What you're actually doing when you sew pants together
Sewing pants together means joining two pieces of fabric along a seam line using a needle and thread—either by hand or machine. The most common seams in pants are the inseam (the line running down the inside of your leg), the outseam (the outside of your leg), and the crotch seam where the front and back pieces meet. Each seam follows the same basic principle: you align two fabric edges, find them with stitches, and finish the raw edges so they don't fray.
The difference between a seam that holds and one that fails is usually thread tension, stitch length, and how straight you keep your line. A sewing machine does this faster and more consistently than hand sewing, but hand sewing works if you don't have access to a machine or are repairing a small section.
Key Takeaways
- Pin or mark your seam line before you start, and use a seam allowance of half an inch unless your pattern specifies otherwise.
- A sewing machine set to a straight stitch with medium tension and a stitch length of 2.0 to 2.5 millimeters works for most pants fabric.
- Backstitch at the beginning and end of every seam to lock the thread and prevent unraveling.
- Finish raw edges with a zigzag stitch or pinking shears to stop fraying, especially on natural fabrics like cotton.
- Try on the pants before you finish the hem or topstitch, because small adjustments to seam width can change the fit significantly.
Preparing your fabric and marking the seam line
Start by laying out your two fabric pieces on a flat surface with the right sides (the finished-looking sides) facing each other. This is called "right sides together." The seam will be hidden inside the pants once you turn them right-side out.
Use pins or fabric chalk to mark where your seam should go. Most pants use a half-inch seam allowance, which means the stitching line sits half an inch from the raw edge of the fabric. If you're working from a pattern, it will tell you the seam allowance. If you're repairing or altering existing pants, measure the original seam allowance and match it.
Pin perpendicular to the seam line—that is, pins running across the line you're about to sew, not along it. Space them about three inches apart. This keeps the fabric from shifting as you feed it through the machine.
Setting up your sewing machine
Thread your machine according to the manual that came with it—the process varies slightly between models, but the basic steps are the same: raise the presser foot, insert thread through the guides and tension discs, and thread the needle from front to back. Use thread that matches your fabric color or is slightly darker; it will be less visible in the finished seam.
Set the stitch type to straight stitch (the default on most machines). Adjust the stitch length to 2.0 to 2.5 millimeters—this is a standard length for woven fabrics like cotton and linen. Shorter stitches (1.5 mm) are stronger but can pucker lightweight fabric. Longer stitches (3.0 mm) are faster but weaker and more visible.
Check your thread tension by sewing a test seam on scrap fabric. The thread should lock evenly between the two layers—you shouldn't see loops on either side. If the top thread is loose and loopy, increase tension slightly. If the bobbin thread is pulling through, decrease tension. Most machines have a dial numbered 0 to 9; start at 5 and adjust from there.
Sewing the seam on a machine
Place your pinned fabric under the presser foot so the needle is about half an inch from the raw edge, aligned with your marked seam line. Lower the presser foot. Backstitch (sew in reverse) for about half an inch to lock the thread, then sew forward along the seam line at a steady pace.
Keep your hands light on the fabric—guide it, don't pull it. The feed dogs (the metal teeth under the presser foot) move the fabric automatically. Pulling causes uneven stitches and can break the needle. Remove pins as you approach them; sewing over a pin can break the needle or bend it out of alignment.
When you reach the end of the seam, backstitch again for half an inch, then lift the presser foot and pull the fabric toward you to cut the thread. Leave about four inches of thread tail so you can rethread easily if needed.
Finishing the raw edges to prevent fraying
Once your seam is sewn, the raw edges of the fabric will fray over time, especially with washing. Finish them with a zigzag stitch or pinking shears. A zigzag stitch is more durable: set your machine to a zigzag pattern with a width of 2.5 to 3.0 millimeters and a length of 2.0 millimeters, then sew along each raw edge about a quarter inch from the seam line.
Pinking shears are scissors with a zigzag blade that cut the edge at an angle, slowing fraying. They work well on tightly woven fabrics but are less effective on loose weaves. Use them after your seam is complete by trimming the raw edges to about a quarter inch from the stitching.
For seams that will take a lot of stress—like the crotch seam—a zigzag finish is stronger. For less-stressed seams, pinking shears are faster and look neater.
Hand sewing a seam if you don't have a machine
Thread a needle with about an arm's length of thread and tie a knot at one end. Bring the needle up through both layers of fabric at the start of your seam line, from the inside (the side that will be hidden). This hides the knot between the layers.
Use a running stitch: push the needle down through both layers about an eighth inch ahead, then bring it back up an eighth inch ahead of that. Keep your stitches small and even. Backstitch every inch or so by reversing direction and sewing back over the last stitch to reinforce the line.
Hand sewing is slower and the stitches are more visible, but it works for repairs and for fabrics too delicate for a machine. The seam will be less strong than a machine seam, so avoid hand sewing stress points like the crotch unless you reinforce it later with a machine stitch.
Checking fit before you finish
Once your main seams are sewn, turn the pants right-side out and try them on. Check the inseam and outseam for straightness and comfort. If the seams pull or twist, you may need to adjust the seam allowance—take in the seam (sew closer to the edge) or let it out (sew farther from the edge) and resew.
Small adjustments at this stage are much easier than after you've hemmed and topstitched. Mark any changes with pins or chalk before you remove the old stitching.
Frequently Asked Questions
What thread should I use for sewing pants?
Use all-purpose polyester or cotton thread in a color that matches your fabric. Polyester is stronger and more forgiving of tension mistakes. For dark fabrics, use dark thread; for light fabrics, use light thread. Avoid cheap thread—it breaks easily and creates weak seams.
Can I sew pants by hand if I don't have a sewing machine?
Yes, but it takes much longer and the seams are less strong. Hand sewing works best for repairs and alterations. For sewing entire pants together, a machine is faster and produces a more durable result. Many libraries and community centers have sewing machines you can use.
What happens if my seam is crooked?
Crooked seams usually mean the fabric shifted as you sewed. Remove the stitches with a seam ripper, repin the fabric, and sew again. Use more pins next time and sew more slowly. If the seam is only slightly off, it may not affect fit—try the pants on before you redo it.
How do I fix a seam that came apart after washing?
Resew the section that failed using the same stitch length and tension you used originally. If the seam keeps failing in the same spot, the thread may be weak or the tension may be wrong. Try a different thread brand or adjust tension up slightly, then sew over the old line again.
Do I need to topstitch my seams?
Topstitching (sewing a visible line parallel to the seam on the outside of the fabric) is optional but adds strength and a finished look. It's common on jeans and work pants. Use thread that contrasts with your fabric, sew a quarter inch from the seam line, and backstitch at both ends.