Bend allowance calculator: bend allowance, bend deduction and flat length

This bend allowance calculator uses the K-factor formula from the SOLIDWORKS help, BA = π(R + K·T)·A/180, and turns your flange lengths into the flat blank length. Example: 2 mm sheet, 2 mm inside radius, K = 0.5, 90° bend, outside dimensions 50 and 30 mm – flat length 76.71 mm.

Example: Flange lengths are measured: outside, to the virtual sharp · Flange length A 50 mm · Flange length B 30 mm · Material thickness T 2 mm · Inside bend radius R 2 mm · Bend angle 90 ° · K-factor 0.5 → Flat length: Flat length 76.71 mm. Source: SOLIDWORKS Help 2022, “K-Factor”: BA = π(R + KT)A/180, K = t/T (location of the neutral sheet), A = bend angle in degrees (read 09.10.2026). Updated: 9 October 2026.

Flange lengths are measured

Flange length A

Flange length B

Material thickness T

Inside bend radius R

Bend angle

the angle the sheet is bent through: 90 for an L-shape

K-factor

0 to 1; 0.5 = neutral axis in the middle of the sheet

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Result

Flat length: Flat length 76.71 mm
Details: Item Value
Bend allowance BA = π(R + K·T)·A/180 4.712 mm
Outside setback (R + T)·tan(A/2) 4.000 mm
Bend deduction BD = 2 × setback − BA 3.288 mm
Flat length A + B − BD 76.71 mm; Calculated with your K-factor – the real neutral axis depends on material, tooling and radius, so check the first part or use your shop’s bend table. Any length unit works if you use it for every field.

Item	Value
Bend allowance BA = π(R + K·T)·A/180	4.712 mm
Outside setback (R + T)·tan(A/2)	4.000 mm
Bend deduction BD = 2 × setback − BA	3.288 mm
Flat length A + B − BD	76.71 mm

How it is calculated

Bend allowance BA = π × (R + K × T) × A ÷ 180. R is the inside radius, T the thickness, A the bend angle in degrees and K the K-factor – the position of the neutral axis as a fraction of the thickness (K = t/T). The bend allowance is the arc length of the bend along that neutral axis. SOLIDWORKS uses the same formula for its K-factor bends.

Flat length from outside or inside dimensions

If your flanges are dimensioned to the flat part only (up to the bend tangent), the flat length is A + B + BA. If they are outside dimensions to the virtual sharp – the usual drawing dimension – the flat length is A + B − BD. The bend deduction BD is twice the outside setback minus the bend allowance; the outside setback is (R + T) × tan(A/2), for a 90° bend simply R + T.

Examples (outside dimensions 50 mm and 30 mm)

T	R	K	Angle	BA	BD	Flat length
2	2	0.5	90°	4.712	3.288	76.71 mm
2	2	0.33	90°	4.178	3.822	76.18 mm
2	2	0.5	45°	2.356	0.958	79.04 mm
2	2	0.5	120°	6.283	7.573	72.43 mm

A 3 mm sheet with R = 4, K = 0.45 and outside legs of 100 and 60 mm gives BA 8.404, BD 5.596 and a flat length of 154.40 mm. Measured to the tangents, the first example has legs of 46 and 26 mm: 46 + 26 + 4.712 = 76.71 mm – the same blank.

Which K-factor?

K = 0.5 puts the neutral axis in the middle of the sheet, K = 0 on the inside face. The real value depends on material, tooling and radius; SOLIDWORKS therefore lets you set K per material in a gauge table or use a K-factor bend table. Bend one test piece, measure it and back-calculate K – the result here is only as good as that value. The arc itself is a circle sector; the blank weight comes from the steel weight calculator.

Frequently asked questions

What is the bend allowance formula?

BA = π × (R + K × T) × A ÷ 180. For T = 2 mm, R = 2 mm, K = 0.5 and 90°: π × 3 × 90 ÷ 180 = 4.712 mm.

What is the difference between bend allowance and bend deduction?

Bend allowance is added to the flat flange parts; bend deduction is subtracted from the outside dimensions. BD = 2 × outside setback − BA, here 2 × 4 − 4.712 = 3.288 mm.

How do I calculate the flat length of an L-bracket?

Outside dimensions 50 and 30 mm, T = 2, R = 2, K = 0.5: 50 + 30 − 3.288 = 76.71 mm.

What K-factor should I use?

The one from your shop’s bend table or from a test bend; 0.5 = neutral axis in the middle of the sheet. Without a measured value the result is an estimate.

Which bend angle do I enter?

The angle the sheet is bent through: 90° for a right-angle bracket, 45° for a 135° inside angle.

What is the outside setback?

The distance from the bend tangent to the virtual sharp on the outside: (R + T) × tan(A/2), 4 mm for R = 2, T = 2 and 90°.

Sources and legal basis

As of: 9 October 2026 · Editorial policy