Seam Allowance & Cutting Calculator
Computes the cutting dimensions of a fabric piece by adding seam and hem allowances to your desired finished size. Use it whenever you need to cut fabric accurately from a custom or self-drafted pattern.
About this calculator
When you cut fabric for a sewn item, the finished piece will be smaller than the cut piece because seam allowances are consumed in the seams and hems. To find the correct cutting dimensions, you must add the seam allowance to both sides of each edge and the hem allowance to the bottom edge. The formula for cutting area is: cutting area = (finishedLength + seamAllowance × 2 + hemAllowance) × (finishedWidth + seamAllowance × 2 + hemAllowance). The seamAllowance is multiplied by 2 because it is added to both sides of the length and both sides of the width. The hemAllowance is added once per edge that will be hemmed. The result is the total cut area in square inches, which you use to mark your fabric before cutting.
How to use
You want a finished panel 18 inches long and 12 inches wide, with a 0.625-inch (⅝") seam allowance and a 1.5-inch hem allowance. Step 1 — cutting length: 18 + (0.625 × 2) + 1.5 = 18 + 1.25 + 1.5 = 20.75 inches. Step 2 — cutting width: 12 + (0.625 × 2) + 1.5 = 12 + 1.25 + 1.5 = 14.75 inches. Step 3 — cutting area: 20.75 × 14.75 = 306.06 square inches. Mark your fabric at 20.75 × 14.75 inches and cut along those lines to achieve a finished panel of exactly 18 × 12 inches after sewing.
Frequently asked questions
What is the standard seam allowance for home sewing patterns?
Most commercial sewing patterns in North America use a 0.625-inch (⅝") seam allowance, which is printed on the pattern pieces and assumed in their cutting lines. European patterns often default to 1.5 cm (about 0.6 inches), which is nearly the same. Quilting patterns typically use a much smaller 0.25-inch (¼") seam allowance because precision is critical when joining many small pieces. Always check your pattern's instruction sheet, since using the wrong seam allowance will make every finished measurement wrong.
How do I calculate seam allowance for a curved seam or neckline?
The mathematical principle is identical — you add the seam allowance distance perpendicularly to the seamline all the way around the curve. In practice, you trace the finished curve, then use a seam gauge or curved ruler to mark the seam allowance parallel to it. The cut piece will look slightly different from the finished piece because curves distort when fabric is stretched or eased. Notching or clipping the seam allowance on inward and outward curves after sewing allows the fabric to lie flat at the finished seam.
Why should I add hem allowance separately from seam allowance?
Seam allowances are folded into the seam and hidden inside the garment, while hem allowances are folded up on the right side or wrong side of a visible edge. They serve different structural purposes and are typically different widths — a narrow rolled hem might be 0.25 inches, while a shirt hem might be 1 inch and a curtain hem 3–4 inches. Combining them into one number would either leave too little hem depth or too much seam bulk. Keeping them separate in the calculator lets you set each one independently for the exact finish you want.