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Bend allowance and K-factor

Enter the sheet thickness, the inside bend radius and the bend angle. Leave the K-factor empty to use the DIN 6935 value for your radius.

Bend allowance is the length of the neutral axis through the bend: BA = pi/180 x A x (R + K x T). For 2 mm sheet, a 2 mm inside radius and a 90° bend with the DIN 6935 K-factor of 0.325, the bend allowance is 4.163 mm and the bend deduction 3.837 mm.

Bend allowance4.163mm

K-factor from DIN 6935: 0.325
Outside setback: 4 mm
Bend deduction: 3.837 mm

Bend allowance = pi/180 x angle x (R + K x T). Outside setback = tan(angle/2) x (R + T). Bend deduction = 2 x outside setback - bend allowance.

CutMeter is software. Check every result before you cut or bend. The person who approves the part is responsible for its dimensions.

K-factor and symbols

If you leave K empty, it comes from the DIN 6935 correction factor: K = (0.65 + 0.5 x log10(R / T)) / 2. CutMeter uses this law from R / T = 0.65, where the DIN 6935 tables start, and takes K = 0.5 from R / T = 5. Below R / T = 0.65 you must enter K. For tight bends, below R / T = 1, confirm K with a test bend.

T = sheet thickness; R = inside bend radius; A = bend angle in degrees, the angle the flange turns through (90 for an L-bracket), not the angle left between the flanges; K = the position of the neutral axis from the inside face as a fraction of T, from 0 to 0.5.

DIN 6935 states its factor k over half the thickness, so K = k / 2. For bend angles from 15° to 90° the bend deduction equals the DIN 6935 compensation value v, which DIN gives with a negative sign. Above 90° the same formulas are used, with flanges measured to the virtual sharp, and the result can differ from DIN 6935.

Worked example

Thickness 2 mm, inside radius 2 mm, bend angle 90°, K-factor left empty.

R / T = 1, so K = (0.65 + 0.5 x log10 1) / 2 = 0.325. BA = pi/180 x 90 x (2 + 0.325 x 2) = 4.163 mm. Outside setback = tan 45° x (2 + 2) = 4.000 mm. BD = 2 x 4.000 - 4.163 = 3.837 mm.

K-factor 0.325 (DIN 6935), bend allowance 4.163 mm, outside setback 4.000 mm, bend deduction 3.837 mm. An L-bracket with outside flanges of 50 and 30 mm has a flat length of 50 + 30 - 3.837 = 76.163 mm.

K-factor by radius to thickness ratio

K from the DIN 6935 law, with bend allowance and bend deduction for 2 mm sheet and a 90° bend
R / T	R for T = 2 mm, mm	K	Bend allowance, mm	Bend deduction, mm
0.65	1.3	0.2782	2.916	3.684
1	2	0.325	4.163	3.837
1.5	3	0.369	5.872	4.128
2	4	0.4003	7.541	4.459
3	6	0.4443	10.821	5.179
4	8	0.4755	14.06	5.94
5	10	0.5	17.279	6.721

Questions about bending

What is the K-factor?

The distance from the inside face of the bend to the neutral axis, the layer that neither stretches nor shrinks, divided by the thickness. K = 0.5 puts the neutral axis in the middle of the sheet.

What K-factor should I use?

The value that matches your press brake, tooling and material, found by bending a test piece and measuring it. Without one, the DIN 6935 law gives 0.278 at R / T = 0.65, 0.325 at R / T = 1 and 0.40 at R / T = 2, and CutMeter uses 0.5 from R / T = 5. Below R / T = 0.65 DIN 6935 gives no value, so enter your own K.

What is the difference between bend allowance and bend deduction?

Bend allowance is the arc length added between the two flat parts. Bend deduction is what you subtract from the sum of the outside flange lengths. Both give the same flat length.

How do I calculate the flat length of a bent part?

Add the outside flange lengths and subtract one bend deduction per bend. For 50 and 30 mm flanges, 2 mm sheet, 2 mm radius and 90°: 80 - 3.837 = 76.163 mm.

Which angle do I enter?

The bend angle, the angle the flange turns through: 90 for an L-bracket, 45 for a flange bent half way. Not the angle left between the two flanges.

Bend allowance

K-factor and symbols

Worked example

K-factor by radius to thickness ratio

Questions about bending

Sources