Introduction
When you bend a sheet of metal, the metal stretches. This means the flat piece you start with must be a little shorter than the two finished legs added together. The amount you take away is called the bend deduction.
Type in your bend angle, material type, thickness, and inside bend radius. The calculator returns the bend deduction, the bend allowance, the K-factor, and the flat blank length. You can work in inches or millimeters.
Why it matters: if your flat pattern is wrong, your part comes out the wrong size after bending. That wastes metal, time, and money. Getting the deduction right the first time means your flanges land on size at the press brake.
The calculator has two modes. Standard mode picks a K-factor for you based on the metal you choose, like mild steel, aluminum, or stainless steel. Advanced mode lets you set your own K-factor and enter both flange lengths to get the full flat blank size.
You also get a step-by-step solution, a chart showing how the numbers change with bend angle, and a table of common angles for your setup. Use it to check your work before you cut.
How to use our Bend Deduction Calculator
Enter your bend angle, material, thickness, and inside bend radius. The calculator gives you the bend deduction, bend allowance, K-factor, and flat blank length for your sheet metal part.
Unit System: Pick inches or millimeters. All inputs and results switch to the unit you choose.
Calculator Mode: Choose Standard for a quick bend deduction. Choose Advanced to set your own K-factor and add flange lengths for a flat blank length.
Bend Angle: Type the angle of your bend in degrees, from 0 to 180. Most parts use 90 degrees.
Material Type: Pick the metal you are bending, like mild steel or 5052 aluminum. This sets the K-factor for that metal.
Material Thickness: Pick your sheet gauge or fraction size in inches. In millimeter mode, type the thickness by hand.
Inside Bend Radius: Pick a common punch radius from the list, or choose "Custom" to type your own. A good start is a radius equal to your material thickness.
K-Factor (Advanced): Type the K-factor for your press brake setup. Most shops use 0.30 to 0.50. Leave the value that loads with your material if you are not sure.
Flange Length A (Advanced): Type the length of the first leg, measured from the bend tangent line to the edge of the part.
Flange Length B (Advanced): Type the length of the second leg the same way. It can be different from Flange A.
Click Calculate to see your results, the step-by-step math, a chart, and a table of common bend angles. Click Reset to Defaults to start over.
What Is Bend Deduction?
When you bend sheet metal, the part gets shorter than the sum of its flat sides. Bend deduction is the amount you subtract from the outside dimensions of a bent part to get the correct flat blank size. Cut the blank to that size, bend it, and the finished part comes out to the right length.
Why Metal Changes Length in a Bend
Metal stretches on the outside of a bend and squeezes on the inside. Somewhere in between is a line that does not stretch or squeeze. That line is called the neutral axis. The steel in the bend follows this line, so the real length of the bend area is not the same as the sharp corner you see on a drawing.
K-Factor
The K-factor tells you where the neutral axis sits inside the material. It is a number between 0 and 1. A K-factor of 0.44 means the neutral axis is 44% of the way in from the inside face. Softer metals like aluminum use a smaller K-factor. Harder metals like stainless steel use a larger one. Typical values fall between 0.30 and 0.50.
Bend Allowance vs. Bend Deduction
Bend allowance (BA) is the length of metal in the bend itself, measured along the neutral axis. You add it to your two flange lengths when the flanges are measured from the bend tangent lines.
Bend deduction (BD) is what you subtract when your flanges are measured to the outside corner of the part. Both give the same flat blank length. Which one you use depends on how your drawing is dimensioned.
The Formulas
- Bend Allowance: BA = (π ÷ 180) × Angle × (R + K × T)
- Outside Setback: OSSB = tan(Angle ÷ 2) × (R + T)
- Bend Deduction: BD = (2 × OSSB) − BA
- Flat Blank: Flange A + Flange B + BA
Here R is the inside bend radius, T is material thickness, and Angle is how far the metal is bent from flat. A 90° bend means the part turns a square corner.
Things That Change Your Numbers
- Material thickness (T): Thicker metal moves more material into the bend.
- Inside bend radius (R): Set by your punch tip and die opening. A common starting point is R equal to T.
- Material type: Mild steel, aluminum, stainless, copper, and brass each behave a little differently.
- Bend angle: Sharper bends remove more length. Deduction grows fast as the angle gets close to 180°.
- Tooling and method: Air bending, bottoming, and coining all give slightly different results.
Tips for Better Flat Patterns
These formulas give a very good starting point, but shop results can drift a little. Bend one test piece, measure it, and adjust your K-factor if needed. Once you find a K-factor that works for a certain material, thickness, and punch, write it down and reuse it. That is how shops keep scrap low and parts accurate.