An overlock machine — called a serger in the United States and an overlocker in the UK, Australia and much of the industrial garment world — is a specialized sewing machine that trims, seams and finishes a fabric edge in a single pass. It is not a replacement for a regular sewing machine; it is a companion machine used alongside one, and it is standard equipment in garment factories for finishing seams on knitwear and preventing fabric edges from fraying.
The naming difference is purely regional, not technical — both terms describe the same machine and the same stitch family, so this guide uses them interchangeably.
What Is an Overlock Machine (Serger)?
An overlock machine sews an overlock stitch: a looped stitch formed at the very edge of the fabric that encases the raw edge in thread while a built-in cutting blade trims away excess fabric just ahead of the needle. This is different from a regular sewing machine's lockstitch or chain stitch, which is formed inside the fabric, not at its edge.
How an Overlock Machine Works
Unlike a lockstitch machine, an overlock machine does not use a bobbin. Instead, one or two needles work with one or two curved loopers fed from separate thread cones. As the needle enters the fabric, it forms a loop; the lower looper sweeps behind the needle and through that loop, and the upper looper then catches both the lower-looper thread and the needle thread, interlacing them over the trimmed fabric edge. This repeats continuously as the fabric feeds through, wrapping the edge in thread.
The number of threads used depends on the stitch type and the job it's built for:
| Thread count | Typical use |
|---|---|
| 1-thread | Butt-seaming two fabric edges edge-to-edge (not a finished seam) |
| 2-thread | Light edge finishing, seaming lightweight knits, narrow hemming |
| 3-thread | Standard edge finishing, rolled hems, decorative edges |
| 4-thread | Seaming plus finishing in one pass; the most common setup on domestic overlockers; suited to higher-stress seams |
| 5-thread | A safety-stitch combination (overlock stitch + a separate straight/chain stitch) for maximum seam strength, common on industrial machines |
Key Parts and Features
Needles and Loopers
One or two needles work above the fabric; the upper and lower loopers work below and behind it. Together they interlace the threads that wrap the fabric edge.
Cutting Knife (Trimmer)
A built-in blade trims the fabric edge just before the stitch forms, so the seam allowance is cut and finished in the same pass — this is what makes an overlock machine faster than sewing and finishing an edge separately.
Differential Feed
Most overlock machines have two separate feed-dog sections — one in front of the needle, one behind — that can be run at different speeds relative to each other. Set correctly for the fabric, differential feed lets the machine feed knit and stretch fabric through evenly without stretching or waving the seam. Pushed to an extreme setting, the same control can deliberately gather or ruffle a lightweight fabric.
Thread Tension Dials
Because an overlock machine balances 2 to 5 threads at once, each thread typically has its own tension dial; correct tension is what keeps the stitch flat against the edge instead of loose or puckered.
Benefits of an Overlock Machine
Compared with finishing an edge on a standard sewing machine, an overlock machine is faster (trimming and finishing happen in one pass), gives a cleaner, more professional edge that resists fraying, handles stretch fabrics far better because of differential feed, and can produce specialty finishes such as rolled hems and decorative edges.
Common Uses of an Overlock Machine
In both home sewing and garment manufacturing, overlock machines are used for: seaming and finishing knit and stretch fabrics in one step; finishing raw edges on woven fabric to prevent fraying; hemming garments; gathering or ruffling lightweight fabric; and decorative rolled-hem edges on fabrics like chiffon.
Overlock Machine vs. Regular Sewing Machine
| Regular Sewing Machine | Overlock Machine (Serger) | |
|---|---|---|
| Stitch location | Inside the fabric | At the fabric edge |
| Thread source | Needle thread + bobbin thread | Needle thread(s) + looper thread(s), no bobbin |
| Cuts fabric while sewing | No | Yes (built-in knife) |
| Best for | Construction seams, topstitching, buttonholes | Seaming knits, edge finishing, preventing fraying |
| Replaces the other machine? | — | No — used alongside a regular machine, not instead of it |
For the full list of a regular machine's components, see parts of a sewing machine and their functions.
How to Use an Overlock Machine
Threading and operating an overlock machine follows a specific order (loopers before needles) because of how the threads interlace. For full step-by-step threading and operating instructions, see how to use a serger.
Frequently Asked Questions
Is a serger the same as an overlock machine?
Yes. "Serger" is the common US term and "overlock machine" or "overlocker" is used in the UK, Australia and in industrial/manufacturing contexts worldwide. They refer to the same machine and the same stitch family.
Can an overlock machine replace a regular sewing machine?
No. An overlock machine cannot sew buttonholes, set zippers, or topstitch the way a regular machine does. It is used alongside a regular sewing machine, mainly for seaming knits and finishing edges.
How many threads does an overlock machine use?
Between 1 and 5, depending on the stitch type. Most domestic overlockers are set up for 3 or 4 threads; industrial safety-stitch machines commonly use 5.
What is differential feed used for?
It adjusts how the two feed-dog sections move fabric relative to each other, which prevents wavy or stretched seams on knit fabric, or, at an extreme setting, creates a deliberate gathered/ruffled effect.