K-Factor Calculator
Enter your material thickness, inside bend radius, and bend angle to instantly find the K-factor, bend allowance, bend deduction, and outside setback for any sheet metal part. Add optional flange dimensions to get the flat blank length. The calculator also looks up a recommended K-factor from an industry reference table based on your bending method and material hardness, so you can cross-check or replace your measured value.
What is the K-factor in sheet metal bending?
The K-factor is the ratio of the neutral axis position (t) to the total material thickness (T): K = t / T. During bending, the outer surface of a sheet is stretched in tension while the inner surface is compressed. Somewhere between those two faces lies a layer where the stress is exactly zero - this is the neutral axis. Because the inner material is compressed and the outer is stretched, the neutral axis shifts toward the inside of the bend, ending up at a distance t that is less than T/2 from the inner surface. K quantifies exactly where that shift puts the neutral axis. A K-factor of 0.50 means the axis sits exactly in the centre of the sheet (pure theory, never quite achieved in practice). Real values for common sheet metals and tooling fall between 0.33 and 0.50. Knowing K is essential because it feeds into every derived quantity: bend allowance, bend deduction, and therefore the flat blank length that determines whether the finished part hits its dimensional tolerances.
Bend allowance, bend deduction, and outside setback
Once K is known, three other values follow directly. The bend allowance (BA) is the arc length of material consumed in the bend zone along the neutral axis: BA = pi x (R + K x T) x (theta / 180), where R is the inside radius and theta is the bend angle in degrees. The outside setback (OSSB) is the distance from the outside bend tangent line to the intersection of the outside mold lines: OSSB = tan(theta / 2) x (R + T). The bend deduction (BD) ties everything together: BD = 2 x OSSB - BA. To find the flat blank length for a single bend, add the two flange lengths measured from the outside tangent points and subtract the bend deduction: Flat blank = Flange A + Flange B - BD. For multiple bends in series, repeat the subtraction once per bend. This calculator handles one bend at a time; for complex parts add additional bends manually or use unfolding software.
What affects the K-factor?
Material hardness is the biggest driver: soft materials such as annealed aluminium and soft copper spring back less and deform more evenly, so their neutral axis stays closer to the inside of the bend (K as low as 0.33). Harder materials such as stainless steel resist deformation and the neutral axis stays closer to the centre (K up to 0.48). The ratio of inside radius to material thickness (Ri/T) also matters: very tight bends (Ri < T) push the neutral axis further toward the inner surface; wide bends (Ri > 3T) let the axis drift toward the centre (K near 0.50). Bending method changes K too: coining, which presses the material fully into the die, produces different neutral axis positions from air bending, where the punch stops before the material contacts the die sides. Grain direction, lubrication, punch and die radii, and temperature are secondary factors. Industry tables give starting K values, but the most accurate K for a production run comes from a physical test bend measured to derive it in back-calculation mode.
How to use this calculator
Select your unit system, enter the material thickness (T), inside bend radius (R), and bend angle. Choose how to supply the K-factor: "Recommend from table" looks up a starting value based on your bending method and material hardness; "Enter manually" lets you type a value from your own test data or material spec; "Back-calculate from measured BA" derives K from a physical test bend (measure the actual flat blank and both flanges to get the real BA, then enter it here). Once K is set, the calculator shows the neutral axis offset, bend allowance, outside setback, and bend deduction. Add optional flange lengths to get the full flat blank dimension. The "Show your work" panel traces every step of the arithmetic so you can verify against your own calculations.
Typical K-factor values by bending method and material
| Bending method | Material | Ri < T (tight) | T <= Ri <= 3T (medium) | Ri > 3T (wide) |
|---|---|---|---|---|
| Air bending | Soft (Al, soft brass) | 0.33 | 0.40 | 0.50 |
| Air bending | Medium (mild steel) | 0.38 | 0.43 | 0.50 |
| Air bending | Hard (stainless) | 0.40 | 0.45 | 0.50 |
| Bottom bending | Soft | 0.42 | 0.46 | 0.50 |
| Bottom bending | Medium | 0.44 | 0.47 | 0.50 |
| Bottom bending | Hard | 0.46 | 0.48 | 0.50 |
| Coining | Soft | 0.38 | 0.44 | 0.50 |
| Coining | Medium | 0.41 | 0.46 | 0.50 |
| Coining | Hard | 0.44 | 0.47 | 0.50 |
Values from the SheetMetal.Me and DurmaPress industry references. Ri = inside radius, T = material thickness.
Frequently asked questions
What is a typical K-factor for mild steel?
For mild steel in air bending with an inside radius between one and three times the material thickness, a K-factor of 0.43 is a common starting point. Tight bends (Ri less than T) push the value down toward 0.38; wide bends (Ri greater than 3T) push it up toward 0.50. These are starting values only; a physical test bend will give you the most accurate number for your specific tooling and batch of material.
How is the K-factor different from the Y-factor?
The Y-factor (sometimes called the Y-coefficient) is the DIN 6935 variant and is related to K by Y = K x (pi / 2). The two are interchangeable; some European press-brake software uses Y while North American CAD tools such as SolidWorks and CATIA use K. Converting between them is straightforward: Y = K x 1.5708.
Why does my flat blank come out wrong when I use the standard K-factor?
Published K-factor tables are averages. Your actual K depends on your punch radius, die opening, tool condition, lubrication, material batch properties, and grain orientation. Use the back-calculate mode: make a test bend with known flange and blank dimensions, measure the result, and derive the actual K for your setup. Repeat whenever material batch or tooling changes.
Can the K-factor be greater than 0.5?
In theory the neutral axis can shift past the centreline of the sheet, which would give K greater than 0.5, but this does not happen in normal press-brake bending. Practical values are between 0.25 and 0.50. A computed K above 0.50 from the back-calculate mode almost always means a measurement error in the bend allowance or flange lengths.
What is the difference between bend allowance and bend deduction?
Bend allowance (BA) is the physical arc length of material used up in the bend zone. Bend deduction (BD) is the shortcut for flat blank layout: BD = 2 x OSSB - BA, where OSSB is the outside setback. When you lay out a flat blank, you measure the flanges to their outside mold lines, add them together, and subtract BD once per bend. Bend allowance is the "add" approach; bend deduction is the "subtract" approach. Both give the same flat blank length when used correctly.
Does the K-factor change with bend angle?
The K-factor itself is a material and tooling constant that does not change with angle for a given setup. However, the bend allowance and bend deduction both depend on the angle because the arc length of the neutral axis through the bend zone grows as the angle increases. This calculator applies the same K to any angle you enter.