What Are the K Factor, Bend Allowance, and Bend Deduction?

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Publish Date: May 7, 2022

K-factor, bend allowance, and bend deduction are important values used to calculate the correct sheet metal flat length or flat pattern.

The length of the sheet in the bending state is different from the length of the flat pattern because the sheet metal will stretch and compress during bending on the press brake.

In this article, we will talk about the K factor, bend allowance, and bend deduction.

Understand Sheet Metal Bending

During the bending process of the metal plate, the outer surface is under tension and the inner surface is compressed.

The neutral axis located between the two surfaces will not be subjected to stress.

The K-factor is the ratio of the neutral axis position to the total material thickness.

The K-factor is used to determine the length of the metal plate stretched during the bending process.

The K-factor can also be used to calculate bending allowance and bending deduction.

The value of the K-factor is influenced by the material properties and the press brake.

What Is K Factor?

First, we need to understand the neutral axis. When sheet metal is bent, the bottom surface is compressed, and the top surface is stretched.

The neutral axis is located inside the metal where it will neither be compressed nor expanded, allowing it to maintain a constant length.

In flat sheet metal, the neutral axis is evenly located at half the thickness of the sheet metal, but it will move during bending.

The position change of the neutral axis is determined by various factors such as the material properties, thickness, bending angle, internal radius, and bending method of the plate.

Mathematically, the K-factor represents the ratio between the position of the neutral axis (t) and the plate thickness (T).

Where t is the distance from the inside surface to the neutral axis and T is the metal thickness(Figure 1).

The K-factor is determined by the physical properties of the material, bending method, bending angle, and other factors.

It typically ranges between 0.30 and 0.50.

In precision sheet metal manufacturing, the K-factor is a crucial factor.

The K-factor is used to calculate the bending flat pattern, which is directly related to the length of the sheet metal stretched during bending.

It is the fundamental value for determining the bending allowance and bending deduction.

Since the ratio of the distance to the neutral axis to the plate thickness determines the position of the neutral axis in the metal plate, knowing the K-factor helps determine the position of the neutral axis after bending.

You can also determine the K-factor using the table below:

You can also use the online & free K Factor Calculator for perfect sheet metal bending

What is Bend Allowance?

The Bend Allowance (BA) is the arc length of bending measured along the neutral axis of the metal plate since the length of the neutral axis does not change after bending.

Once the bend allowance is calculated, it should be added to the flat length to determine the required sheet metal length needed to form the desired workpiece.

As is shown in the below figure:

The following formula is used to calculate the flat length:

Sheet Metal Flate Length=Leg Length 1 +BA + Leg Length 2

Bend Allowance Calculation Formula


A - Bend angle

R - Inside radius

K - K factor

T - Material thickness

What Is Bend Deduction?

After bending, the flange length (i.e., the total length) is greater than the length of the sheet metal flat pattern.

Bend deduction is the length of material that we need to remove from the total length of the plate to obtain the correct flat pattern.

Bend Deduction Calculation Formula:

According to the figure above, the bend deduction is the difference between the bend allowance and twice the outside setback.

Bend Deduction = 2 × OSSB - BA

Bend Deduction Calculation Formula:


The K-factor, bend allowance, and bend deduction are crucial factors to consider when designing and bending sheet metal parts.

By calculating these values, the material length and workpiece size can be determined, resulting in more accurate bending and reduced material waste.

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The K-factor represents the position of the neutral axis relative to the thickness of the sheet.

The value of the K-factor is influenced by factors such as the characteristics of the metal sheet and the bending radius.

The bending allowance is used to calculate the length of the unfolded bending area.

The bending deduction is determined by the difference between the total length of the metal plate and the total length after bending.

The three factors of K-factor, bending allowance, and bending deduction help to ensure the accuracy of the bent workpiece.

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