Seam Allowance Calculator
A finished dimension is what you see after sewing; the cut dimension is larger because every sewn edge swallows a seam allowance of fabric. Get the cut size wrong and pieced blocks no longer fit together. This calculator converts finished width and length into cut width and length by adding your chosen seam allowance for each sewn edge. You control how many edges are sewn across the width and down the length, so it handles squares sewn on all sides, strips sewn on two, and anything in between. The seam allowance is your input, matching quilting, garment, or any other convention.
Seam allowance formula
Added width = seam allowance * edges across width
Added length = seam allowance * edges down length
Cut width = finished width + added width
Cut length = finished length + added length
Set edges to 2 for a piece sewn on both sides of that dimension, 1 for a single sewn edge, or 0 for a folded or selvedge edge that takes no allowance.
Sewing context
- Quilting piecing uses a 1/4 inch seam allowance by convention; garment seams are often 5/8 inch.
- Each sewn edge consumes one seam allowance, so opposite sewn edges add two allowances to that dimension.
- Small cutting errors compound across a pieced block, so cut to the exact figure where your ruler allows.
- The inch is defined by the NIST Office of Weights and Measures.
- Folded or selvedge edges that are not sewn take no allowance: set those edges to 0.
Seam allowance: frequently asked questions
How do I add seam allowance to a finished size?
Add the seam allowance once for every edge that will be sewn. A piece sewn on all four sides gains two seam allowances across its width and two down its length. With a 1/4 inch allowance, a 6 inch finished square is cut at 6.5 inches square (6 plus 0.25 plus 0.25).
What seam allowance should I use?
Quilting uses 1/4 inch by convention; garment sewing commonly uses 5/8 inch. The allowance is a choice that depends on your pattern and technique, so it is a user input here. Enter the allowance your pattern specifies and the calculator adds it on the correct number of edges.
Why multiply the allowance by the number of edges?
Each sewn edge consumes one seam allowance of fabric. If both the left and right edges of a piece are sewn, the cut width must include two allowances. The calculator lets you set how many edges are seamed across the width and down the length, then adds the allowance for each.
Does this handle a 1/2 inch grid for cutting?
It returns the exact cut dimensions. Many rotary rulers are marked in eighths, so the calculator's exact figure can be rounded to the nearest convenient line. For accuracy, cut to the exact figure when your ruler allows it, especially for pieced blocks where small errors accumulate.
How do I go from cut size back to finished size?
Subtract the seam allowance for each sewn edge. This calculator focuses on the common direction, finished to cut, because that is what you do when cutting from a pattern. To reverse, simply subtract the allowances that the cut size includes.
Official sources
- NIST: Units of length (inch definition).
- NIST: Office of Weights and Measures.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.