam allowances.

You can use whatever seam allowance you prefer.

The purpose of a seam is simply to join two pieces of fabric without exposing the raw edges. The seam allowance is just the extra fabric so the stitching isn’t too close to the raw edges and doesn’t come undone. So what seam allowance is best? First, let’s think about why different patterns have different seam allowances.

Some Typical Seam Allowances

Patchwork typically uses 1/4″ seams.

The size of the seam allowance can vary wildly depending on type of sewing, purpose and designer (especially since the internet has enabled so many independent designers to create patterns). But here are some typical sizes:

3mm (1/8″)Tiny seams for tiny pieces – I’ve seen this occasionally for rag dolls or dolls clothing. Often it’s sewn at 1/4″ and then cut back to 1/8″, but I have seen it sewn at 1/8″ too. This is used in particular for small fiddly pieces that require turning eg fingers, so you need to reduce the bulk as much as possible.
6mm (1/4″)Patchwork is of course the home of the 1/4″ seam. Typically you’re working with nicely-behaved, thin but not flimsy, cottons that don’t unravel too much, plus sewing straight seams. You also want to minimise wastage when sewing lots of small pieces together. Soft toys and dolls often use 1/4″ (more commonly than 1/8″). Small intricate pieces with minimal bulk when turned. I have sometimes seen purses or bags sewn at 1/4″. These generally tend to be sewn in quilting cottons rather than bulkier fabrics (so all the points about patchwork apply) and tend to be smaller simpler styles.
1-1.3cm (3/8″-1/2″)Metric patterns tend to use 1cm; imperial or US patterns use 1/2″, which is slightly larger. Many bag patterns use this allowance, especially larger, more solid structured bags with interfacing. This is suited to the thicker fabrics and multiple layers when 6mm (1/4″) would get lost. Also common among independent clothing designers, rather than the commercially used 16mm (5/8″), especially for looser-fitting clothing such as elasticated waists, or for children’s clothes. These designs tend to be in lightweight dressmaking fabrics, rather than heavy tailoring fabrics. Finally, this is the most common choice for home decorating projects such as cushions, pillows, slipcovers and bedding, when fabrics may be heavier, but seams are typically straight. They may also include piping in the seams.
16mm (5/8″)The commercial standard for clothing pattern companies (Vogue, McCalls, Butterick, Simplicity). This wider seam allowance allows room for alteration to get a good fit. It’s also suitable for a wide range of fabrics, including less stable or loose weave fabrics that may unravel. Some pattern pieces may be cut on the bias, so the seams end up narrower once cut.
Larger seam allowancesHaute couture can use even larger seam allowances. Again this is to allow for perfecting the fit, dealing with troublesome fabrics, allowing extra for bias seams and extra for special seam finishings. For very small fiddly pieces, such as dolls of soft toys, sewing two identical pieces together is sometimes done before cutting out the piece at all, and then the seam allowance cut back afterwards.

What’s the Best Seam Allowance?

So the seam allowance, then, depends on the following:

1. The type of fabric you’re working with. Use larger seam allowances for:

  • heavy or bulky fabrics
  • fabrics that unravel easily
  • unstable fabrics (eg slippery satins) or pieces that are cut on the bias
  • multiple layers

2. The type, shape and size of pattern pieces:

  • Straight seams are easier to deal with – you can get away with smaller seam allowances
  • Very small pieces or details may need small allowances to remove bulk, especially if pieces are to be turned (eg doll’s fingers)
  • Smaller seam allowances can help manipulate tricky pieces, for example sewing a curved concave piece to a convex piece.
  • You can use larger seam allowances on awkward shapes, or difficult-to-pin pieces and then cut back later.

3. The need for adjustments:

  • if you’re fitting as you go along (the obvious example is clothing, but it could also apply if you’re, say, making a case that needs to fit a particular item), allow extra room for adjusting the seams
  • Likewise, if you need to check that pieces fit together (for example if you’ve changed things), allow extra
  • If you know pieces fit together well (eg you’ve sewn it before) you can reduce larger seam allowances

French seams in a bag. Pattern seam allowance 15mm (5/8″); finished seam 10mm (just under 1/2″).

4. The type of seam finish:

  • some finished may need larger seam allowances eg French seams

5. Personal preference:

  • This is the last on the list – use the criteria above first, and don’t reduce the seam allowances too small
  • what you’re used to
  • what you’re comfortable with

So when should you change the seam allowance on a given pattern?

  • If you’re using a different fabric to the one recommended.
  • If you’ve changed the size, especially if it’s much smaller and you’ll need to reduce bulk.
  • Use a larger seam allowance if you might need to adjust the seams. Or use a smaller if you know the measurements are correct.
  • If you think the designer has made a poor choice taking into account the considerations above.
  • If you just prefer something different (although don’t go smaller than needed to satisfy other criteria).

Changing or Adding Seam Allowances

1. Determine the existing seam allowance. First you should check your pattern carefully to see what, if any, seam allowance is used. Read carefully; sometimes patterns use different seam allowances for different pieces. Some patterns provide pieces without any seam allowances at all.

Three pattern pieces: no seam allowances, cutting and stitching lines shown, cutting line only

2. Remove seam allowance from pattern pieces If the pattern doesn’t include any seam allowance, you can skip this step. If the seam allowance is marked on the pattern piece already (I always mark the stitching line on my patterns) then trace the stitching line.

If the pattern pieces does not show the stitching line, you’ll need to draw it on first. Measure the distance of the seam allowance inside the cutting line at several points and join them up.

Measure in from the edges to find the stitching line.

3. Add new seam allowance to pattern pieces Go round the stitching line, marking the new seam allowance at regular intervals, and then join up the points. Some people like to just eyeball this, but I prefer to measure as it’s more accurate, especially if you have lots of small intricate pieces.

Measure out from the stitching line to find the new cutting line.

4. Note the new seam allowance!Finally, remember to make a note somewhere about your new seam allowance (for example, on the new pattern piece).If you pack away, then when you return you’re liable to forget and start sewing the original seam allowance. This is particularly a danger if you’ve changed away from your preferred allowance and are sewing on autopilot.

If you’re using larger seam allowances then it’s tempting to use the existing pattern pieces and just cut the fabric with a little extra room all round. This never works for me – I always manage to forget and cut along the actual pattern piece on at least one portion. The chance of messing up increases with the number and complexity of pattern pieces!


I always work in metric when sewing, as I can’t be bothered keeping track of fractions.

(It’s a myth that everyone in the UK is metric – it’s far more complicated than that. We use a mixture of different measurements depending on age, type of measurement, scale, where we learnt… I cook in pounds and ounces, check the weather in centigrade, travel in miles, estimate small distances in millimeters – and use centimeters, meters, feet and yards interchangeably for intermediate distances, depending on my mood. The standard warning about not mixing measurements is meant for me!)

For the record, unless I’m sewing clothing when I go for the standard 16mm (5/8″) allowance, I like 1 cm best. Small enough that you don’t feel you’re wasting fabric, but large enough to allow for a bit of imprecise cutting, ravelly fabrics and tweaking if necessary. I’ll use 1/4″ seams on patchwork projects because there are so many seams and anything larger results in a lot of wastage, but it always makes me a little nervous…

So go on, you know you want to. What’s your favorite seam allowance?

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