What you need to sew a onesie
A onesie is a one-piece garment that covers the torso and legs in a single connected pattern. To sew one, you need a pattern (either purchased or drafted), fabric suitable for the style you want, thread that matches your fabric, and basic sewing tools. Most onesies are sewn from knit or woven cotton, though fleece and jersey work well too.
Start by gathering: a commercial pattern sized to your measurements, or a pattern you draft yourself; 1.5 to 2.5 yards of fabric depending on pattern size; all-purpose or ballpoint thread; scissors or a rotary cutter; a measuring tape; pins; a sewing machine; and a serger or zigzag stitch for seam finishing if you want a polished look. If you are new to sewing, a commercial pattern is faster and more reliable than drafting your own.
Key Takeaways
- A onesie pattern consists of front and back body pieces, two leg pieces, and usually a crotch gusset that prevents the crotch seam from tearing under movement.
- Knit fabrics like jersey and fleece require ballpoint needles and a stretch stitch or narrow zigzag to prevent seams from breaking when the fabric stretches.
- The crotch seam is the most stressed part of a onesie, so reinforce it with a second row of stitching or a serger seam for durability.
- Elastic at the wrists, ankles, and waist keeps the garment fitted and prevents it from riding up or sliding down during wear.
Choosing and preparing your pattern
Commercial patterns for onesies are sold by Simplicity, McCall's, Butterick, and Vogue, usually in the children's or sleepwear section. Take your measurements (chest, waist, hip, inseam, and shoulder width) and compare them to the pattern envelope before buying. Patterns come with multiple sizes printed on the same tissue, so you can trace only the size you need onto pattern paper or directly onto your fabric.
If you are drafting your own pattern, start with a fitted t-shirt pattern for the upper body and add length, then attach a pant leg pattern at the waist. Many sewers find this approach slower and more error-prone than using an existing pattern. Once you have your pattern pieces, lay them out on your fabric according to the layout diagram on the pattern envelope. This diagram shows you how to arrange pieces to use fabric efficiently and account for grain line direction.
Cutting and marking your fabric
Lay your fabric flat on a clean, large surface. Pin pattern pieces to the fabric following the layout diagram, making sure grain lines are straight and pieces are not placed on the fold unless the diagram says they should be. Use sharp scissors or a rotary cutter to cut around each piece, cutting through both layers if your fabric is folded. Cut slowly and follow the pattern edge exactly—mistakes here are hard to fix later.
After cutting, transfer pattern markings (darts, seam allowances, notches, and placement lines for elastic) to your fabric using tailor's chalk, a fabric pen, or thread tracing. Notches are small cuts or marks that help you align seams correctly. If your pattern includes a crotch gusset—a small diamond or oval piece that sits at the crotch seam—cut it carefully and mark its placement on both the front and back pieces.
Sewing the body and legs together
Start by sewing the front and back body pieces together at the shoulder seams. Place pieces right sides together (the pretty sides facing each other), pin along the seam line, and sew using a straight stitch with a 5/8-inch seam allowance. If you are using knit fabric, use a ballpoint needle and a narrow zigzag stitch or a stretch stitch instead of a straight stitch—this allows the seam to stretch with the fabric without breaking.
Next, sew the two leg pieces together along the inseam (the seam that runs up the inside of the leg). Pin right sides together and sew from the crotch area down to the ankle. Then attach each leg to the body by sewing the leg opening to the body opening, matching notches and distributing the fabric evenly. This is where the crotch gusset comes in: if your pattern includes one, sew it into the crotch seam where the front, back, and legs meet. The gusset prevents stress tears in this high-movement area.
Reinforcing the crotch seam
The crotch seam bears the most stress in a onesie because it is pulled in multiple directions every time the wearer moves. After your first seam is sewn, sew a second row of stitching 1/4 inch away from the first seam, on the seam allowance side. This double-stitching distributes the load and makes the seam much more durable.
If you have a serger, use it to finish the seam allowances at the crotch and anywhere else seams might fray or tear. A serger trims, stitches, and finishes in one pass. If you do not have a serger, use a zigzag stitch on your regular machine set to a medium width and length, stitching along the raw edges of the seam allowance to prevent fraying. Trim away any loose threads after serging or zigzagging.
Adding elastic to wrists, ankles, and waist
Elastic keeps a onesie fitted and prevents it from riding up or sliding down. Most patterns call for 1/2-inch to 1-inch wide elastic sewn into a casing (a tunnel of fabric) or applied directly to the edge. To explore elastic directly, cut elastic to the measurement specified in your pattern (usually 2 to 4 inches shorter than the opening it will finish). Pin the elastic to the wrong side of the fabric opening, stretching it evenly as you pin so it lies flat when relaxed.
Sew the elastic down using a zigzag stitch or a stretch stitch, stretching the elastic as you sew so it stays flat. The zigzag allows the elastic to move without the thread breaking. Fold the elastic to the inside and sew again from the right side, catching the elastic in the seam. This creates a neat, finished edge that looks professional and wears well.
Finishing hems and trying on
Hem the leg openings and wrist openings after the elastic is in place. Fold the edge up 1/2 inch to the wrong side, pin, and sew using a straight stitch or a narrow zigzag. For knit fabrics, a zigzag or stretch stitch is better because it flexes with the fabric. Try on the onesie before cutting threads and making any permanent adjustments—if the inseam is too long or too short, you can adjust it now.
Check that the crotch seam sits comfortably and that the legs do not twist. If the fit is off, mark where adjustments are needed and try again. Once you are satisfied, trim all loose threads and press the finished onesie with a warm iron. Wash and dry it once before wearing to pre-shrink the fabric and test the durability of your seams.
Frequently Asked Questions
Can I use woven fabric instead of knit for a onesie?
Yes, but woven fabric will be less stretchy and may feel restrictive. Woven onesies work better as pajamas or loungewear than as everyday wear. Use a straight stitch instead of a zigzag, and expect the garment to be stiffer and less forgiving if the fit is slightly off.
What if I do not have a serger?
A serger is helpful but not required. Use a zigzag stitch on your regular machine to finish seam allowances and prevent fraying. Set the stitch width to medium (about 3 to 4) and length to medium (about 2 to 2.5), and sew along the raw edges of the seam allowance.
How do I know what size pattern to buy?
Measure your chest, waist, hip, and inseam, then compare those numbers to the size chart on the pattern envelope. Buy the size that matches your largest measurement. You can always take in seams if the pattern is too big, but adding fabric is much harder.
Why does the crotch seam keep ripping?
The crotch seam tears because it bears constant stress and a single row of stitching is not strong enough. Reinforce it with a second row of stitching, use a stretch stitch instead of a straight stitch, and make sure your needle is the right type for your fabric (ballpoint for knits, universal for wovens).
Should I pre-wash my fabric before cutting?
Yes, especially for cotton and other natural fibers that shrink. Wash and dry your fabric the same way you plan to wash the finished onesie, then press it flat before cutting. This prevents the finished garment from shrinking after you sew it.