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Sheet Metal Bend Allowance Calculator – Online K-Factor Tool

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Sheet Metal Bend Allowance Calculator

Precision K-Factor bend calculations for fabrication engineers — compute Bend Allowance, Bend Deduction & flat pattern lengths instantly

Industry-Standard Formulas
Input Parameters
mm
Enter sheet metal gauge thickness
mm
Typically 0.5×T to 3×T depending on material
°
0.15 (inner) 0.33 (typical neutral) 0.50 (center)
Optional — Flat Pattern Calculation
mm
From bend apex to edge
mm
From bend apex to edge
Calculation Results
Bend Allowance (BA)
—
mm
Bend Deduction (BD)
—
mm
Outside Setback (OSSB)
—
mm
Total Flat Length
—
mm
BA = (π/180) × A × (R + K×T)  |  BD = 2 × OSSB − BA  |  OSSB = tan(A/2) × (R + T)
T R Neutral Axis A

Schematic diagram — bend angle 90°

K-Factor Reference by Material
Material Typical K-Factor Recommended Min. R/T Ratio Neutral Axis Position Notes
Aluminum 5052-O 0.33 1.0T 33% from inner face Soft temper, excellent formability
Aluminum 6061-T6 0.35 1.5T – 2.0T 35% from inner face Heat-treated, moderate formability
Stainless Steel 304 0.40 – 0.45 1.0T – 1.5T 40–45% from inner face Springback compensation needed
Mild Steel CRS 0.38 – 0.42 0.5T – 1.0T 38–42% from inner face Most common; K≈0.40 default
Copper C110 0.33 – 0.38 0.8T – 1.0T 33–38% from inner face Soft; tight radii possible
Brass C260 0.33 – 0.37 0.8T – 1.0T 33–37% from inner face Cartridge brass, good formability
Spring Steel 0.45 – 0.48 2.0T – 3.0T 45–48% from inner face High springback; larger radii required

T = Material thickness. Actual K-factor varies with tooling, grain direction & bend method. Always validate with test bends.

Frequently Asked Questions
What is Bend Allowance (BA) in sheet metal?

Bend Allowance (BA) is the arc length of the bend measured along the neutral axis of the material. It represents the amount of material consumed by the bend itself. When creating a flat pattern, BA is added to the sum of the straight flange lengths to determine the total blank length. The formula is: BA = (π/180) × A × (R + K×T), where A is the bend angle in degrees, R is the inside bend radius, K is the K-factor, and T is the material thickness. The neutral axis is the plane within the material that neither compresses nor stretches during bending.

What is the K-Factor and why does it matter?

The K-Factor is the ratio of the neutral axis position relative to the material thickness, measured from the inner bend face. K = distance from inner face to neutral axis / T. It typically ranges from 0.30 to 0.50. A K-factor of 0.50 means the neutral axis is exactly at the mid-thickness; lower values mean it shifts toward the inner bend radius. K-factor depends on material type, bending method (air bending, coining, bottoming), tooling, grain direction, and the R/T ratio. Accurate K-factor values are essential for precise flat pattern development.

How is Bend Deduction (BD) calculated?

Bend Deduction (BD) is the amount subtracted from the total of the two flange lengths (measured from the bend apex) to get the correct flat pattern length. The formula is: BD = 2 × OSSB − BA, where OSSB = tan(A/2) × (R + T). For a 90° bend, this simplifies to BD = 2 × (R + T) − BA. The flat pattern length = Flange A + Flange B − BD. BD accounts for the material stretching and the geometry of the bend apex.

What is Outside Setback (OSSB)?

Outside Setback (OSSB) is the distance from the bend apex to the tangent point where the bend begins, measured along the outside flange. Formula: OSSB = tan(A/2) × (R + T). For a 90° bend, OSSB = R + T (since tan(45°) = 1). OSSB is used to determine the mold line distances and is essential for calculating Bend Deduction. Two times OSSB minus BA equals BD.

How to choose the right K-Factor for my material?

Selecting the correct K-factor requires considering: (1) Material type — softer materials have K closer to 0.33; harder materials approach 0.45. (2) Bending method — air bending shifts the neutral axis inward (lower K); coining/bottoming shifts it outward (higher K). (3) R/T ratio — tighter radii push the neutral axis inward. (4) Grain direction — bending across the grain typically yields a slightly different K-factor than bending with the grain. The most reliable approach is to perform test bends, measure the resulting flat pattern, and back-calculate the exact K-factor for your specific setup.

What's the difference between BA method and BD method for flat pattern?

Both methods yield the same flat pattern length but use different reference points: BA Method: Flat = L1_inside + L2_inside + BA (flange lengths measured from the bend tangent lines on the inside). BD Method: Flat = Flange_A + Flange_B − BD (flange lengths measured from the bend apex/vertex on the outside). The BD method is more commonly used in industry because flange dimensions on drawings are typically given from the bend apex. Both methods are mathematically equivalent when applied correctly.

Why does my 90° bend not come out exactly 90 degrees?

This is due to springback — the material's elastic recovery after bending. Springback causes the bend angle to open slightly (e.g., a 90° tool might produce an 88° bend). Springback is more pronounced in harder materials (stainless steel, spring steel) and larger bend radii. To compensate, operators over-bend the material slightly. The amount of over-bend depends on material properties, thickness, and R/T ratio. K-factor calculations assume the final desired angle; springback compensation is a separate adjustment made at the press brake.

Can I use the same K-Factor for all bend angles?

In theory, the K-factor should be consistent across bend angles for the same material and R/T ratio, because the neutral axis position is a material property. However, in practice, slight variations occur — especially at very acute angles (<30°) or very obtuse angles (>150°). For critical applications with multiple bend angles, it's advisable to validate the K-factor at each angle through test bends. Most manufacturers use a single calibrated K-factor per material/thickness combination, which provides sufficient accuracy for the majority of applications.

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