Craft & Making
Seam Allowance Calculator
Enter finished length and width plus seam allowance to see cut size.
What size do I cut?
Patterns mix finished and cut sizes. For a rectangular piece, cut size = finished size + twice the seam allowance on each dimension, because you add allowance to both opposite edges. Enter finished length and width plus the allowance your pattern uses (commonly 1–1.5 cm, or ⅝″ / ¼″ in imperial sewing). Shaped garment pieces follow the pattern’s own seam lines — this tool is the rectangle rule for patches, cushions, quilt blocks and bags. Plan overall fabric with fabric meterage once you know cut sizes.
Worked example
A 40 × 30 cm finished rectangle with 1.5 cm seam allowance cuts at 40 + 2×1.5 = 43 cm by 30 + 2×1.5 = 33 cm.
Limits and assumptions
Rectangles only. Curved seams, French seams, bindings and asymmetric hems need the pattern method, not a uniform add-on.
Frequently asked questions
Why multiply allowance by two?
Each dimension has two opposite edges; both receive the same seam allowance.
Is ¼″ the same as 1.5 cm?
No — ¼″ is about 0.6 cm. Enter the allowance your pattern actually specifies.
Do hems use the same rule?
Single-fold hems often add once the hem depth plus seam; check the pattern. This tool assumes seam allowance on all four sides.
What about quilt ¼″ seams?
Enter 0.6 cm (or ¼″ via unit prefs) as the allowance; finished block size still follows your block maths.