Sheet metal bend allowance calculator

Sheet Metal Bend Allowance Calculator | K-Factor & Bend Deduction

Bend Allowance · Bend Deduction · K-Factor · Press Brake

Sheet Metal Bend Allowance Calculator

Calculate the neutral-axis radius, bend allowance, outside setback, and bend deduction from sheet thickness, bend angle, inside radius, and K-factor. Use the result for flat-pattern estimating and press brake setup, then verify production values with the applicable bend table and a measured test bend.

mm
deg
mm
ratio
Starting values only. Confirm the final factor with your bend table or a test coupon.
Interactive sheet metal bend diagram The sheet angle, thickness, inside radius, and dotted neutral axis update as the calculator inputs change. t Angle 90° Neutral axis r = 3.66 mm BA = 5.749 mm

Cross-sectional bend reference. The plate angle, thickness, bend radius, and dotted orange neutral axis update in real time.

Bend Allowance
5.749 mm
Bend Deduction
4.251 mm
Outside Setback
5.000 mm
Neutral-Axis Radius
3.660 mm
K-Factor Used
0.33

Fix the highlighted field above to see the bend calculation.

Bending Fundamentals

What Controls Bend Allowance

A bend stretches the material near the outside surface and compresses it near the inside surface. Between those zones is the neutral axis, where the developed length is used for flat-pattern calculations. The K-factor estimates where that axis sits through the sheet thickness.

Sheet Thickness

Use the measured material thickness rather than relying only on the nominal gauge. Small thickness differences affect neutral-axis position, setback, bend deduction, and press brake behavior.

Bend Angle

The bend angle controls the arc length consumed by the bend. The calculator accepts angles greater than zero and below 180 degrees because the outside-setback formula approaches infinity at 180 degrees.

Inside Bend Radius

The inside radius is measured to the inside surface of the formed sheet. Tooling, material behavior, and bend method determine the actual radius produced on the press brake.

K-Factor

The K-factor is the neutral-axis distance from the inside surface divided by sheet thickness. The quick guide provides starting values, but shop-specific test data should control production work.

Neutral-Axis Radius

The neutral-axis radius equals the inside radius plus K-factor multiplied by sheet thickness. Bend allowance is measured along this theoretical radius.

Press Brake Verification

Actual flat patterns can differ because of tooling, springback, material temper, grain direction, die opening, bend method, and measured thickness. Validate the calculation with a test bend before releasing production blanks.

Calculation Reference

Bend Allowance and Deduction Formulas

The calculation first locates the neutral axis. It then calculates the developed arc length through the bend, the outside setback from the virtual sharp intersection, and the bend deduction used with outside flange dimensions.

  1. Locate the neutral axis through the sheet thickness. r = R + (K × t)
  2. Calculate the developed arc length along the neutral axis. BA = (π ÷ 180) × θ × (R + K × t)
  3. Calculate the outside setback from the virtual sharp corner. OSB = tan(θ ÷ 2) × (R + t)
  4. Convert the two setbacks and arc length into bend deduction. BD = (2 × OSB) − BA
  • Neutral Radiusr = R + Kt
  • Bend AllowanceBA = (π/180)θ(R + Kt)
  • Outside SetbackOSB = tan(θ/2)(R + t)
  • Bend DeductionBD = 2OSB − BA
  • t Sheet thickness
  • θ Bend angle in degrees
  • R Inside bend radius
  • K K-factor
  • r Neutral-axis radius
Worked Example — 2 mm Sheet, 90° Bend
  • Inside radius3.0 mm
  • K-factor0.33
  • Neutral radius3 + (0.33 × 2) = 3.66 mm
  • Bend allowance(π/180) × 90 × 3.66 = 5.749 mm
  • Outside setbacktan(45°) × (3 + 2) = 5.000 mm
Bend deduction = 10.000 − 5.749 = 4.251 mm

Shop Procedure

Using Bend Allowance for Press Brake Setup

Use the calculated values as an engineering starting point. Reliable production requires measured material, known tooling, and a verified bend result rather than assuming one universal K-factor for every job.

1. Measure the Actual Sheet

Check thickness with a calibrated micrometer or vernier caliper. Nominal gauge and delivered thickness can differ enough to shift the flat pattern and final flange dimensions.

2. Confirm Tooling and Bend Method

Record punch radius, die opening, bend method, and intended inside radius. Air bending, bottoming, and coining do not produce the same material response or neutral-axis position.

3. Run a Test Coupon

Bend a short strip from the production sheet, measure the final angle and flange dimensions, and adjust the K-factor or bend deduction until the flat pattern matches the real result.

4. Lock the Shop Value

Store the approved bend deduction or K-factor by material, thickness, tooling, grain direction, and machine setup. Recheck when any of those conditions change.

Theoretical Results Need Production Verification

Material temper, springback, coating, grain direction, actual thickness, punch condition, die opening, and brake calibration can all change the finished bend. Use the calculator for layout and estimating, but control final blanks with your verified shop bend table and inspection measurements.

Frequently Asked Questions

Sheet Metal Bend Calculator FAQ

These answers explain the calculation basis and the production checks that remain necessary before cutting a final flat pattern.

What is bend allowance in sheet metal?
Bend allowance is the developed length of the neutral axis through the bend. It represents the material length consumed by the curved portion and is calculated from the bend angle, inside radius, sheet thickness, and K-factor.
What does the K-factor represent?
The K-factor describes the neutral-axis position through the material thickness. A value of 0.33 places the neutral axis at approximately one-third of the thickness measured from the inside bend surface. It should be verified against the actual material, tooling, bend method, and test pieces.
How is bend allowance calculated?
Bend allowance equals pi divided by 180, multiplied by the bend angle in degrees, multiplied by the inside bend radius plus K-factor times sheet thickness.
How is outside setback calculated?
Outside setback equals the tangent of half the bend angle multiplied by the sum of the inside bend radius and sheet thickness.
How is bend deduction calculated?
Bend deduction equals two times the outside setback minus the bend allowance. It is used to subtract the bend contribution from the sum of the outside flange dimensions when calculating a flat blank.
Which K-factor should I use for cold rolled steel?
A K-factor of 0.33 is a common starting value for cold rolled steel in estimating. Actual production values can change with material grade, hardness, bend radius, grain direction, tooling, and whether the process is air bending, bottoming, or coining.
Why can the calculated flat pattern differ from the press brake result?
Differences can come from actual sheet thickness, material temper, inside radius, punch and die geometry, springback, bend method, grain direction, tooling wear, and machine setup. A test bend and measured coupon should control final production settings.
Can this calculator replace a bend table or test bend?
No. It provides a theoretical layout value. For production work, verify the result against the shop bend table, tooling data, material certificate, press brake program, and a measured test bend.