Sheet Metal Bend Calculator — Bend Allowance, Tonnage, V-Die & Flat Pattern

Bend allowance (BA) = π/180 × (R + K × T) × A, where R is the inside bend radius, K is the K-factor (neutral-axis position as a fraction of thickness), T is the material thickness, and A is the bend angle in degrees. The term (R + K × T) is the radius of the neutral axis — the layer of metal that neither stretches nor compresses. Use the Bend Allowance tab above to compute it with any units.

Bend allowance (BA) is the arc length of the neutral axis through the bend — you ADD it to the flat leg lengths measured to the mold (tangent) lines. Bend deduction (BD) is the amount you SUBTRACT from the sum of the outside flange dimensions: BD = 2 × OSSB − BA, where OSSB (outside setback) = (R + T) × tan(A/2). Both give the same flat length; shops use whichever matches how they dimension drawings.

The K-factor is the neutral axis position as a fraction of thickness (0 = inside surface, 0.5 = middle). Practical values: 0.33 for sharp bends (R < T), 0.38 for R/T 1–2, 0.41 for R/T 2–3, 0.43 for R/T 3–4, 0.45 for R/T 4–10, and 0.50 for large radii. Softer materials (aluminum, copper) run slightly higher; hard stainless slightly lower. For critical work, bend a test coupon, measure it, and back-calculate K — the K-Factor tab estimates it from your R/T ratio.

For air bending: F (kN) = 1.42 × Rm × S² × L / V, where Rm is the material tensile strength in MPa, S is thickness in mm, L is bend length in meters, and V is the die opening in mm. The imperial equivalent is the exact unit conversion of the same formula: P (US tons) = 1.236 × Rm × S² × L / V with S and V in inches and L in feet (for mild steel at 450 MPa this simplifies to ≈ 556 × S² × L / V). Bottoming needs roughly 4× and coining roughly 5× the air-bending force. Compare the result against your press brake's load chart and never exceed machine or tooling capacity.

Air bending presses the sheet into the V-die without touching the bottom — one punch/die set makes many angles, but springback must be compensated. Bottoming (bottom bending) forces the sheet against the die walls for a precise angle at roughly 4× the air-bending tonnage. Coining stamps the radius into the sheet at roughly 5× tonnage with almost no springback, but it needs a high-tonnage press and wears tooling fast. The Tonnage tab shows all three side by side.

The standard rule for mild steel is V = 6 × thickness for sheet ≤ 3 mm, V = 8 × thickness for 3–10 mm, and V = 12 × thickness above 10 mm. For stainless steel, use about 1.5× the opening you'd pick for mild steel (or accept higher tonnage). The resulting inside bend radius in air bending is roughly V/6. A V that is too narrow spikes tonnage and can crack the sheet; too wide wastes force and accuracy. The V-Die tab applies these rules automatically.

A flange shorter than about 70% of the V-die opening (0.7 × V) will fall into the die instead of bridging it, so the bend can't form properly. For example, a 12 mm V-die needs flanges at least ~8.4 mm long. If your part needs a shorter flange, use a narrower V (which raises tonnage), a rotary bender, or a different forming method.

Flat length = (sum of all outside flange dimensions) − (sum of each bend's bend deduction). Measure every flange as an outside dimension, compute BD for each bend from its angle, radius, K-factor and thickness, then subtract. Equivalently: flat = (sum of mold-line leg lengths) + (sum of bend allowances). The Flat Pattern tab accumulates any number of bends and shows the running total plus a per-bend breakdown.

Punching force = cut perimeter × thickness × material shear strength. For a round hole: F = π × d × S × τ. Example: a 10 mm hole in 2 mm mild steel (shear ≈ 300 MPa) needs π × 10 × 2 × 300 / 1000 ≈ 18.9 kN. Rectangular holes use perimeter 2 × (w + h). Ironworker and punch-press ratings are just this formula with the machine's shear-strength assumption — the Punching section computes it for round, rectangular, and custom perimeters in metric or imperial.

Honest answer: published tonnage factors (575, 650, 660…) each assume a different tensile strength for 'mild steel' and different safety margins, so single-number calculators can disagree by 10–20%. This tool avoids the hidden assumption by asking for the material's actual tensile strength (Rm) and applying one consistent physics formula in both unit systems. Treat every tonnage number as a planning estimate and confirm against your press brake's own load chart before running the job.

Tonnage scales directly with tensile strength: stainless 304 (≈585 MPa) needs about 1.3× the force of mild steel (≈450 MPa) for the same bend, aluminum 5052 (≈215 MPa) about half. Punching force scales with shear strength the same way. The K-factor shifts slightly too — soft aluminum runs a touch higher, hard stainless a touch lower. Pick the material in each tab (or enter custom Rm/τ from your mill certificate) and every result updates.

Aircraft work typically uses tight inside radii (often 1–2× thickness), which puts the K-factor near 0.33–0.38. Aluminum 2024-T3 also has strict minimum bend-radius limits to avoid cracking — consult the aircraft structural repair manual (SRM) for the exact radius tables for your alloy and temper. This calculator handles the math; the airworthiness limits come from the manual, not from any calculator.

Yes — the Reference Chart section below contains a printable 90° bend-allowance and bend-deduction table for common thicknesses and radii (K = 0.40), plus the V-die selection chart. Use your browser's print function (or the Print button) with the chart section — it is formatted to print cleanly on one page for the shop wall.

Either — the unit toggle at the top converts every input and result, and the underlying formulas are identical physics, so metric and imperial results always agree exactly. US fab shops typically work in inches, gauge, and US tons; most of the rest of the world uses mm and kN. Note that sheet 'gauge' numbers are not linear — enter the actual decimal thickness, not the gauge number.

These are planning estimates for quoting, tooling selection, and process planning. Before bending production parts: verify speeds, feeds, tonnage, and K-factors against Machinery's Handbook and your tool manufacturer's data; confirm tonnage against your press brake's load chart and tooling limits; never exceed machine capacity; wear eye protection and cut-resistant gloves when handling sheet (edges are razor sharp); keep hands clear of the point of operation; and never wear gloves near rotating machinery.

Yes. This tool runs 100% in your browser — every calculation happens locally in JavaScript on your device. No inputs, dimensions, or results are ever sent to a server, stored, or tracked. You can even save the page and use it offline.