How to sew a fly-front zip

A fly front closes trousers with a zip sewn to a fly facing on one front and a fly shield on the other, finished with a curved line of topstitching. Left and right below mean as worn. Follow your pattern's markings throughout.

Pinny's tip
Use a zip longer than the fly and wait until the waistband is on before cutting it.

What you'll need

• Trouser zip: about 15cm (6in), or longer to trim later.
• Zip foot: to sew close to the teeth.
• Interfacing: for the fly facing and fly shield.
• Fabric marker: to draw the topstitching line.

Step by step

1. Finish the centre front edges of both fronts and transfer the pattern marks.
2. With right sides together, stitch the centre front from the inside leg seam to the dot with a 1.5cm seam allowance.
3. Interface the fly facing and finish its curved edge. Stitch it to the left front from the top edge to the dot, then trim and press the allowances towards the front.
4. Lay the zip face down on the facing with the tape against the front seam and its end just above the dot. Stitch the tape to the facing, turn the facing in and press lightly.
5. Interface the fly shield, fold it in half lengthwise, stitch the bottom edge, turn it out, press and finish the raw edge.
6. Press the right front 5mm to the inside. Baste the zip to that fold and the shield to the zip, then stitch close to the fold through all layers from the top to the dot.
7. Pin the shield out of the way, close the zip and draw the topstitching line using the front pattern piece or the facing as a guide. Topstitch through the front and facing.
8. Unpin the shield and make bar tacks at the bottom of the topstitching and at the curve, this time stitching through the shield too.

Common mistakes

• Catching the shield in the topstitching. Pin it out of the way first.
• Cutting the zip too soon. Wait until the waistband is on.
• Sewing the zip to the trouser front. Check it is stitched only to the facing or extension.

Watch it done

• How To Sew A Fly Front Zipper | Sewing Technique For Beginners by Thuy sewing on YouTube. Opens on YouTube; the video belongs to its creator.
• How to Sew the Fly/Zipper on Pants by Cornelius Quiring on YouTube. Opens on YouTube; the video belongs to its creator.

Sources

All checked 1 November 2026. We read these; we did not test the products ourselves.

1. Pauline Alice: How to sew a fly front zipper
2. Threads: A Step-by-step Method to Sew a Fly-Front Zipper
3. YouTube: How To Sew A Fly Front Zipper | Sewing Technique For Beginners (Thuy sewing)
4. YouTube: How to Sew the Fly/Zipper on Pants (Cornelius Quiring)