Seam Allowance Converter – Imperial & Metric Quick Reference

The most common standard seam allowance in commercial sewing patterns is 5/8 inch (approximately 16 mm). This is widely used in garment construction across the US and many other countries. Some pattern companies also use 1/2 inch (13 mm) or 3/8 inch (10 mm) for specific areas like necklines and curves. In metric countries, 15 mm is the common standard, which is very close to 5/8 inch.

To convert inches to millimeters, multiply the inch value by 25.4. For example: 5/8″ = 0.625 × 25.4 = 15.875 mm. In practice, sewists often round to the nearest millimeter or half-millimeter. Our converter above handles all common fraction formats (like 1/4, 5/8, 1 1/2) and provides both exact and practical conversions instantly.

5/8 inch (16 mm) became the standard because it provides enough fabric beyond the stitching line to allow for fitting adjustments, seam finishes, and durability without excessive bulk. It's wide enough to trim, grade, or finish seams neatly, yet not so wide that it wastes fabric or creates bulk in curved areas. This standard was widely adopted by major pattern companies like Simplicity, McCall's, and Butterick.

Quilting typically uses a 1/4 inch (6 mm) seam allowance. This narrower allowance reduces bulk at seam intersections and helps achieve precise piecing. Most quilting sewing machines come with a special 1/4″ presser foot. In metric terms, this is approximately 6 mm, though some quilters prefer exactly 6.35 mm for maximum precision.

For most home sewing projects, converting to the nearest millimeter is perfectly adequate. A difference of ±0.5 mm is rarely noticeable in finished garments. For example, 5/8″ (15.875 mm) is commonly rounded to 16 mm, and 1/4″ (6.35 mm) is rounded to 6 mm. However, for precision work like tailored garments or when matching plaids, using the exact conversion can be beneficial.

Seam allowance is the fabric between the stitching line and the raw edge of the fabric. It's crucial because it provides the margin needed to construct a durable seam, allows for fitting adjustments, and enables various seam finishing techniques. Without adequate seam allowance, seams can fray, tear, or fail to accommodate body adjustments. Different projects require different allowances—garments typically use wider allowances than quilts.

Inch fractions represent divisions of an inch. The denominator (bottom number) tells you how many equal parts the inch is divided into. For example, 1/8 means the inch is divided into 8 parts, and you count 1 part. 5/8 means 5 of those 8 parts. On a standard ruler, the longest marks are inches, the next longest are 1/2″, then 1/4″ and 3/4″, then 1/8″ marks (5/8, 3/8, 7/8), and the smallest are 1/16″ marks. Our converter helps you find the millimeter equivalent without manual calculation.