Seam Allowance Calculator

Frequently Asked Questions

How do I add seam allowance to a pattern?

Cut size = finished size + seam allowance on each edge being sewn. A 12 in finished width with 5/8 in allowances on both side seams is cut at 12 + 2 × 0.625 = 13.25 in.

What are common seam allowance widths?

Home garment patterns usually use 5/8 in (1.6 cm), quilting uses 1/4 in, and many commercial/industrial patterns use 1 cm or 3/8 in. Always check the pattern's stated allowance.

How does seam allowance affect total fabric?

Each seam adds allowance on both joined pieces, so a panel sewn on all four edges grows by twice the allowance in both width and length. Plan yardage from the cut size, not the finished size.

Should hems be treated like seam allowances?

No - hems need a separate, usually larger allowance (often 1–2 in) folded once or twice. Add the hem allowance to the bottom edge in addition to any side-seam allowances.

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How This Calculator Works

A pattern's finished size is what you measure on the completed garment or item; the cut size is the bigger rectangle you actually scissor out of the fabric, because every seam swallows a strip of cloth along each edge it touches. This calculator turns a finished width and height into the cut dimensions by adding your chosen seam allowance to both opposing edges of each measurement. It works in inches or centimetres and offers the seam-allowance values you meet most often: ¼ inch for patchwork piecing, ⅜ inch for knits and four-thread sergers, ½ inch for bags and crafts, the classic ⅝ inch garment standard from the Big Four pattern companies, and a clean 1 cm for European drafts. Pick a preset or type a custom value, and the tool reports the cut rectangle plus exactly how much fabric the allowance adds in each direction so you can lay out a marker and check yardage with confidence.

Formula / Method

For a single dimension the rule is cut = finished + (2 × seam allowance), because the allowance appears on the leading edge and the trailing edge alike. So cut width = finished width + 2 SA and cut height = finished height + 2 SA. The total fabric added per dimension is therefore 2 × SA - a ⅝ inch allowance adds 1¼ inch to the width and another 1¼ inch to the height. Centimetre entries are converted to inches by ÷2.54 for the internal math and converted back for display, and a 1 cm preset is stored as 1÷2.54 ≈ 0.394 inch so a metric workflow stays exact. Blank, zero, or negative finished sizes are rejected with a friendly message rather than returning a meaningless cut size, and a negative custom allowance is refused outright.

Worked Example

You want a finished cushion panel 12 × 8 inches at the garment-standard ⅝ inch allowance. Cut width = 12 + (2 × 0.625) = 13.25 inch; cut height = 8 + 1.25 = 9.25 inch, so you cut a 13¼ × 9¼ inch rectangle and the allowance has added 1¼ inch each way. Switch to patchwork at ¼ inch and the same finished block needs 12.5 × 8.5 inch - only ½ inch added per side. In metric, a finished 30 × 20 cm panel at a 1 cm allowance becomes 32 × 22 cm. Multiply the added amounts across all the pieces in your layout to see how seam allowances quietly inflate total yardage.

Common Mistakes

Adding the seam allowance only once instead of to both edges - you must add 2 × SA per dimension, not 1.

Mixing standards within one project: piecing a quilt at ¼ inch but assembling the border at ⅜ inch throws off every measurement.

Forgetting that hems, casings, and facings need their own (often larger) turn-under allowance separate from the seam.

Cutting to the finished line because the pattern "looks right," then ending up with a garment a full inch too small all round.

Assuming a 1 cm and a ⅜ inch allowance are interchangeable - 1 cm is ≈ 0.394 inch, slightly larger.

Tips & FAQ

Why is ⅝ inch the garment standard? It gives enough cloth to handle, press open, and let out later if a fit adjustment is needed; quilters use ¼ inch because hundreds of seams would otherwise bury too much fabric. What allowance for knits on a serger? Most sergers trim at ¼–⅜ inch, so ⅜ inch is a safe default that the blade tidies as it stitches. Does this include hems? No - this sizes seam edges only; add a separate hem allowance (commonly 1–2 inch) to the bottom edge of garments. How do I keep allowances consistent? Mark the seam line as well as the cut line on tricky curves, or use a guide on the machine bed. Can I use a 0 allowance? Yes for appliqué or fused edges where no seam is taken, but for any sewn join you need real fabric to grip. Should I add ease too? Ease is built into the finished pattern dimensions, not the seam allowance, so do not double-count it here. Treat the output as a cutting guide and always verify against your specific pattern's printed instructions.

Frequently Asked Questions

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