Bend Allowance Calculator

Calculate how much material a sheet metal bend consumes along its neutral axis from thickness, inside radius, bend angle, and K-factor.

Quick Facts

Formula
BA = B(rad) × (R + K·T)
B = bend angle, R = inside radius, K = K-factor, T = material thickness.
K-factor range
Typically 0.33 to 0.50
Lower values suit tight bends (R < 2T); higher values suit large-radius bends.

Your Results

Calculated
Bend Allowance (BA)
-
Neutral-axis arc length added per bend
Bend Deduction (BD)
-
Subtract from outside flange sum for flat length
Setback (SB)
-
Mold line to tangent-point distance
Total Bend Allowance
-
BA multiplied by the number of bends

Ready

Enter thickness, inside radius, bend angle, and K-factor, then press Calculate.

About the Bend Allowance Calculator

Sheet metal does not fold for free — the material along a bend arcs and stretches slightly as it is formed, which changes the developed (flat) length of the part compared with simply adding up the finished flange lengths. Bend allowance quantifies exactly how much length to add at each bend so a flat pattern folds up to the correct finished dimensions.

Understanding the formula

The standard bend allowance formula measures the arc length of the neutral axis — the layer inside the sheet that neither stretches nor compresses during forming:

BA = B × (π/180) × (R + K × T)

where B is the bend angle in degrees (how far the material is bent away from flat), R is the inside bend radius, T is the material thickness, and K is the K-factor — the fraction of thickness at which the neutral axis sits, measured from the inside face of the bend.

Bend deduction and setback

Bend deduction (BD) is the companion figure used when flange lengths are measured to the outside mold line (where the flat flange faces would intersect) rather than along the neutral axis. Setback (SB) is the distance from that mold-line intersection back to the tangent point where the flat material begins to curve: SB = (R + T) × tan(B/2). Bend deduction is then BD = 2 × SB − BA. Subtracting BD from the sum of the outside flange lengths gives the flat pattern length.

Choosing a K-factor

The K-factor is not a fixed constant — it shifts with material, bend radius, thickness, and forming method (air bending, bottoming, or coining). A widely used rule of thumb is K = 0.33 for tight bends where the inside radius is less than twice the material thickness, and K = 0.50 for larger-radius bends where the radius is at least twice the thickness. For precise fabrication work, use a K-factor measured or supplied for your specific material and process rather than the rule-of-thumb value.

Working with units

Enter thickness and radius in the same length unit — commonly millimeters or inches — since the calculator does not convert between systems; results are reported in that same unit. The bend angle is always in degrees, measured as the amount the material is bent away from flat, so a simple right-angle bracket uses a 90-degree bend angle.

Frequently Asked Questions

What is bend allowance?
Bend allowance is the length of material consumed along a bend's neutral axis, the line inside the sheet that neither stretches nor compresses when the metal is formed. Adding it to the flange lengths measured along that neutral line gives the flat, unbent length of a sheet metal part before forming.
What is the difference between bend allowance and bend deduction?
Bend allowance describes length added along the neutral axis. Bend deduction applies when flange lengths are measured to the outside mold line instead; it equals twice the setback minus the bend allowance (BD = 2 x SB - BA), and is subtracted from the summed outside flange lengths to get the flat pattern length.
How do I choose a K-factor?
K-factor depends on material, bend radius, thickness, and bending method (air bending, bottoming, or coining). A common approximation is K = 0.33 for tight bends where the inside radius is less than twice the thickness, and K = 0.50 for larger-radius bends. For precise work, use a K-factor measured for your specific material and process.
What angle should I enter for the bend angle?
Enter the amount the material is bent away from flat, in degrees, not the included angle between the finished flanges. A simple 90-degree corner bracket uses a bend angle of 90 degrees.

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