Flat Pattern Calculator (Multi-Bend)
Flat blank length for a part with several bends: flanges, angles, radii, thickness and K-factor → bend allowance and deduction per bend, developed length.
The flat length, the sum of the bend deductions (outside dimensions) or allowances (inside), and a per-bend table with BA, setback, BD, neutral radius and R/T — with notes where the K-factor rule is being stretched.
Example: A U-channel 50 / 100 / 50 outside in 2 mm with two 90° bends at R2 and K 0.44: BA 4.524 and BD 3.476 per bend, flat 193.05; the same channel dimensioned inside is 80 + BA per bend.
The bend allowance,
applied bend by bend.
How each bend is turned into an allowance or a deduction, and what K stands for.
Per bend
Each bend has an angle θ, an inside radius R, and the sheet thickness T. The neutral axis sits at K·T from the inside face, so the arc it travels is BA = θ(R + K·T). The outside setback — from the mould line (where the two outside faces would meet) to the tangent — is tan(θ/2)(R + T) for bends up to 90°, and R + T beyond that, where the tangent length no longer grows. The bend deduction is BD = 2·setback − BA.
Adding up
With flanges dimensioned outside (to the mould lines), the flat is Σ flanges − Σ BD, because each flange pair overlaps by two setbacks and the material actually needed is the arc. With inside dimensions (to the tangents), the flat is simply Σ flanges + Σ BA. Bend order does not change the total; it changes the layout.
K is yours
The K-factor comes from your material, radius, tooling and grain direction — a bend test or the shop table — and is entered, not assumed. The page notes bends over 90° (setback rule changes), tight bends (R/T under 1) and large radii (R/T over 5) where a single K is least reliable. Springback and grain are not modelled. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Bend allowance BA = θ(R + K·T), outside setback tan(θ/2)(R + T) for θ ≤ 90° and R + T beyond, bend deduction 2·OSSB − BA — the standard press-brake flat-pattern arithmetic; K-factor is yours
Last reviewed 20 September 2026. How results are checked: How we verify.