Skip to content

Mastering Bend Allowance Calculations

By meticulously following each step outlined in this guide, you can enhance the accuracy and efficiency of your bending operations. Maybe you also want to check out our Section Bending Machine page.

In the intricate world of sheet metal bending, mastering the concept of the k-factor is crucial for achieving accurate and precise results. The k-factor, representing the ratio of the location of the neutral axis to the material thickness, plays a pivotal role in determining bend allowances and ensuring optimal bending outcomes. Let’s delve deeper into the factors influencing the k-factor and explore methods for its calculation and application.

The Influence of Bend Radius and Forming Method

The k-factor is profoundly influenced by the bend radius and the chosen forming method. Altering the inside bend radius relative to the material thickness can lead to significant shifts in the k-factor. For instance, reducing the inside bend radius may induce cracking on the outer surface of the bend, causing the neutral axis to shift inward and decreasing the k-factor.

Similarly, transitioning between different forming methods, such as air forming, bottoming, or coining, can impact the k-factor. Deformation and thinning of the bend radius during bottoming result in an increased k-factor, whereas coining, which relieves stress, leads to a decrease in the k-factor as the neutral axis shifts towards the inner surface of the bend.

Effect of Material Thickness and Tooling

Changes in material thickness and tooling configurations also influence the k-factor. Thicker and harder materials tend to decrease the k-factor, whereas adjustments in tooling, such as using narrower die widths, can increase the k-factor. Moreover, maintaining a constant material thickness while altering tooling setups affects bending force and consequently influences the k-factor.

Levels of Accuracy in K-Factor Determination

Achieving precise bend calculations requires an understanding of the variables influencing the k-factor. While an average k-factor value of 0.4468 suffices for many applications, more accurate results may necessitate alternative methods of k-factor determination.

One approach involves establishing the k-factor based on the radius-to-material thickness relationship. For instance, if the bend radius is less than double the material thickness, the k-factor is typically 0.33, while a radius greater than double the material thickness corresponds to a k-factor of 0.5. Additionally, referencing k-factor charts provides further accuracy in selecting appropriate k-factor values for specific bending scenarios.

Measuring Test Pieces for Precise K-Factor Determination

For utmost accuracy, calculating the k-factor from test bends offers unparalleled precision tailored to specific material grades and bending conditions. This method entails measuring test pieces, including the bend allowance (BA) and the inside radius (Ir), to derive the k-factor formula.

Accurate measurement techniques, such as employing pin gauges, radius gauges, or optical comparators, ensure precise determination of the Ir. Measuring the BA, which represents the arc length of the neutral axis, requires careful assessment before and after bending to ascertain accurate results.

Bend Allowance Calculation for 90-Degree Bends

In the case of 90-degree bends, determining the bend allowance involves measuring the total outside dimension of the formed part and subtracting the material thickness (Mt) and the measured inside radius (Ir) from the outside flange dimension. Adding the two inside leg dimensions together and subtracting the flat dimension yields the bend allowance, providing a clear understanding of the bending dynamics at this specific angle.

Achieving precision in metal fabrication demands a thorough understanding of bend allowance calculations. In this comprehensive guide, we'll delve into the intricate process of determining bend allowance step by step. Whether you're a seasoned professional or a novice in the field, this article will equip you with the knowledge and skills needed to excel in metal bending operations.

If your bend equals 90 degrees, you can measure the total outside dimension of the formed part, then subtract the Mt and the measured Ir from the outside flange dimension; this gives you the inside leg dimension. Add your two inside leg dimensions together, then subtract the flat dimension, and you get the BA:Inside leg dimension for 90-degree bend = Outside dimension – Mt – Ir
Measured inside leg dimensions – Measured flat = BA

Again, this equation works only for 90-degree bends, basically because of how the radius and leg dimensions relate at a 90-degree angle. Technically speaking, it’s because the flat leg length meets the Ir at the tangent point.

Greater or Less Than 90 Degrees: Step-by-Step Guide

To measure the Bend Allowance (BA) for bends with angles greater or less than 90 degrees, trigonometry comes into play. This involves a bit more complexity but is essential for accurate calculations.

It’s important to note that while the trigonometric equations provided here are effective, they are not the only options available. You can consult any trigonometry resource, whether online or in your library, to explore alternative equations for solving various sides and angles of a right-angle triangle.

Let’s begin by addressing an external angle less than 90 degrees. Let’s take the example of a 60-degree external bend angle as illustrated in Figure 2. The following steps correspond directly to the referenced figure, and these steps must be repeated for the second inside leg dimension.

Mastering Bend Allowance Calculations: A Comprehensive Guide
Figure 1: The terminology used for this discussion is presented here.

Step 1: Measure Dimension A

The journey begins with precise measurements. Using a calibrated measuring tool, carefully measure dimension A on the test piece. Accurate measurements lay the foundation for successful bend allowance calculations.

Step 2: Add Material Thickness (Mt) to Dimension A

Dimension A serves as the starting point. By adding the material thickness (Mt) to dimension A, you obtain dimension B, which sets the stage for further calculations.

Figuring k factor in sheet metal bending
Figure 2: This shows one way in which you can use right-angle trigonometry to “walk through the triangles” and calculate the inside leg dimension (dimension F) of a bend with an external angle of 60 degrees.

Step 3: Measure the Inside Bend Radius (Ir)

Utilize specialized instruments such as a pin gauge, radius gauge, or optical comparator to measure the inside bend radius (Ir) accurately. The inside bend radius plays a crucial role in bend allowance calculations, influencing the final outcome of the bending process.

Step 4: Calculate the Outside Setback (OSSB)

The next step involves computing the outside setback (OSSB), a fundamental parameter in bend allowance calculations. Employ the following formula to determine OSSB:
OSSB = [tangent (external bend angle/2) × (Mt + Ir);
Understanding the geometry of the bending operation is essential for accurate OSSB calculation. Visualize the green triangle formed by OSSB, with angles C and B guiding the determination of side b.

Step-by-Step Guide

To calculate the Outside Setback (OSSB), we’ll utilize trigonometric functions to determine the dimensions of the green triangle. Here’s a step-by-step guide:

  • Step 1#: Calculate OSSB:
    Use the formula: OSSB = [tangent (external bend angle/2) × (Mt + Ir)].
    OSSB represents side ‘a’ of the green triangle.
  • Step 2#: Determine Angles of the Green Triangle:
    Given that the external bend angle is 60 degrees, angle C of the green triangle is 30 degrees, and angle B is 60 degrees.
  • Step 3#: Solve for Side b of the Green Triangle:
    Use the formula: b = a × sine B.
    Side b corresponds to dimension C, which extends to the tangent point on the material’s outside surface.
  • Step 4#: Adjust for True Position of Dimension C:
    Note that at this particular bend angle, dimension C may closely match or be very close to the material thickness (Mt). However, dimension C will vary depending on the bend angle.
    Therefore, we use the OSSB to accurately calculate dimension C’s true position.

By following these steps, we can accurately determine the Outside Setback (OSSB) and ensure precise measurements for further calculations in the bending process.

Step 5: Determine Dimension D

Dimension D corresponds to side c of the red right-angle triangle. With side a (hypotenuse) representing the material thickness (Mt), angle B of the purple triangle is derived from the external bend angle. Employing trigonometric principles, calculate dimension D using the cosine of angle B.

Step-by-Step Guide

To determine dimension D, which corresponds to side c of the red right-angle triangle, we’ll follow these steps:

  • Identify Angles and Sides:
    Side a (the hypotenuse) is equivalent to the material thickness (Mt).
    Angle B of the purple triangle represents the external bend angle, which is 60 degrees. This angle ensures that angle C of the purple triangle is 30 degrees, given the sum of angles in a triangle is 180 degrees.
    With the material edge forming a 90-degree angle, angle B of the red triangle is 60 degrees.
  • Calculate Side c of the Red Triangle:
    Utilize the formula: c = a × cosine B.
    Side c, also known as dimension D, represents the length from the material’s edge to the tangent point on its outside surface.

By applying trigonometric principles and the known angles and sides of the triangles involved, we can accurately compute dimension D, facilitating precise measurements in the bending process.

Step 6: Calculate Dimension E

Dimension E serves as a pivotal parameter in bend allowance calculations. By subtracting the sum of dimensions C and D from dimension B, you obtain dimension E, which contributes to the overall accuracy of the bending process.

Step 7: Solve for Dimension F

Dimension F, the inside leg length, is derived from the purple triangle’s angles and dimensions. Employing cosine C, calculate dimension F to ascertain the precise inside leg length of the bend.

Handling External Bend Angles Greater Than 90 Degrees: For workpieces featuring external bend angles exceeding 90 degrees, a similar procedure is followed. Commence with measured dimensions on the test piece and navigate through right triangles to determine the inside leg dimension. Replicate this process for the opposite leg to ensure comprehensive accuracy.


Mastering bend allowance calculations is indispensable for achieving precision in metal fabrication. By meticulously following each step outlined in this guide, you can enhance the accuracy and efficiency of your bending operations. Armed with a thorough understanding of bend allowance principles, you’re well-equipped to tackle complex bending challenges with confidence and expertise.

Works Cited and Img Resources: Analyzing the k-factor in sheet metal bending