Free K-Factor Calculator for Sheet Metal Bending

What is K-Factor?

The K-factor is a ratio that represents the position of the neutral axis (the plane within the sheet that neither compresses nor stretches during bending) relative to the material thickness. It is defined as:

The K-factor always falls between 0 and 1. A value of 0.5 means the neutral axis sits exactly at the midpoint of the material thickness. In practice, the neutral axis shifts inward during bending, so K-factors are typically less than 0.5 for tight bends.

The K-factor depends on material properties (hardness, ductility), bending method (air bending, bottoming, coining), the ratio of bend radius to thickness (R/T), and grain direction. Getting it right is critical for accurate flat pattern development.

Bend Allowance Formula

Bend Allowance (BA) is the arc length of the neutral axis through the bend. It represents the amount of material consumed by the bend, and is used to calculate the total flat pattern length.

Related formulas:

Both methods produce the same flat length. Use Bend Allowance when your CAD software requires it (Autodesk Inventor, SolidWorks), or Bend Deduction if you work with outside dimensions.

K-Factor Reference Tables

Standard K-factor values based on bending method, material hardness, and the ratio of bend radius to material thickness (R/T). Soft materials include aluminium and copper. Medium includes mild steel and brass. Hard includes stainless steel and spring steel.

Air Bending

Most common press brake method. The punch does not force the material fully into the die.

Bottoming

Material is pressed fully into the die. Higher tonnage, tighter tolerances than air bending.

Coining

Maximum tonnage. Punch penetrates the material for the tightest radii and highest accuracy.

Tips for Accurate Bending

• 1.Always test bend first. Run a sample bend on scrap material from the same batch before cutting your production parts. Material properties vary between suppliers and even between batches.
• 2.Measure actual bend radius. The inside bend radius from your press brake may differ from the punch tip radius, especially in air bending where the radius is partly determined by die width.
• 3.Account for springback. Material springs back after bending. Overbend slightly, or use bottoming/coining for tighter angle control. Springback increases with higher tensile strength and larger R/T ratios.
• 4.Consider grain direction. Bending perpendicular to the rolling grain produces better results and reduces the risk of cracking, especially for hard materials like stainless steel.
• 5.Reverse-engineer for production. For production runs, use the Reverse Engineer tab above to calculate the exact K-factor from a physical test bend. This accounts for your specific material, tooling, and machine characteristics.
• 6.Minimum bend radius. As a general rule, the minimum inside bend radius should be at least equal to the material thickness for soft materials, and 2-3 times thickness for hard materials. Going tighter risks cracking on the outside of the bend.
• 7.Use consistent units. Ensure all your measurements are in the same unit system (mm or inches) throughout your calculations. Mixing units is one of the most common sources of error.

How to Reverse-Engineer K-Factor

Table values are starting points. For production-quality accuracy, reverse-engineering the K-factor from a physical test bend is the gold standard. Here is the process:

1. 1.Cut a test coupon from the same material batch you will use in production. Measure and record the total flat length precisely (Lt).
2. 2.Bend the coupon on your press brake using the exact tooling (punch and die) you plan to use for production. Aim for a single bend at your target angle (typically 90 degrees).
3. 3.Measure both legs after bending. Record the outside dimension of each leg (X and Y) from the outside of the bend to the end of the leg.
4. 4.Measure the actual inside bend radius using radius gauges or a CMM/3D scanner. Do not assume it matches the punch radius.
5. 5.Enter measurements into the Reverse Engineer tab above. The calculator will derive the actual K-factor for your specific setup. A valid result falls between 0 and 1.
6. 6.Use this K-factor in your CAD software (Inventor, SolidWorks, Fusion 360) for accurate flat pattern development for that specific material, thickness, and tooling combination.

Remember: K-factor is specific to the combination of material, thickness, tooling, and bending method. If any of these change, you should run a new test bend.

Learn More About K-Factor

For a deeper dive into K-factor theory, how it relates to Y-factor and bend tables, and practical tips from our press brake operators, read our full guide: