Seam Allowances
Seam allowance is the margin of fabric added beyond the stitching line (seamline) to construct a garment. While adding a uniform offset around a pattern piece appears simple, improper corner geometry, mismatched widths, and misunderstanding curve behavior are among the most common causes of puckered seams, distorted silhouettes, and hems that do not lie flat.
In pattern drafting, the seamline is the true geometry of the garment. The cut edge exists solely to provide additional fabric for sewing.
1. Standard Seam Allowance Widths
Using the same width around an entire pattern piece often creates unnecessary bulk or makes sharp curves difficult to sew cleanly. Seam allowances should vary according to the mechanical function of the seam.
| Seam Type | Common Applications | Standard Width | Functional Reason |
|---|---|---|---|
| Enclosed / Curved | Collar edges, neckbands, cuffs, lapels, pocket flaps | 1/4" (6 mm) | Eliminates the need to trim away bulk after sewing; turns cleanly without bubbling. |
| Standard Structural | Bodice side seams, shoulder seams, sleeve seams | 1/2" (12 mm) or 5/8" (15 mm) | Provides stability and mechanical strength for standard assembly and minor fitting adjustments. |
| Armscyes / Set-in Sleeves | Armhole seams joining sleeve to body | 3/8" (10 mm) to 1/2" (12 mm) | Reduces binding under the arm and prevents bulky ridge formation inside the armhole. |
| Straight / Folded Hems | Shirt hems, jacket hems, straight skirt/pant hems | 1" to 1-1/2" (25 to 38 mm) | Adds weight for clean drape and accommodates blind-hemming or double-folding. |
| Curved / Flared Hems | Circle skirts, curved shirt tails | 1/4" to 1/2" (6 to 12 mm) | Narrow margins prevent excess fabric from bunching when pressed to the inside of a curve. |
You may also want to widen the seam allowances on fabrics that have a tendency to fray.
2. Corner Geometry: Angles, Hem Truing, and Stepping
When two seamlines meet at anything other than a right angle (90°), simply tracing parallel lines out to an intersection creates cut edges that do not align once folded or stitched.
A. Tapered Hems & Seam Allowance Reflection
Woven Fabrics
When a side seam tapers inward or flares outward toward a hem, the hem allowance must not continue along the seam's angle. It must be mirrored across the hem fold line.
- The Problem: If a leg or skirt tapers toward the hem and the hem allowance is drafted straight down, folding the hem allowance up pulls against the wider leg width, causing the garment to pucker or cup.
- To Draft: Fold the hem allowance up along the finished fold line on the paper pattern, trace the side seam through the folded layers, and cut. When unfolded, the hem allowance will flare outward at the exact reciprocal angle required to match the garment circumference.
Knit Fabrics
Unlike wovens, tapered hems on knit garments are generally drafted straight down at a 90° angle from the finished hemline rather than mirrored or flared outward.
- Why it Works: Knit fabrics possess inherent stretch and recovery. When the hem allowance is turned up to the inside, the fabric easily stretches to accommodate the slightly wider circumference of the tapered garment without pulling, cupping, or distorting the exterior.
- To Draft: Square a line down at a 90° angle from the side seam at the finished hem fold line, extending downward for the desired hem depth.
The Add Hem tool in PatternMaker Pro has options for both woven and knit. The hem extension will automatically be drawn for the selected fabric.
B. Acute and Obtuse Corners
- Acute Angles (<90°): When two seamlines meet at a sharp point (such as collar points, lapels, or deep V-necklines), standard offsets create long, extended points. Square off (blunt) the apex of the cut edge perpendicular to the seamline to give machine feet an accurate starting point and reduce interior bulk.
- Obtuse Angles (>90°): Ensure seam allowances intersect squarely so the cut edges line up evenly without creating false notches or overhangs.
3. Curved Seams: Convex vs. Concave
The single greatest source of fitting and sewing confusion on curved seams is the difference in circumference between the seamline and the cut edge.
- Convex Curves (Outward): The outer cut edge is longer than the actual stitching line. A wide seam allowance forces excess fabric into a smaller space when turned inside, resulting in ripples. Keep seam allowances narrow (1/4" to 3/8" or 6 to 10 mm) or clip small wedges up to the seam line. The wedges will close up as the seam line is shaped.
- Concave Curves (Inward): The inner cut edge is shorter than the actual stitching line. If the seam allowance is too wide, the cut edge will restrict the seam from opening flat. Clip in toward the seamline to relieve tension. These clips will open up when the seam line is shaped.
4. Joining Seams with Different Seam Allowance Widths
Pattern instructions may direct you to join pattern pieces along edges that carry different seam allowance widths (for instance, a collar stand with a 1/4" (6 mm) seam allowance attaching to a bodice neckline with a 1/2" (12 mm) seam allowance).
The Golden Alignment Rule
Always match pieces by their internal seamlines, never by their cut raw edges.
Seams with different seam allowance widths are the result of poorly thought out pattern drafting. You should avoid providing mismatched seam allowances on your patterns.
Using PatternMaker Pro
After opening a new draft, you'll want to set the default seam allowance value for the draft with the Set Seam Allowance Defaults tool. To create seam allowances in PatternMaker Pro, you'll use the Add Seam Allowance to Pattern tool. You can customize the seam allowance on any line with the Change Line/Curve Seam Allowance tool.