Which Width? A Breakdown of Seam Allowances
OVERVIEW
Seam allowances are one of the key foundations of garment construction. Knowing which width of allowance to apply to a seam can streamline the sewing process significantly, and can help define design choices such as raw edge finishes and topstitching, as well as contributing to the longevity of the garment.
This guide will discuss the applications for the different seam allowance widths commonly used in sewing. This guide will then provide a material list and methodology for copying a pattern to fresh paper in order to apply a seam allowance.
DEFINITIONS
Pattern piece - The paper piece of a sewing pattern.
Cutwork - The fabric piece that is cut using a pattern piece. Refers to an individual piece, or a collection of pieces.
Raw edge (or cut edge) - The outer perimeter of cutwork. The edge is cut and does not have a seam or hem treatment applied, and is therefore unfinished.
Stitch line - The line along the pattern that is sewn. This is the line where seam allowance width is measured from.
Seam allowance - The distance from the stitch line to the raw edge on both a pattern piece and on cutwork.
Thread count - The number of threads woven into one square inch of fabric. A higher thread count means a higher density of threads, and a more tightly woven fabric. It is calculated by adding the total number of threads along the warp (vertical) and weft (horizontal).
Notch - A point marked along a stitch line on a pattern piece that provides a guideline for sewing. The notch is transferred to cutwork with either a convex triangle cut along the raw edge, or a single snip (always at least ⅛” away from the stitch line) along the seam allowance that is perpendicular to the stitch line. Notches can be used to align matching raw edges along two pieces of cutwork.
SEAM ALLOWANCES AS A TOOL
Why are seam allowances necessary?
Seam allowances give the sewing machine needle the material it needs to “grab” the fabric and create a stitch. Different seam allowances will lend themselves best to a variety of fabric thicknesses, thread counts, and other characteristics. Defining a seam allowance while patterning will also help to decide how the garment is ultimately finished, as certain widths will lend themselves better to certain finishes. The two goals of a properly applied seam allowance are first to guide the stitch line accurately, and second to aid in the structural integrity of the garment.
Seam Finishes
Seam finishes are one aspect to consider when deciding what seam allowance to apply. Different finishes work best with different widths due to the structure of the finish or the type of machine used. Think of these as options for joining two pieces of cutwork.
Turnback: ¼” of the seam allowance is folded back toward the fabric and stitched to the remaining seam allowance, keeping the raw edge tucked between the garment and the seam allowance. Repeated on each side.
Recommended seam allowance: at least ½”
Serged (merrowed): The seam is run through a serger, also called a merrow machine, to cut and bind the raw edge while simultaneously stitching the seam in place. Can be used as a raw edge finish in addition to a straight stitch, or can be used to sew cutwork together as is. Knit or stretch fabrics must be sewn with this stitch (or a zig zag stitch on a standard home sewing machine) to allow the seam to expand and contract along with the fabric without breaking.
Recommended seam allowance: ⅜”
French: The cutwork is placed wrong sides together and sewn with a ¼” seam allowance. The resulting seam is trimmed to a ⅛” seam allowance, then folded inward to place the right sides together of the fabric and ironed. The seam is finished with a second ¼” seam allowance stitch, following the pattern piece stitch line. This fully encases the cutwork raw edge. Best used for straight seams or seams with a gentle curve. Best for unlined garments of delicate fabrics that tend to fray.
Recommended seam allowance: ½”
Flat felled: Fabric is placed wrong sides together and sewn along the stitch line. One side of the seam allowance is trimmed to half the height of the original, and the remaining taller allowance is folded over the shorter one and topstitched down. This is most commonly found as a finish on workwear and the inseam of jeans.
Recommended seam allowance: at least ⅜”
Hong Kong: Bias tape is applied to each side of the seam allowance, encasing the raw edge. ¼” bias tape is recommended to avoid a bulky resulting seam. This can use a matching bias tape color for uniformity, or a contrast color for a design element. Used in unlined garments.
Recommended seam allowance: ½”
Hem Finishes
Hem finishes are another way to think about different seam allowances. Like seams, hems need different allowance widths to accommodate different finishes. Think of these as ways of creating a clean edge on a single piece of cutwork.
Bias hem: Bias tape is applied to the hem, using either a matching color for uniformity or a contrast color for a design element.
Recommended seam allowance: at least 1/4” (bias tape width)
Raw hem: A straight stitch is applied ¼” away from the raw edge, typically in a matching color. This creates a barrier for the raw edge and any fraying that is intended for the garment’s final design, and contributes to the longevity of the garment.
Recommended seam allowance: ¼”
Turnback hem: The seam is folded back, then back a second time to encase the raw edge inside the hem, and finished with a straight stitch. Smaller versions of this hem are also called a rolled hem.
Recommended seam allowance: ½”
SEAM ALLOWANCES
1/4”
Recommended for smaller pieces of garments such as pocket flaps, shirt cuffs, collars, and collar stands. This is the narrowest recommended seam allowance for machine sewing, as narrower allowances may not feed into the machine properly for a clean and tightly sewn seam.
3/8”
The “production standard” for most mass produced clothing. This is the standard seam allowance of feeding fabric into a serger, and is therefore best used for knit and stretch garments. Also good for curved seams, as there is slightly less fabric to be pulled around the curve at the raw edge. Typically used with a lining for woven garments, as many seam finishes may not easily be applied to this width.
1/2”
The most common seam allowance for mockups, seam allowance finishes, and home patterning. This is the most universal seam allowance to use as a default, as it allows for many different ways to finish the raw edge while still allowing some flexibility around gentle curves.
5/8”
The standard seam allowance found in mass produced home sewing patterns. This is unlikely to be found elsewhere, but finishes that can be applied to ½” allowance can work for ⅝”. Often this can be too bulky to allow for clean curves without notching.
3/4”
Commonly found in theatre costuming, this allows for extra fabric in case of fit revisions. Best applied to straight seams.
1” or more
Mostly used in mockup garments only, as this width is too bulky for curves and can interfere with the overall look of the finished garment.
USING SEAM ALLOWANCES WHILE SEWING
Notches
When placing and cutting notches, cut only to ⅛” away from the stitch line. Any closer will weaken the seam and may cause a hole to develop, or can break the seam. Align the cutwork notches before pinning, and take care to match them as the seam is sewn. A single notch indicates that a seam is on the front of the garment, while two parallel notches close together indicate that a seam is on the back of the garment.
Releasing a curve
If a curved seam is sitting oddly after sewing, identifying where the seam allowance needs to be cut (or “released”) may help it lay flatter. Check for points where the raw edge appears to be buckling, and snip inward perpendicular to the stitch line to allow the seam to fall naturally.
Fitting or taking in/letting out seams
Take care to only let out a seam to a ¼” seam allowance and no narrower. Blend the new stitch line into the ends of the cutwork if revising the fit of the entire seam, or back into the existing stitch line if only revising a section. Sew the revised stitch line before taking out the previous stitching.
ADDITIONAL TOOLS
Magnetic seam allowance gage: A sewing machine add-on tool that creates a barrier to guide the raw edge of the cutwork as it is sewn. It is placed at the desired seam allowance marker on the machine’s plate, and holds the fabric down for additional control. Shopping option (not sponsored)
Adjustable guide sewing feet: A presser foot with an attached sliding guide to set the desired seam allowance, for additional visual aide. Shopping option (not sponsored)
APPLYING A SEAM ALLOWANCE
MATERIALS
Pattern block, homemade pattern, or store-bought pattern
Paper (pattern paper or medical table paper)
Mechanical pencils
Erasers
Sewing pins
Clear ruler, preferably 2” wide and 18-24” long
Tracing wheel
French curve and hip curve, as necessary
METHOD
Preparing the pattern and paper: Iron the paper and pattern to ensure you are working on a flat surface. Use the paper with the grid facing up, if using pattern paper.
Marking the paper: Draw a vertical line taller than the pattern piece itself, and mark it as the center front or center back of the pattern piece (depending on the pattern piece used). Mark a horizontal, perpendicular line in the center of the vertical line and mark it as the chest level. Check that the chest level is marked on the original pattern piece - if not, mark a horizontal line 1” below the bottom of the armhole curve. If copying a pattern piece for a skirt or pants, mark the low hip level (the fullest part of the garment). These lines will act as reference points for the copied pattern shape.
Placing the paper: Place the paper over the original pattern piece. Align the pattern piece with the center front/back line and the chest level, and use silk pins to secure the pattern to the paper. Silk pins are finer than ball point pins, and hold the paper in place without a ball point pushing or warping the paper. The pattern piece should show through the pattern paper or medical paper.
Tracing the pattern: Use a ruler to trace the pattern piece straight stitch lines. Do not trace the seam allowances. For curves, mark a series of short dashes along the curve and connect them with a French curve to create a smooth shape. Another option is to trace over key points with a tracing wheel, but be mindful that excessive use of a tracing wheel can weaken a pattern piece along the stitch line due to too much perforation of the paper. For this method, trace over key points such as notches or corners, then use a ruler to connect them. Be sure to trace all style elements such as darts.
Truing the pattern: Mark the grainline of the pattern piece and bring the line all the way to the edges of the pattern piece, for easier alignment to the fabric grain when cutting later. Square off all corners at least 1/4” away from the corner points (the pattern piece should create a right angle at these points, or close to it, to allow for a smooth overall shape to the garment when sewn).
Notching: Add all notches from the original pattern piece by placing them perpendicular to the stitch line, and checking that they still align with their corresponding notches on the rest of the pattern. Take care to follow the standard of a single notch indicating the front of the garment and two parallel notches indicating the back of the garment, for quick reference when sewing up the garment.
Applying the seam allowance: Align your ruler along the stitch line of the pattern piece, and follow the width of your desired seam allowance to trace a parallel line around the outer edge of the pattern piece. This will be your seam allowance. Mix and match seam allowances according to the needs of each seam. In this pattern piece, the side and center front seam both have 1/2” seam allowance, while the armhole has 3/8” to account for the curve of the seam, and the neckline has 1/4” to minimize bulk at the tighter curve of that seam.
Labeling: Mark the center front and side seams of the pattern piece. Label every pattern piece with the garment name, the pattern piece name, the size or intended wearer, type of fabric to cut, and number of pieces to cut. A good tip is to circle only the number of pieces to cut, to give a quick reference when working with the pattern later.
CLOSING THOUGHTS
Seam allowances are often overlooked as a key part of garment construction, but they allow for a multitude of design choices and options for garment construction. By learning to use the best widths for each seam, a garment can quickly be elevated to something well finished and long lasting!
