Adding Seam Allowances, Hem Allowances, and Trueing Pattern Pieces

Why Seam and Hem Allowances Matter

A pattern piece drafted to finished dimensions is a seamline map, not a cutting guide. Before fabric is ever touched by shears, every edge of every pattern piece must carry the correct amount of extra material so seams can be stitched and finished cleanly and hems can be turned without distortion. Getting these allowances right the first time prevents costly re-cuts and is a core competency tested by both the Custom Tailors & Designers Association credential and the Marine Fabricators Association Master Fabric Craftsman (MFC) examination.

Standard Seam Allowances by Edge Type

Trade practice assigns different allowances to different edge types based on the stress the seam will bear, the finish method used, and the precision required during fitting.

• Straight side seams and shoulder seams: 5/8 inch is the dominant standard in US ready-to-wear and custom tailoring drafting systems, including the Armstrong method. This width gives adequate fabric for alterations during fitting.
• Curved seams (armhole, neckline, princess seam): 1/2 inch is common because a tighter curve is harder to ease and clip; extra width beyond 1/2 inch adds bulk after clipping.
• Neckline edges: 3/8 inch to 1/2 inch, depending on the facing or binding treatment specified.
• Hem allowances: Variable by garment type. A tailored jacket hem typically carries 1 to 1-1/2 inches; a trouser hem 1-1/2 to 2 inches; a straight skirt hem 1 to 1-1/2 inches; a flared or circular skirt hem 1/2 to 3/4 inch because excess fabric would ripple.
• Marine and upholstery fabric seams: Heavy canvas and vinyl applications typically call for 1/2 inch to 5/8 inch, sewn with a flat-felled or double-stitched seam for waterproofing and durability. Always follow the project specification sheet or client work order.

Always add allowances with a seam gauge held against the seamline, walking it along the curve or straight edge in short increments. Never estimate by eye on curved edges.

Trueing Pattern Pieces

Trueing means verifying that every seam that will be joined in construction is equal in length on both mating pieces, and that the seamline shapes blend smoothly into one another. If the bodice front side seam measures differently from the bodice back side seam, the assembled garment will pucker or drag.

How to true a seam pair:

1. Place the two pattern pieces seamline-to-seamline with the notches and endpoints aligned.

2. Pin through both layers at matching notches and at the endpoints (usually marked with a drill hole or cross mark).

3. Walk a flexible ruler or simply hold the pieces together and measure the seamline length on each piece from endpoint to endpoint.

4. Any discrepancy greater than 1/16 inch must be corrected by adding to or reshaping the shorter piece. Never shorten the longer piece by cutting into the seamline — you may destroy fitting ease.

5. After correcting length, re-blend any curve that was disturbed using a French curve or hip curve so the seamline flows without a flat spot or bump.

Trueing also applies to the outer cut edge. After you correct the seamline, re-measure the seam allowance on the corrected section to make sure it is still uniform.

Notch and Drill-Hole Placement

Notches are small triangular or rectangular marks cut into the seam allowance (pointing outward beyond the cut line) that guide matching during assembly. The Armstrong convention uses:

• Single notch: front of a garment section (e.g., front armhole, single notch on sleeve cap front).
• Double notch: back of a garment section (e.g., back armhole, double notch on sleeve cap back).
• Center-fold line: a double-headed arrow or the word FOLD printed parallel to the grain line on any piece cut on the fold.
• Drill holes (punch marks): placed 1/4 inch inside the seamline (inside the seam allowance, into the body of the piece) to mark dart points, pocket placement, and zipper stops. Because they land inside the finished seam, they do not show on the finished garment.

Notches must appear on the cut line, not on the seamline. If you add seam allowances after notching the seamline, transfer each notch outward by the allowance distance to the new cut line.

Safety at the Drafting Table

Pattern making is lower-risk than machine work, but injuries still occur. Keep all awls, tracing wheels, and push-pin points directed away from your body. When using a rotary cutter to trim paper pattern copies, always cut away from yourself on a self-healing cutting mat — never use your palm or a book as a backing surface. Store sharp drafting tools point-down in a holder or capped when not in use. Keep the work surface clear of clutter so pattern pieces cannot slip, causing a reaching injury.

Checking Your Work

After completing any pattern set:

1. Lay all pieces out and confirm every edge carries an allowance — no bare seamlines remain.

2. Check curved edges with a flexible ruler to ensure the allowance is consistent, not varying from 3/8 inch at one point to 3/4 inch at another.

3. Pair and walk every mating seam. Record your measurements.

4. Verify every notch is transferred to the cut line and that front/back designation (single/double) is correct.

5. Confirm all drill holes sit inside the seam allowance area.

A complete, trued pattern set is the foundation of accurate cutting and efficient production — in custom tailoring and in marine fabrication alike.