What the Bend Allowance Calculator does
This tool gives the numbers a press brake operator or sheet metal designer needs to turn a folded part into a flat blank. Enter the material thickness, inside bend radius, bend angle and K-factor, and it returns the bend allowance, the bend deduction, the outside setback, and, if you add the two flange lengths, the flat pattern length to cut. It is built for fabricators making enclosures, panels, brackets and machine parts.
The result is the standard neutral-axis calculation, so it matches what your CAD and press brake expect. Everything runs in your browser and updates as you type.
How to use it
- Enter the material thickness and the inside bend radius in millimetres.
- Set the bend angle (90 for a right angle).
- Set the K-factor (0.38 is a reasonable default for steel; use your measured value if you have one).
- Optionally enter the two outside flange lengths to get the flat length to cut.
- Read the bend allowance, bend deduction and setback, plus the flat length.
Why use it here
It is free, instant and private, with no sign-up and nothing uploaded. It gives bend allowance, deduction, setback and flat length together, which is exactly the set a fabricator needs to mark out a blank. For multi-bend parts, subtract one bend deduction per bend from the outside-to-outside dimensions.
Tip: to size the machine for the job, use the press brake tonnage calculator, and to weigh the finished sheet, use the metal weight calculator.
Frequently asked questions
- How is bend allowance calculated?
- Bend allowance is the arc length of the neutral axis through the bend: BA = (π/180) × angle × (inside radius + K-factor × thickness). The tool also returns the outside setback, OSSB = (radius + thickness) × tan(angle/2), and the bend deduction, BD = 2 × OSSB − BA.
- How do I get the flat length of my part?
- Enter the two outside flange lengths. The flat pattern length is the sum of the flanges minus the bend deduction: Flat = A + B − BD. Add one bend deduction for each bend in a multi-bend part.
- What K-factor should I use?
- The K-factor is where the neutral axis sits as a fraction of thickness, between 0 and 0.5. A common starting value for steel is 0.33 to 0.42 (this tool defaults to 0.38). Soft materials and tight radii run lower, large radii higher. Use the figure your shop has measured for the best accuracy.
- Is the bend angle the included angle or the angle of bend?
- Enter the angle the material is bent through. A square corner is a 90 degree bend, a gentle bend is a smaller number. For a 90 degree bend the setback equals radius plus thickness.
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Last updated: June 23, 2026