At the cutting table

Box Bag Math.

One rectangle for the outer and one for the optional lining. Edit the pattern or the finished dimensions to calculate in either direction.

Press Enter or leave a field to calculate. Inches accept decimals or fractions such as 3/8 or 1 1/2. Rounding changes the display only; calculations retain full precision.

Pattern & construction

Pattern length Along the zipperPattern width Wraps around the bag Coil zipper sizeClearance per side (% of teeth) Zipper hem per edge 0 for raw or bound edgesEnd seam allowance Corner seam allowance Corner cut depth

Finished bag / editable

Your finished size.

Length Width Height

From rectangle to box.

Solid lines: fabric edges. Dashed orange lines: folds or stitching. Dotted lines: trim after sewing. Dimensions follow your units and rounding; detail views are schematic.

1 / Full pattern

2 / Folded with zipper

3a / Pre-cut corner

3b / Sew, then trim

Flat zipper panels laid out beside the sewing machine — Zipper panels at the workbench, before the bag is boxed.

Finished blue dopp kit with black zipper and end tabs — A finished blue dopp kit with boxed corners and end tabs.

Cutting notes

Keep square uses the exact calculated corner depth. Rounding cuts independently can make the end slightly rectangular. Estimates ignore fabric thickness and turn of cloth; test your construction in scrap fabric.

Construction & formulas

Length runs along the zipper. Width wraps around the closed bag. Zipper exposure equals nominal teeth width plus clearance on both sides. Zipper size numbers are treated as nominal millimeters, not exact measured YKK specifications.

Center the zipper on the flattened tube. Measure corner depth C from the folded edge. At each end, measure C from the end stitching line, or C + end seam allowance from the raw end. The raw-edge notch is therefore rectangular; it is square relative to the stitching line.

Effective circumference = pattern width − 2 × zipper hem + zipper exposure.
Height = 2 × (C + corner seam allowance).
Width = circumference ÷ 2 − height.
Length = pattern length − 2 × end seam allowance − height.

Without pre-cutting: flatten each corner with the end seam aligned to the bottom fold. Mark a stitch line perpendicular to that seam, half the finished height from the tip. Its length (the triangle long side) equals the finished height. Sew first, then trim toward the tip, leaving the corner seam allowance.

For a square end: height = width = circumference ÷ 4. Corner depth C = height ÷ 2 − corner seam allowance.

Reverse: pattern length = finished length + height + 2 × end allowance. Pattern width = 2 × (width + height) + 2 × zipper hem − zipper exposure. With Keep square selected, editing width or height changes both together.