Hem Allowance Calculator
Calculates the total fabric length you need to cut, factoring in your chosen hem depth and top seam allowance. Ideal for skirts, trousers, curtains, or any project where precise cutting matters.
About this calculator
When cutting fabric for a garment or home-dec project, the finished length is never the cut length. You must add extra for the hem at the bottom and any seam at the top where the piece attaches to a waistband or header. The formula is: cutLength = finishedLength + (hemType × 2) + seamAllowance. The hem value is doubled because a turned hem folds back on itself twice—once to hide the raw edge and once to form the visible fold. Common hem depths are 0.5 inches (narrow), 1 inch (standard), and 2 inches (deep or curtain hem). The top seam allowance is typically 0.5–0.625 inches depending on your pattern. Getting this calculation right before cutting prevents costly fabric waste.
How to use
Say you are hemming a skirt with a finished length of 24 inches. You choose a standard 1-inch double-fold hem (hemType = 1) and a top seam allowance of 0.625 inches. Plug into the formula: cutLength = 24 + (1 × 2) + 0.625 = 24 + 2 + 0.625 = 26.625 inches. Cut your fabric panel at 26.625 inches (approximately 26⅝ inches). After sewing the top seam and folding and stitching the hem, your finished skirt will measure exactly 24 inches.
Frequently asked questions
How much hem allowance should I add for a double-fold hem?
A double-fold hem requires the fabric to be folded twice, so the allowance is the hem depth multiplied by two. A narrow 0.5-inch double-fold hem needs 1 extra inch of fabric; a standard 1-inch hem needs 2 extra inches; a deep 2-inch hem needs 4 extra inches. Choosing a deeper hem adds weight that helps a skirt hang better and allows future length adjustments. Always press the fold before stitching for a crisp, professional finish.
What is the difference between hem allowance and seam allowance in sewing?
Seam allowance is the fabric between the cut edge and the stitch line where two pieces are joined together—typically 0.5 to 0.625 inches. Hem allowance is the extra fabric folded under at a single raw edge to create a finished, neat bottom or end. Both must be added to the finished measurement before cutting, but they serve different structural roles. Confusing the two is one of the most common reasons garments end up shorter than intended.
Why do I need to add extra fabric length when sewing a waistband or casing?
When a fabric panel is sewn to a waistband, header tape, or casing, the top seam consumes a small amount of the panel's length—usually 0.5–0.625 inches. If you cut to the exact finished length without including this seam allowance, the final garment will be shorter than planned by exactly that amount. Adding the top seam allowance in the formula ensures the finished measurement is accurate after construction. This is especially important for trouser legs or curtains where precise length is critical.