Sheet Metal Box & Pan Flat Pattern Developer

Enter the finished outside length, width and uniform wall height, then the sheet thickness, actual formed inside radius and a K-factor checked against your material and tooling.

The four rectangular corner cutouts extend through the intersecting bend zones and into the straight base by the entered clearance. This conservative open-corner layout removes more material than a close-fitting welded pan. It does not make watertight or sealed corners.

Dimensions: outside setback S = radius + thickness for each 90-degree bend. Bend allowance BA = (π / 2) × (radius + K × thickness), and bend deduction BD = 2S − BA. Blank length = outside length + 2 × wall height − 2BD; width follows the same rule.

The distance between opposite bend centers is outside length − BD. The straight base between bend tangent lines is outside length − 2S. Each flange runs wall height − BD / 2 from its bend center to the cut edge. These are different measurement references.

Worked check: 300 × 200 mm outside dimensions, 50 mm walls, 1.5 mm sheet, 2 mm radius and K = 0.33 give BA ≈ 3.919 mm and BD ≈ 3.081 mm. The overall blank is 393.8 × 293.8 mm. With 3.5 mm clearance beyond the tangent, the remaining flange widths are 286 × 186 mm. The corners remain open after forming.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Outside length

300 mm

Outside width

200 mm

Outside wall height

50 mm

Material thickness

1.5 mm

Inside bend radius

2 mm

K-factor

0.33

Clearance past bend tangent

3.5 mm

Primary result: Blank length: 393.8 mm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This covers a single-piece, four-flange box with equal wall height on all sides and open (relieved, unwelded) corners only. It does not cover unequal wall heights, welded or hemmed corners, a lid, or multi-piece construction.

Dashed lines show bend centers in the developed neutral surface, not machine back-gauge settings. Check the K-factor, bend sequence, tool clearance, minimum flange and springback with your fabricator. The tool rejects dimensions with no straight base or straight wall remaining.

Inside face spacing subtracts twice the thickness. Rounded internal corners reduce the flat floor available for a rectangular item. Clearance is an open notch beyond the bend tangent, not a guaranteed assembled gap. Print at actual size and measure the PDF calibration mark before cutting.