Start with a pattern and fabric that match your skill level

Sewing slacks means choosing between two paths: making them from a commercial pattern, or altering a pair you already own. If you are new to sewing, altering an existing pair is faster and more forgiving—you are working with a finished garment and only changing the fit. Making slacks from scratch requires understanding how pattern pieces fit together, which takes practice.

For a first pair from a pattern, look for one marked "beginner" or "straightforward" on the envelope. Simplicity, McCall's, and Vogue all make slack patterns with clear instructions. Choose a pattern one size larger than your usual size, because patterns assume you will add seam allowance and the fit is often snug. Buy fabric the yardage the pattern calls for—usually 1.5 to 2.5 yards depending on the style—and pick a stable fabric like cotton twill, linen, or a cotton-poly blend. Avoid stretchy knits until you have made slacks in woven fabric first.

Key Takeaways

  • Altering an existing pair of slacks is the fastest way to learn, because you only need to adjust the inseam, side seams, or waistband rather than construct the whole garment.
  • Making slacks from a pattern requires cutting fabric pieces accurately, sewing a fly front or elastic waist, and attaching a waistband—plan for 6 to 10 hours for your first pair.
  • The inseam is the most common alteration: measure from your crotch to your ankle bone, then have a tailor shorten it, or fold and stitch the hem yourself if you have a sewing machine.
  • A serger (overlock machine) finishes seams neatly and prevents fraying, but a regular sewing machine works fine if you finish raw edges with pinking shears or a zigzag stitch.
  • Press seams as you go—use a hot iron to flatten each seam before crossing it with another, or the garment will look homemade and wrinkled.

Altering the inseam on an existing pair

The inseam is the seam that runs up the inside of your leg. Shortening it is the easiest alteration and the one most people need. Put on the slacks with the shoes you will wear them with, and have someone mark where the hem should hit—usually at the top of your shoe or just touching it. Measure from that mark to the crotch seam and write down the number. That is your new inseam length.

Take off the slacks and lay them flat on a table. Measure from the crotch seam down the inside leg to where you marked, and add 1.5 inches for the hem allowance. Mark a line in chalk or tailor's pencil all the way across both legs at that point. Cut along the line with sharp fabric scissors, cutting through both layers at once. Fold the raw edge up 1.5 inches, press it with an iron, then fold it up again and press. Pin the fold in place, then stitch it down with a needle and thread using a blind stitch (a stitch that runs inside the fold so it does not show on the outside), or use a sewing machine set to a straight stitch 0.25 inches from the fold.

Adjusting the side seams for a slimmer or looser fit

If the slacks are too tight in the thigh or hip, or too loose everywhere, you can take in or let out the side seams. Try on the slacks inside out and have someone pin the side seams to the fit you want. The pins should run from the hip down to the knee. Mark the new seam line with tailor's chalk, then take off the slacks.

Sew along the chalk line on both legs using a sewing machine set to a straight stitch with a stitch length of 2.5 millimeters. Start at the hip and sew down to the knee, then backstitch to lock the thread. Try on the slacks again to check the fit before you trim the excess fabric. Once you are happy with the fit, trim the seam allowance to 0.5 inches and finish the raw edges with pinking shears or a zigzag stitch on your sewing machine to prevent fraying.

Making slacks from a pattern: cutting and preparing the pieces

Lay out your pattern pieces on the fabric according to the layout diagram on the pattern envelope. Most slack patterns include a front piece, a back piece, a waistband, and a fly facing (the piece that hides the zipper). Pin the pattern pieces to the fabric with the grain line running straight down the length of the fabric—the grain line is marked on the pattern with an arrow. Cut around each piece with sharp scissors, cutting through both layers of fabric at once if the pattern says to.

Mark the notches (small triangles on the pattern edges) and any other markings like pocket placement or dart lines with tailor's chalk or a fabric pen. These marks tell you where pieces connect to each other. Transfer the marks to both sides of the fabric so you can see them when you are sewing. Separate the pattern pieces from the fabric and set them aside—you will not need them again until you are checking the instructions.

Sewing the darts and assembling the front and back

Darts are small folds sewn into the fabric to shape it around your body. Most slack patterns have darts at the waist and sometimes at the hip. Fold the fabric along the dart line with the right sides together (the pretty sides facing each other), and pin along the stitching line marked on the pattern. Sew from the wide end of the dart to the point using a straight stitch, then backstitch at the end to lock the thread. Remove the pins and press the dart flat with an iron, pressing it toward the center back or side seam depending on where it is.

Once the darts are sewn, sew the front pieces together at the center front seam if your pattern has two front pieces. Pin the pieces with right sides together, matching the notches, and sew using a straight stitch with a stitch length of 2.5 millimeters. Backstitch at the beginning and end. Press the seam open with an iron. Repeat this step for the back pieces if needed. Now sew the front to the back at the side seams, pinning and sewing from the waist down to the ankle, then press the seams open.

Installing the fly front and attaching the waistband

A fly front is the overlapping flap that hides the zipper. Pin the fly facing (a small piece of fabric) to the right front piece with right sides together, matching the notches. Sew along the marked line, then press the facing to the inside and topstitch (sew a visible line of stitching) 0.25 inches from the seam to hold it flat. Insert a zipper into the fly opening by pinning the zipper tape to the seam allowance and sewing it down with a zipper foot on your sewing machine.

For the waistband, fold it in half lengthwise with wrong sides together (the ugly sides facing each other) and press. Pin the waistband to the waist opening with right sides together, matching the notches and the center marks. Sew around the entire waist using a straight stitch. Fold the waistband to the inside and pin it in place, then stitch it down by hand using a blind stitch, or use a sewing machine to stitch in the ditch (sew directly in the seam line so the stitching does not show on the outside).

Hemming the legs and finishing the garment

Try on the slacks with the shoes you will wear them with and have someone mark where the hem should hit. Measure from the crotch seam to that mark on both legs and write down the numbers—they may be different if one leg is longer than the other. Add 1.5 inches to each measurement for the hem allowance, then mark a line in chalk across each leg at that point. Cut along the line, fold the raw edge up 1.5 inches, press it, then fold it up again and press.

Pin the hem in place and stitch it down using a blind stitch by hand, or use a sewing machine set to a blind hem stitch if your machine has one. Press the finished slacks with a hot iron, pressing the legs flat and the seams to one side. Try them on one more time to check the fit and make sure the hem is even.

Frequently Asked Questions

Do I need a serger to sew slacks?

No. A regular sewing machine works fine. A serger finishes the raw edges neatly and prevents fraying, which makes the inside look more professional, but you can finish edges with pinking shears or a zigzag stitch on a regular machine instead.

What is the difference between a fly front and an elastic waist?

A fly front has a zipper and a button, like jeans. An elastic waist is simpler—you sew the side seams, attach elastic to the waist opening, and hem the legs. Elastic waist slacks are faster to make and a good choice for your first pair.

Can I use a pattern that is too small and just add seam allowance?

No. Patterns are designed with a specific amount of ease (extra room) built in. A pattern that is too small will be tight even if you add seam allowance. Buy the size the pattern recommends based on your measurements, which are usually listed on the envelope.

How do I know if my seams are straight?

Use the seam guide on your sewing machine—a line marked on the throat plate that shows where 0.5 inches is. Keep the edge of the fabric aligned with that line as you sew. Practice on scrap fabric first if you are new to sewing.

What should I do if the waistband is too tight after I sew it on?

You can let out the side seams slightly to add room at the waist, or you can remove the waistband and sew it back on with a slightly larger seam allowance. It is easier to prevent this by trying on the slacks before you attach the waistband and adjusting the side seams if needed.