Practical tools for measured work
Sheet Metal Bend Allowance Calculator
Calculate sheet-metal bend allowance, deduction and setback, or find K-factor from a measured test bend.
Changing mode resets the inputs to that mode's example.
0 to 180 exclusive. Opening = 180 minus this value.
The radius actually formed in the part, not necessarily the tool nose radius.
0 to 1. Position of neutral axis as fraction of thickness.
How it works
Angle convention
The bend angle is the rotation from flat. A 90-degree bend means the sheet has been rotated 90 degrees from its original flat position. The opening (the angle you see between the two flanges) equals 180 minus the bend angle.
Inside radius
Enter the radius actually formed in the part. This is not automatically the tool nose radius. Springback, material behavior, and tool geometry all affect the final inside radius. Measure or verify the formed radius on a sample part.
Outside flange datum (calibration mode)
The outside flange dimension is measured from the virtual sharp corner (the mold-line intersection where the two flat faces would meet if the bend had zero radius) to the part edge. In practice, extend the two flat faces until they intersect; that intersection point is your datum. This is the same reference used in flat-pattern drawings.
Formulas
Forward (allowance from K):
- Neutral radius = R + K x T
- Bend allowance = radians(angle) x neutral radius
- Outside setback = tan(radians(angle)/2) x (R + T)
- Bend deduction = 2 x setback - allowance
Inverse (K from measurement):
- Bend deduction = flange A + flange B - flat length
- Bend allowance = 2 x setback - deduction
- K = (allowance / radians(angle) - R) / T
Worked example (allowance mode)
Suppose a part has tangent legs of 20 mm each, R = 2 mm, T = 1.5 mm, angle = 90 deg, K = 0.33. Setback = tan(45 deg) x (2 + 1.5) = 3.5 mm. Outside legs = 20 + 3.5 = 23.5 mm each. Allowance = (pi/2) x (2 + 0.33 x 1.5) = 3.919 mm. Flat length = 2 x 20 + 3.919 = 43.919 mm.
Now consider a different part where the outside legs are 20 mm each (tangent legs = 16.5 mm). Flat length = 2 x 16.5 + 3.919 = 36.919 mm. Same bend parameters, different part geometry, different flat length.
Calibrated example
A=25, B=25, flat=47.1416, R=1, T=2, angle=90. Setback = 3. Deduction = 50 - 47.1416 = 2.8584. Allowance = 6 - 2.8584 = 3.1416 = pi. K = (pi / (pi/2) - 1) / 2 = 0.5.
K-factor transferability
K depends on the specific combination of machine, material, tooling, and bend radius. A K-factor calibrated on one setup does not universally transfer to another. Always verify with a test bend for production-critical parts.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Bend angle (rotation from flat)
- 90 deg
- Inside radius (actual formed)
- 2 mm
- Sheet thickness
- 1.5 mm
- K-factor
- 0.33
Primary result: Bend allowance: 3.9191 mm.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
This calculator assumes a single uniform bend with constant radius and thickness.
It does not account for springback, multi-bend interactions, or non-uniform material properties.
The K-factor model is an approximation. Check a representative test bend before cutting a production batch.
If calibration is rejected, check the measurements and angle convention. This tool accepts K between 0 and 1 and rejects a zero-radius, zero-allowance calibration.
Frequently asked questions
Why does my calibrated K-factor differ from the table value?
K is not a material constant. It shifts with tool radius, press stiffness, material lot, and bend angle. A table value is a starting estimate. Your calibrated value from an actual test bend on the same setup is the one to trust for that specific part.
Can I use the same K-factor for a 30-degree bend if I calibrated at 90 degrees?
It is a reasonable first approximation, but K does vary with angle because the neutral axis position shifts as the bend becomes sharper or more gradual. For production work, calibrate at the actual bend angle you will use.
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