A French seam is sewn twice on an ordinary sewing machine, so the raw edges end up sealed inside the seam. An overlocked seam is made in one pass: an overlocker trims the edge, stitches it and wraps it in thread.
Use a French seam on fine, non-stretch fabrics, especially sheer or unlined ones where the inside will show. Use an overlocked seam on jersey and other stretch fabrics, or when you want to get through everyday sewing quickly. You don't always have to pick one, because a single garment can use both.
What Is the Difference Between a French Seam and an Overlocked Seam?
The difference is where the raw edge goes. A French seam hides it inside a narrow channel of fabric, which is why Oak National Academy's textiles lesson describes it as a clean, enclosed seam for lightweight fabrics. An overlocked seam leaves the edge where it is and covers it in thread.
"Overlocked seam" can mean two things, so check which one a pattern or tutorial has in mind:
- A seam sewn entirely on the overlocker. The machine joins the fabric, trims the allowance and finishes the edge in one go. This is the usual route on knits.
- A sewing machine seam with overlocked edges. You stitch the seam as normal, then run the allowances through the overlocker to stop them fraying. This is the usual route on wovens.
Our guide to what an overlocker does explains the machine itself. The number of threads matters too: the University of Kentucky's guide to seams and seam finishes recommends a 3-thread seam where a knit needs give, and the more stable 4-thread seam for wovens and some knits.
Seeing the seams side-on shows why a French seam looks neat from both sides, while overlocking relies on thread to protect the edge.
Which Fabrics Suit Each Seam?
Stretch and weight decide most of it. A French seam suits fine wovens, and an overlocked seam suits anything that stretches.
The fabrics we'd French-seam first are the ones you can see through or that fray badly: chiffon, georgette, organza, voile, fine silk and cotton lawn. If the inside of the garment will show, that's where we'd start. It's worth deciding on the seam when you choose your dress fabrics, because a sheer fabric commits you to a neat inside.
The needle sizes below come from SCHMETZ's needle and fabric chart, and you'll find them in our range of Schmetz sewing machine needles.
| Fabric | Usual choice | Why | Needle for a French seam |
|---|---|---|---|
| Chiffon, organza | French seam | The seam shows through | Universal or Microtex 60 to 70 |
| Georgette | French seam | Frays and shows through | Universal or Microtex 60 to 80 |
| Voile | French seam | Lightweight and often unlined | Universal 60 to 80 |
| Fine silk | French seam | Edges fray and the inside may show | Microtex, from size 60 |
| Cotton poplin | Either | Depends on whether the inside shows | Microtex or Universal 60 to 80 |
| Linen, mid-weight cotton | Either | Check the bulk on a scrap first | Universal 70 to 90 |
| Jersey, ribbed knits | Overlocked seam | The seam needs to stretch | Not suitable |
| Denim, heavy wool | Not a French seam | Too bulky, so use a flat-felled seam or a plain seam with neatened edges | Not suitable |
Asking about stretch first stops you sewing a French seam on a fabric that needs to give.
How Do You Sew a French Seam on a 1.5cm Seam Allowance?
Split the 1.5cm between two lines of stitching: a 1cm seam first, then a 5mm seam.
- Stitch with wrong sides together. It feels backwards, but this is correct for a French seam. Sew 1cm from the edge.
- Press and trim. Press the seam flat, then trim the allowance to about 3mm.
- Fold and press. Fold along the seam so the right sides are together, with the stitching sitting exactly on the fold.
- Stitch again. Sew 5mm from the fold, sealing the raw edge inside.
- Press to one side. The right side now looks like a plain seam, and the inside shows a slim, enclosed ridge.
Stray threads poking through on the right side mean the first trim wasn't close enough. Unpick the second seam, trim a little closer and sew it again.
Check the allowance before you cut. If your pattern uses less than 1.5cm, add the difference to the cutting line. You can sew a French seam by hand, but a machine keeps long seams more even.
Fine fabrics can slip under the foot, so read our tips for sewing delicate fabric before you start, and our guide to stopping fabric puckering if the seam won't lie flat.
Most of the fabric goes into the first seam, which leaves a narrow second seam wide enough to hold the trimmed edge.
When Does a French Seam Beat an Overlocker?
We'd suggest a French seam in three situations:
- The inside will be seen. An unlined blouse or a sheer sleeve shows every seam, and an enclosed seam is tidy from both sides.
- Overlocking thread would show through. On a see-through fabric, a thread-wrapped edge can appear as a visible line from the right side.
- You only sew wovens now and then. A French seam needs nothing more than the straight stitch on your sewing machine, so a tidy inside doesn't have to mean buying a second machine.
Where Does a French Seam Let You Down?
It struggles with stretch, tight curves and bulk.
Stretch is the clearest limit. The seam is built from straight stitching, which can snap when jersey is pulled. If you don't own an overlocker, our guide to sewing stretch fabric without a serger covers the stitches that work.
Curves are less clear-cut. The University of Kentucky's guide says a French seam can follow a curve if you clip the enclosed allowance before pressing, but it suggests a mock French seam for the armhole of a sheer garment. That version starts as a plain seam: you turn in both allowance edges and stitch them together.
Bulk catches people out where seams cross, so test those junctions on a scrap. Alterations are another consideration. Both seams trim fabric away, so neither leaves much to let out, while a plain seam with neatened edges keeps more of the allowance.
Overlocking doesn't always look wrong on light fabric, either. The same guide lists some lightweight fabrics where a serged seam suits because it needs to stay inconspicuous.
Do You Still Need an Overlocker if You Can Sew French Seams?
That depends on whether you sew jersey.
This conversation does sometimes lead to an overlocker, but jersey usually decides it rather than French seams. If you regularly make T-shirts or loungewear, a French seam isn't an option and an overlocker earns its place. If you mostly sew wovens now and then, the sewing machine you already have may be all you need. We give the same advice about any machine: choose for how you sew now, not what you feel you should own.
Owning an overlocker doesn't retire the French seam, because the two work well together:
- Use both in one garment. French-seam the side seams of a chiffon blouse, then finish the hem with a rolled hem if your overlocker has that setting.
- Speed up the first pass. Sew the first seam on the overlocker with wrong sides together, so the blade trims as it stitches. Then fold and finish on your sewing machine, testing first because the enclosed thread adds a little bulk.
A coverstitch machine is a different purchase. It's designed for hems and has no blade to cut seam allowances, so it won't replace either seam.
If you do add an overlocker, allow for overlocker threads too, since each needle and looper takes its own. You can see and test machines before you buy at our stores.
Try Both Seams on a Scrap Before You Cut Out
The quickest way to decide is to sew both seams in the fabric you plan to use.
- Cut four strips from a scrap of your fabric.
- Join two strips with a French seam. Join the other two with a plain seam and overlocked edges, or zigzagged edges if you don't have an overlocker.
- Hold both samples up to the light and look at them from the right side.
- Pull each seam gently, then wash the samples as you'd wash the finished garment.
The sample that still looks right and holds its shape is your answer. Bring both into a store if you'd like a second opinion, or look at our classes and courses for hands-on help with your machine. You'll find everything else you need at Franklins.
