sewcalcs.uk
Seam Allowance Fabric Calculator — Extra Fabric for Pattern Pieces
Calculate how much extra fabric is needed when adding seam allowances to all pattern pieces, and see the total fabric required with cutting layout.
How We Calculate This
Each pattern piece is enlarged by the seam allowance on every seamed edge and by the hem allowance on hemmed edges.
Cut dimensions per piece
Each rectangular piece has two width-running edges (top and bottom) and two height-running edges (left and right). The seamed edges you enter are spread across these pairs, and every seamed edge adds one seam allowance while every remaining edge adds the hem allowance.
Cut width = Finished width + allowance on each left/right edge
Cut height = Finished height + allowance on each top/bottom edge
Cutting layout
The calculator works out how many cut pieces fit across the fabric width, taking layout efficiency into account, then multiplies the rows needed by the cut height. The finished-size (no-allowance) layout is built with the very same packing rules, so the difference between the two is the seam allowance cost on its own.
Waste percentage
Waste = (Total fabric area − Total piece area) / Total fabric area × 100. It counts the layout-efficiency margin and any empty space in a partial final row, so it runs higher than the 15–25% norm for projects with only a handful of pieces.
Working figures for this are collected in our Seam Allowance Chart reference table.
Frequently Asked Questions
Pattern pieces are drawn to their finished (sewn) size. When you cut fabric, you must add seam allowance beyond the sewing line so there is fabric to stitch. Forgetting to account for seam allowances is one of the most common reasons sewists run short of fabric — especially on projects with many small pieces.
Most UK commercial dressmaking patterns include 1.5 cm (⅝ inch) seam allowance. Independent designers often use 1 cm. Quilters commonly use 6 mm (¼ inch). Japanese patterns frequently include no seam allowance and expect you to add your own. Check your pattern instructions before calculating.
Each seamed edge adds one seam allowance width to the cut piece. A simple rectangle sewn on all four sides gains seam allowance on every edge, significantly increasing its cut size compared with its finished size. This calculator lets you set the number of seamed edges per piece to reflect patterns with mix of seamed and hemmed edges.
Fabric waste is the proportion of the purchased rectangle of fabric that is not part of any finished piece — it ends up as off-cuts and selvedge. A waste percentage of 15–25% is typical for a whole-garment marker laid out by an experienced cutter. This calculator uses a simple rectangular-grid layout, so projects with only a few pieces (where the last row sits half-empty) will read higher than 25% — that empty fabric is real off-cut you still pay for. Simpler shapes, wider fabric and nesting more pieces together all reduce the figure.
No — this calculator gives a baseline fabric requirement for plain or all-over fabrics with no directional layout restriction. For directional fabrics, nap or large pattern repeats, add 15–30% extra fabric to the result. Use the Nap Layout Calculator or Pattern Matching Calculator for those adjustments.
Related Calculators
Seam Allowance Calculator
Calculate seam allowance amounts and adjust pattern pieces accordingly.
Hem Calculator
Calculate hem allowances and fabric needed for different hem types.
Gather Calculator
Calculate fabric width needed for gathered sections based on fullness ratio.
Tuck Calculator
Calculate fabric needed for pin tucks, released tucks and decorative tucks.
Pleat Calculator
Work out fabric needed for box pleats, knife pleats and inverted pleats.
Was this calculator helpful?
Last updated: March 2026
All calculations are estimates. Always verify quantities before cutting or purchasing fabric.