Bend Allowance Calculator

Solve using: Chooses whether the calculator starts from K-factor or from Known bend deduction (selected unit).

Bend angle (degrees): The entered angle uses the selected convention. The formulas use bend rotation: 180 degrees minus the included angle, or the angle from flat directly.

Inside radius (selected unit): The bend radius on the inside face after forming.

Material thickness (selected unit): The sheet thickness used with radius and K-factor.

K-factor: A unitless number that tells where the neutral axis sits through the thickness.

Known bend deduction (selected unit): A measured or table value for one bend that can be used instead of entering K-factor.

Bend allowance: The arc length of material in the bend region along the neutral axis [1].

Outside setback: The distance from the bend tangent point to the outside mold line intersection, used when finding bend deduction [2].

Bend deduction: The amount subtracted from the sum of two outside flange lengths to get flat length for one bend [2].

Flat length: The estimated blank length for the two entered outside flanges and one bend.

Neutral axis radius: Inside radius plus K-factor times thickness. This is the radius used in the bend allowance formula.

Assumption note: A short message that flags unusual input ranges or geometry that may not match shop results exactly.

Mistake: Entering a bend angle from the brake note into Bend angle (degrees) while leaving Angle entry means on included angle.
Fix: If your angle is measured from the original flat sheet, change Angle entry means to match so the calculator converts it before finding Bend allowance .

Mistake: Mixing inches and millimeters between Inside radius (selected unit) , Material thickness (selected unit) , and Flange length 1 (selected unit) .
Fix: Set Length unit set first, then enter every length in that same unit so Flat length and Outside setback stay consistent.

Mistake: Typing a value into K-factor when Solve using is set to known deduction mode, or typing Known bend deduction (selected unit) while in K-factor mode.
Fix: Use the input that matches Solve using . The inactive mode input is ignored, so only the visible one should be checked.

Mistake: Using inside flange dimensions when Flange length 1 (selected unit) and Flange length 2 (selected unit) are meant to be outside flange lengths.
Fix: Enter outside flange lengths for this one-bend formula, or your Bend deduction and Flat length will not match the part layout basis.

Mistake: Treating an unusual K-factor used as automatically correct because the math ran.
Fix: Read the Assumption note . If the inferred or entered K-factor is outside the usual 0 to 0.5 range, verify material, tooling, process, and measurement basis.

Mistake: Entering 0, a negative value, or a near-180 angle in Bend angle (degrees) or a nonpositive Material thickness (selected unit) .
Fix: Use an included angle strictly between 0 and 180 degrees after any conversion, keep Material thickness (selected unit) greater than 0, and keep Inside radius (selected unit) at 0 or more.

• This calculator is for one bend with two outside flange lengths. It does not sum multiple bends or build a full part flat pattern.
• Flat length assumes Flange length 1 (selected unit) and Flange length 2 (selected unit) are outside dimensions. If your drawing uses inside dimensions or tangent dimensions, results will differ.
• K-factor is process-dependent. Air bending, bottoming, coining, tooling, grain direction, material type, and thickness can change the real bend result.
• The known-deduction mode infers K-factor used from the entered geometry. If that inferred value is outside the usual 0 to 0.5 range, the math still works but the assumptions may not match your shop setup.
• All length inputs must use one consistent unit chosen by Length unit set. Mixing units makes every length output wrong.
• The angle must convert to an bend rotation greater than 0 degrees and less than 180 degrees. Bend rotations near 180 degrees make the tangent term unstable for Outside setback.
• Inside radius (selected unit) must be 0 or more, and Material thickness (selected unit) must be greater than 0.
• Rounding only changes displayed values. Internal calculations should keep full precision until final formatting, so small display differences can appear if you compare rounded intermediate steps.

Angle convention

The formulas use bend rotation A in degrees. For an included interior angle, A = 180 - included angle. For an angle measured from flat, A is the entered angle.

BA = (pi / 180) * A * (R + K*T)

OSSB = (R + T) * tan(A*pi/360)

BD = 2*OSSB - BA; flat length = L1 + L2 - BD

R is inside radius, T is thickness, K is the neutral-axis factor and L1/L2 are outside flange lengths. In known-deduction mode, BA = 2*OSSB - BD and K = (BA / (A*pi/180) - R) / T.

Worked example

With a 120-degree included angle (60 degrees from flat), R = 2 mm, T = 1 mm, K = 0.33 and two 25 mm outside flanges, BA is 2.440 mm, setback is 1.732 mm, deduction is 1.024 mm and flat length is 48.976 mm. The 90-degree case gives BA 3.660 mm and flat length 47.660 mm.

These one-bend estimates depend on material and tooling. Use measured shop data where available.