Bend Allowance Calculator
Calculate bend allowance, bend deduction and flat length from thickness, inside radius, angle and k-factor. Checks the radius against forming limits.
Sets the k-factor and the minimum bend radius from our DFM rule set.
Degrees bent away from flat. A right angle is 90.
Comma separated, in mm. One more leg than bends, so three legs means two bends.
Leave blank to use Aluminum 5052-H32 at 0.33. Enter a measured value if you have one.
Radius is within normal forming limits
1.00x thickness, at or above the 1x preferred for Aluminum 5052-H32.
Developed flat
Bend geometry
Bend allowance is the arc length along the neutral axis of a bend, and it is what turns a folded part back into a flat blank. This calculator computes bend allowance, bend deduction, outside setback and developed flat length from thickness, inside radius, bend angle and k-factor, then checks the radius against the minimum our DFM engine enforces for that alloy.
How to read the result
A press brake stretches the outside of a bend and compresses the inside. Somewhere between the two sits a neutral axis that neither stretches nor compresses, and the k-factor says where: it is the distance from the inside surface to that axis, as a fraction of material thickness. A k-factor of 0.33 puts the neutral axis a third of the way through.
Get the k-factor wrong and every flange on the part is wrong by the same amount, which is why the flat pattern, not the folded model, is where sheet metal parts are usually lost. If you have a real bent sample, measure its flat and work backwards to the k-factor your press and tooling actually produce, then use that number rather than a table value.
The material list and the minimum bend radius for each alloy are read from the same rule set our automated DFM review uses, so a radius this tool calls too tight is the radius a DFM run will flag.
Frequently asked questions
- What is the difference between bend allowance and bend deduction?
- Bend allowance is the arc length of material consumed by the bend along the neutral axis, so you add it to the flat leg lengths measured to the bend tangent. Bend deduction is the amount you subtract from the summed outside dimensions. They describe the same bend from two directions: BD = 2 x outside setback - BA.
- What k-factor should I use for aluminum?
- Between 0.33 and 0.35 for most common sheet alloys, which is what our rule set carries for 1100, 3003, 5052, 6061, 6063 and 7075. The k-factor rises as the inside radius grows relative to thickness, so a generous radius pushes it toward 0.5. If tolerances are tight, measure a sample bend rather than trusting any table.
- What is the minimum bend radius for sheet metal?
- It depends on the alloy and temper, not on thickness alone. Soft aluminum such as 5052-H32 forms down to about 0.5 times thickness on thin gauge (1 times is the safer default), cold-rolled steel to about 0.5, stainless 304 to about 1.0, and hard tempers such as 6061-T6 and 7075-T6 need 2 to 3 times thickness on thin gauge, and more on heavier gauge, before they crack at the outside of the bend.
- Why does my flat pattern not match the shop's?
- Almost always a k-factor difference. The value depends on tooling, die width, forming method and the material batch, so two shops forming the same drawing legitimately produce different flats. Send the folded 3D model and the bend specification rather than a flat DXF, and let the shop develop the blank on its own press.
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