Free calculator

Bend allowance calculator

Enter the thickness, inside radius and K-factor, then the outside size of each flange. The calculator gives the flat length to cut, and the bend allowance, setback and deduction for every bend.

How the flat length is worked out

When sheet bends, the inside squeezes and the outside stretches. Somewhere between them is a neutral axis that stays its original length. The K-factor says where it sits: the distance from the inside face divided by the thickness.

Bend allowance (BA) is the length of the neutral axis through the bend: BA = (π ÷ 180) × A × (R + K × T).
Outside setback (OSSB) runs from the start of the bend to where the outside faces would meet: OSSB = tan(A ÷ 2) × (R + T).
Bend deduction (BD) is what each bend takes off the outside dimensions: BD = 2 × OSSB − BA.
Flat length = the outside flange sizes added up, minus the bend deduction for every bend.

A is the bend angle (90° for a right angle), R the inside radius and T the thickness. Use one unit throughout: millimetres or inches both work.

Worked example: a 50 × 100 × 50mm channel in 2mm sheet, inside radius 2mm, K-factor 0.44. Each 90° bend: BA = 1.5708 × (2 + 0.88) = 4.52mm; OSSB = 1 × 4 = 4mm; BD = 8 − 4.52 = 3.48mm. Flat length = 200 − 2 × 3.48 = 193.05mm.

Which K-factor?

The K-factor depends on the material, the radius, the tooling and how the part is bent, so treat these as starting points:

Inside radius	Starting K-factor
Less than the thickness	0.33
Between 1 and 3 × the thickness	0.40 to 0.45
More than 3 × the thickness	0.50

Find your own: bend a strip of known length on your press brake and measure both outside flanges. Bend deduction = flange 1 + flange 2 − strip length. Change the K-factor above until the calculator shows the same bend deduction, and use that for that material, thickness and tooling from then on.

Inside radius when air bending: as a rule of thumb, mild steel bends to an inside radius of about 16% of the V-die opening, and the die opening is often 6 to 8 times the thickness. A test bend beats any rule.

Then nest the blanks

Blanks to the sheet

Enter the blank width and quantity, and open the blanks in the CutNest app to nest them on your sheet size.

Grain and rolling direction

Lock parts so bends run across the rolling direction where it matters, and the nest keeps every blank that way round.

Cut sheets and DXF (Pro)

Print a cut sheet for every sheet, labels for every blank, or export DXF for the laser.

Price the job (Pro)

Quote from the layout: material, cutting time and your margin, with the bending added as a line.

Questions

What is the difference between bend allowance and bend deduction?

Bend allowance is the length of material in the bend itself, measured along the neutral axis. Bend deduction is what you take off the outside flange sizes to get the flat length. If you draw to inside dimensions you add bend allowances; if you draw to outside dimensions you subtract bend deductions.

What K-factor should I use?

Around 0.33 for tight bends where the inside radius is less than the thickness, 0.40 to 0.45 for radii of one to three times the thickness, and 0.50 for large radii. A test bend on your own brake gives the right figure for your tooling.

Does it work in inches?

Yes. Choose inches, or simply use one unit for every size. The formulas are the same.

Are the flange sizes inside or outside?

Outside: from the outside face of one bend to the outside face of the next, or to the edge. For angles other than 90°, measure to where the outside faces would meet.

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