Bend Allowance Calculator
Find the exact amount of material consumed in a sheet metal bend and work backwards to the correct flat blank size. Enter your material thickness, inside bend radius, bend angle, and K-factor (or pick a preset material), and the calculator returns the bend allowance, bend deduction, outside setback, and total flat length for a two-leg part. Switch between millimetres and inches as you work.
What is bend allowance and why does it matter?
When a press brake bends a flat sheet of metal, the inner face of the bend is compressed and the outer face is stretched. Somewhere in between lies the neutral axis, a theoretical surface that neither compresses nor stretches. The bend allowance (BA) is the arc length of the neutral axis through the bent zone. It tells you exactly how much flat material is consumed by the bend itself. Without it you cannot calculate the correct flat blank size, and every part you cut will come out the wrong length after forming. Fabricators add bend allowance to the flat dimensions of each leg to find the total flat blank length, or subtract the bend deduction from the outer mould line dimensions to arrive at the same figure.
The bend allowance formula and what each variable means
The standard formula is: BA = (pi / 180) x bend-angle x (r + K x T). The inside bend radius (r) is measured from the punch tip to the inside face of the sheet. The material thickness (T) is the stock gauge in consistent units. The K-factor (K) is a dimensionless ratio that locates the neutral axis: K = distance-from-inside-face / T. A K-factor of 0.5 places the neutral axis exactly at mid-thickness. Most real bends fall between 0.30 and 0.50, with harder materials at the low end because compression on the inside face is more severe. The bend deduction (BD) is the correction to use when you measure dimensions from outside mould lines rather than tangent points: BD = 2 x OSSB - BA, where OSSB = tan(angle/2) x (r + T). The flat blank length for a single-bend part is then: Leg 1 + Leg 2 + BA (measuring from tangent point) or Leg 1 + Leg 2 - BD (measuring from outside mould lines).
Choosing the right K-factor for your material and process
The K-factor is the single biggest source of error in bend allowance calculations. It is not purely a material property; it also depends on the forming process, die width, lubrication, and how much springback the part exhibits. For air bending (the most common process), K typically runs from 0.30 for very hard, high-strength steels to 0.45 for soft aluminium. Bottoming and coining compress the material more and push K lower than air-bending values for the same material. The best practice is to bend a test piece at your planned angle with your actual tooling, measure the flat length consumed, and back-calculate K. Once established for a material-tooling combination, that K-factor can be used for production. The reference table on this page gives industry starting points for the most common materials.
Inside radius, minimum bend radius, and springback
The inside bend radius controls both the risk of cracking and the accuracy of the K-factor. As a rule of thumb, the minimum safe bend radius for most ductile metals is 0.5 to 1 times the material thickness (r/T >= 0.5). Below that ratio, tensile stress on the outer face can exceed the material tensile strength and initiate cracking or fracture along grain boundaries. Harder materials require a larger minimum radius. After the press brake releases, elastic springback causes the bend angle to open slightly. The amount of springback increases with yield strength and decreases with bend radius. To compensate, fabricators over-bend by a calculated amount, which is why press brake operators enter a corrected angle rather than the final target angle.
Typical K-factor values by material
| Material | K-factor | Notes |
|---|---|---|
| Soft aluminium (2024-O, 6061-O) | 0.45 | Highly ductile; neutral axis shifts outward |
| Soft brass (half-hard) | 0.42 | Similar ductility to soft aluminium |
| Mild steel (low-carbon) | 0.44 | Most common fabrication material |
| Stainless steel (304, 316) | 0.38 | Work-hardens quickly; K shifts lower |
| Spring steel (AISI 1075+) | 0.33 | High strength; significant springback |
| Hard/hardened steel | 0.30 | Minimum ductility; risk of cracking at tight radii |
These are starting-point K-factors for air bending with standard tooling. Verify against test bends for critical work, as tooling radius, die width, and lubrication all shift the effective K-factor.
Frequently asked questions
What is the difference between bend allowance and bend deduction?
Bend allowance (BA) is the arc length of the neutral axis through the bend zone - the amount of flat material consumed by the bend. Bend deduction (BD) is the correction applied when you dimension from the outside mould lines of the finished part. If you measure your legs to the outer corners (mould lines), you subtract the bend deduction from their sum to get the flat blank length. If you measure to the bend tangent points instead, you add the bend allowance. Both methods produce the same flat blank; the choice depends on how your drawing is dimensioned.
How do I find the correct K-factor for my material?
Start with the tabulated values for your material type (see the reference table on this page). Then cut a test strip from your actual stock, bend it at a known angle with your actual tooling, and measure the flat length consumed. From that measurement, calculate: K = (BA / (pi/180 x angle) - r) / T, where BA is the measured arc length, r is the inside radius, and T is the thickness. Repeat at two or three angles and average the results. This empirical K-factor will be more accurate than a lookup value because it incorporates your specific tooling geometry and friction conditions.
What does the outside setback (OSSB) represent?
The outside setback is the horizontal distance from the mould line (the projected outside corner of the finished bend) to the point where the bend arc begins (the tangent point) on the outside surface of the sheet. It is calculated as tan(bend-angle / 2) x (inside-radius + thickness). You use OSSB when converting between mould-line dimensions and tangent-point dimensions, and it appears directly in the bend deduction formula: BD = 2 x OSSB - BA.
Can I use this calculator for curved or multi-bend parts?
Yes, for multi-bend parts, apply the calculator once for each individual bend. Each bend zone has its own bend allowance based on its angle, inside radius, thickness, and K-factor. The total flat blank length is the sum of all the leg lengths plus the sum of all the individual bend allowances. If the inside radius or angle varies between bends (which is common), run separate calculations for each and add the results.
What is springback and how does it affect bend allowance?
Springback is the elastic recovery that causes a bent part to partially open after the press brake releases. It means the actual included angle of the part is larger than the die angle. Bend allowance calculations use the final target angle of the finished part, not the over-bent angle used at the press brake. So if you need a 90-degree part but your tooling must bend to 87 degrees to compensate for springback, enter 90 degrees in this calculator to get the correct flat blank length, then program the press brake to 87 degrees.
Why does the bend deduction sometimes come out negative?
At very obtuse bend angles (close to 180 degrees, nearly flat), the outside setback becomes very small while the bend allowance remains substantial, so BD = 2 x OSSB - BA can become negative. A negative bend deduction is mathematically valid: it means the flat blank must be longer than the sum of the mould-line dimensions. This is not an error; it simply reflects the geometry at very shallow bends. In practice, obtuse bends near 180 degrees are uncommon in sheet metal fabrication and the bend-allowance approach (measuring from tangent points) is clearer in those cases.
Does material thickness affect the K-factor?
Thickness affects the K-factor indirectly. For a given material, thicker stock typically has a lower K-factor because the neutral axis shifts proportionally closer to the inside face as the ratio of inside radius to thickness decreases. If you are working at the same r/T ratio with different thicknesses of the same material and tooling, the K-factor will be similar. However, because minimum inside radii are often fixed by tooling rather than scaled with thickness, thinner sheets often have a higher effective r/T ratio and therefore a higher K-factor than thicker sheets of the same material.