TOOLS / SHEET METAL

Multi-bend flat pattern calculator

Work out the flat blank length for a U channel or tray with 2, 3 or 4 bends, instead of adding single bends by hand.

Flat pattern length192.92 mm
Bend deduction per bend3.54 mm

Sum of outside legs: 200.00 mm. Bend allowance per bend: 4.46 mm; outside setback per bend: 4.00 mm. Flat = 200.00 - 2 x 3.54 = 192.92 mm.

All bends are assumed to share the same angle, radius and K-factor. For mixed bends, work bend by bend. Confirm K-factor with a test bend for the actual material and tooling before cutting production blanks.

Formula

Flat length = sum of outside leg lengths - (number of bends x bend deduction). Per bend: outside setback OSSB = (inside radius + thickness) x tan(angle / 2); bend allowance BA = angle in radians x (inside radius + K-factor x thickness); bend deduction BD = 2 x OSSB - BA.

Standard / source

Sheet-metal bend development on the DIN 6935 background, as summarised in the MetalCalz tool-expansion research (2026-10-27). This extends the existing single-bend allowance tool to multiple bends; it does not replace a test bend for the actual material and tooling.

Worked example

U channel, legs 50 + 100 + 50 = 200 mm, thickness 2 mm, inside radius 2 mm, K-factor 0.42, angle 90 degrees, 2 bends: OSSB = (2 + 2) x tan(45 degrees) = 4.00 mm. BA = (pi / 2) x (2 + 0.42 x 2) = 4.46 mm. BD = 8.00 - 4.46 = 3.54 mm per bend. Flat = 200 - 2 x 3.539 = 192.92 mm. These are the calculator defaults.

Safety note

Planning estimate for cutting blanks. All bends are assumed to share the same angle, radius and K-factor; mixed bends must be worked bend by bend. Confirm the K-factor with a test bend for the actual material and tooling before cutting production blanks.