Workshop and Metalwork Calculators
Bend allowance.
Enter the sheet thickness, inside bend radius and bend angle, then choose a K-factor from the rule-of-thumb table or type your own. Add the two flange lengths to get the flat blank length. Runs in your browser; nothing is sent or stored.
Inputs
Result
Fill the form and press Calculate.
Answer first. Bend allowance BA = angle (in radians) x (inside radius + K x thickness). The flat length is the flange lengths (to the tangent points) plus BA. The K-factor is only a rule of thumb; bend a test piece to confirm.
Worked example
Sheet 2 mm thick, inside radius 2 mm, bent 90 degrees by air bending in a medium material. The radius is within 0 to 1 x thickness, so K = 0.38 from the table. Bend allowance = 90 x (pi/180) x (2 + 0.38 x 2) = 4.34 mm, bend deduction = 2 x (2 + 2) x tan 45 - 4.34 = 3.66 mm. Wikipedia's own check: angle 90, radius 10, thickness 10, K 0.33 gives a bend allowance of 20.89.
How it works
Formulas. BA = A x (pi/180) x (R + K x T), with A the bend angle in degrees, R the inside radius, T the thickness and K the K-factor. Bend deduction BD = 2 x (R + T) x tan(A/2) - BA, where (R + T) x tan(A/2) is the outside setback. Both come from the Wikipedia article on bending.
Which flat length? If your flange lengths are measured to the start of the bend (the tangent points), flat length = flange 1 + flange 2 + BA. If they are measured to the outside apex of the bend, flat length = flange 1 + flange 2 - BD. The tool shows both so you can pick the one that matches your drawing.
K-factor. K is the position of the neutral line as a fraction of thickness. The table in the source is labelled a rule of thumb whose actual results "may vary remarkably", and it is more likely to be accurate for air bending. The three columns are the source's soft, medium and hard material groups. If the radius is more than 3 times the thickness, the table gives 0.50 in every case.
Angle here is the angle the sheet is bent through (a flat sheet is 0, a right-angle corner is 90), not the included angle between the flanges.
Safety. Press brakes and bending tools can crush fingers; use guards, keep hands clear of the die and wear eye protection. Material springback and tooling change the final angle. This is an estimate, not professional fabrication advice.
Sources
- Bending (metalworking) - Wikipedia (opened 2026-10-08)
The formulas and the generic K-factor table are from the page above (the table is described there as a rule of thumb).
FAQ
What is bend allowance?
The length of the arc of the neutral line through the bend. Adding it to the flange lengths (measured to the tangent points) gives the flat pattern length.
What K-factor should I use for steel?
The source table gives 0.38 for medium materials in air bending when the radius is between 0 and 1 times the thickness, rising to 0.50 when the radius is more than 3 times the thickness. Treat it as a starting point and confirm with a test bend.
What is the difference between bend allowance and bend deduction?
Bend allowance is added to the flanges measured to the tangent points. Bend deduction is subtracted from flanges measured to the outside apex. They describe the same bend.
Is the bend angle the included angle?
No. It is the angle the material turns through. A flat strip bent into an L has a bend angle of 90 degrees.
Why does my part come out the wrong length?
K-factor, springback, die width and material grain all matter. Bend one test piece, measure the flat length, and back-calculate K with the source formula K = (BA / (angle x pi / 180) - R) / T.