How is the bend correction calculated?

Use the formula BA = Angle x (π/180) x (Radius + K-factor x Thickness). Enter the bend angle in degrees, inside radius, k-factor and material thickness to get the length of the neutral axis arc through the bent area calculated by the tool.

What should I use for my K-factor?

The K-factor almost always falls between .3 and .5. It varies with material, bending method, inside radius, and angle. The most reliable values come from the supplier or by measuring test bends. If no data is available, a value near .33 is a common starting point.

What is the difference between curvature correction and curvature compensation?

The bend allowance is the arc length of the neutral axis. When added to the inner flange length it will give a flat plate. The bend deduction is a value that is subtracted from the sum of the outside profile lengths to also get a flat plate. Both values represent the same bend, but from different reference points.

How to determine the length or dimensions of a flat plate?

Add the lengths of the two outer flanges and subtract the bend deduction: Flat = La + Lb - BD. The length of the flat plate is the dimension that will be cut before bending. This tool mode for a flat plate length is suitable for a single bend joining two flanges.

Is it possible to determine a K-factor from measured bending?

Yes. Switch to the K-factor mode and enter the measured bend allowance, bend angle, inside radius and thickness. The tool will then calculate K = (BA / (angle x pi/180) - radius) / thickness. Save this result and use it for the same material and shape.

Bend Allowance Calculator

Bend setup

What do you want to calculate?

Bend allowance & deduction

Flat length (blank size)

Find the K-factor

Bend angle (degrees)

Inside bend radius

K-factor

Material thickness

Enter the bend angle, inside radius, K-factor, and thickness to get the bend allowance and deduction.

Result

Bend allowance

Bend deduction

Bend allowance

Bend deduction: mm
Setback (OSSB): mm

Neutral-axis radius: mm

Bend allowance chart

Your own bend allowance chart: values in mm for the inside radius, K-factor, and thickness above.
Bend angle	Bend allowance (mm)	Bend deduction (mm)
15°	0.958	0.358
30°	1.916	0.763
45°	2.875	1.268
60°	3.833	1.941
90°	5.749	4.251
120°	7.665	9.655
150°	9.582	27.739

The bend allowance tool shows the changes that occur when sheet metal is bent. When an identical piece of sheet metal is bent, the metal on the inside of the bend will be compressed while the outside will stretch. Between these areas there is a line that maintains its original length. This line is the neutral axis and the length of this axis that extends around the bent area is the bend allowance.

If this value is calculated correctly then a sheet of the correct size can be cut out so that the bent part matches exactly what was intended. If the calculation is incorrect, all the parts which are bent will either be too long or too short. This tool has three applications; calculating bend compensation and deduction values for bend radii, determining sheet dimensions from flange lengths, and back-calculating K-factors from measured bend results.

What is Bow Correction?

When sheet metal is bent the inner surface compresses while the outer surface stretches. The neutral axis is in between and forms a layer that neither stretches nor shrinks. Bend allowance is the length of the arc traveled by the neutral axis.

Since the neutral axis maintains its length, the bend allowance is equal to the actual material length corresponding to the curve. When you add this to the straight section of the flange, you get the total length of the sheet metal.

K-factor

The neutral axis is not exactly at the center of the material. Its position is indicated by a factor K which represents the ratio of the distance from the inner face to the neutral axis over the thickness of the material.

K = \frac{t}{T}

The K-factor is almost always between .3 and .5. It varies with material, bending method, inside radius, and angle. If a supplier has tested a particular configuration they can provide reliable values. If no data exists then a value near .33 is a common starting point for cold rolled steel.

Situation	Typical K-factor
Soft material, tight radius	0.30 to 0.35
Medium material, air bending	0.38 to 0.45
Hard material, large radius	0.45 to 0.50

Bending correction formula:

The bend correction is calculated directly from the arc length of the neutral axis.

BA = \theta \cdot \frac{\pi}{180} \cdot \left(r + K \cdot T\right)

BA is the bend allowance, theta is the bend angle in degrees, r is the inside radius, K is the K-factor and T is thickness. As the angle is given in degrees it must be multiplied by pi over 180 to convert it into radians which is required for calculating arc length.

For example: At a bend angle of 45 degrees, an inner radius of 2mm, material thickness of 5mm and a k-factor of 0.35 the bend allowance is:

BA = 45 \cdot \frac{\pi}{180} \cdot \left(2 + 0.35 \cdot 5\right) \approx 2.945 \text{ mm}

Bend correction and offsetting

The bend deduction is a related value. When the finished product is measured at the outside corners, the outer shape seam extends to the actual outside of the bend. The bend deduction is the amount that must be subtracted from the overall length of the outside part in order to obtain a flat sheet metal pattern.

This value is determined by the offset. The offset is the distance between the bend line and the outer seam.

SB = \left(r + T\right)\cdot \tan\!\left(\frac{\theta}{2}\right)

BD = 2 \cdot SB - BA = 2\left(r + T\right)\tan\!\left(\frac{\theta}{2}\right) - BA

At a bend of 30 degrees r=5mm, t=15mm, k=0.43, the bend allowance is approximately 5.995mm and the bend deduction is approximately 4.723mm.

Determination of length of flat sheet metal

The length of the flat sheet, or developed length, is the dimension that is cut prior to any bends. There are two methods for calculating this length and they both produce the same result: Either measure from the flange to the inside bend line and add the bend allowance, or measure to the outside seam and subtract the bend deduction.

\text{Flat} = L_a + L_b - BD

If one flange is 50mm wide and the other is 30mm with a standard bend of 90 degrees, then the resulting sheet length will be approximately 75.75mm. This is slightly shorter than the simple sum of the two flanges (80mm) as the metal in the area of the bend is shared between both sides.

Calculation of the K-factor

If you have already bent a test piece and measured the actual bend correction, then you can work backwards to find out what K-factor is for that particular setting. The formula used to calculate the bend allowance is:

K = \frac{1}{T}\left(\frac{BA}{\theta \cdot \frac{\pi}{180}} - r\right)

Turn the tool on to K-factor mode, enter the measured bend allowance and then the angle, radius and thickness and the K-factor will be calculated. Save this value and use it for subsequent material cuts with the same material and shape to get accurate results from the start.

Why suppliers provide crosstabs

In practice, bend charts are usually used. These charts contain a row per angle for a particular material, radius and method. The chart is essentially the result of repeated calculations of this formula. The tool generates a small bend chart that shows a list of bend deductions and bend allowances for common angles based on the values entered. This allows you to look up the required values directly without having to recalculate them.

The results shown by this calculation tool are estimates. The actual deformation may vary due to material lots, tools, press settings etc. Therefore, before cutting materials in large quantities, please check important parts with measurement values or a bending table provided by the supplier.

Frequently asked questions

How is the bend correction calculated?: Use the formula BA = Angle x (π/180) x (Radius + K-factor x Thickness). Enter the bend angle in degrees, inside radius, k-factor and material thickness to get the length of the neutral axis arc through the bent area calculated by the tool.
What should I use for my K-factor?: The K-factor almost always falls between .3 and .5. It varies with material, bending method, inside radius, and angle. The most reliable values come from the supplier or by measuring test bends. If no data is available, a value near .33 is a common starting point.
What is the difference between curvature correction and curvature compensation?: The bend allowance is the arc length of the neutral axis. When added to the inner flange length it will give a flat plate. The bend deduction is a value that is subtracted from the sum of the outside profile lengths to also get a flat plate. Both values represent the same bend, but from different reference points.
How to determine the length or dimensions of a flat plate?: Add the lengths of the two outer flanges and subtract the bend deduction: Flat = La + Lb - BD. The length of the flat plate is the dimension that will be cut before bending. This tool mode for a flat plate length is suitable for a single bend joining two flanges.
Is it possible to determine a K-factor from measured bending?: Yes. Switch to the K-factor mode and enter the measured bend allowance, bend angle, inside radius and thickness. The tool will then calculate K = (BA / (angle x pi/180) - radius) / thickness. Save this result and use it for the same material and shape.

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Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

Wikipedia: Bending (metalworking)
Overview of sheet-metal bending, neutral axis, bend allowance, and K-factor.
SendCutSend: Bend Allowance and how to apply it
Fabricator's guide to bend allowance, deduction, setback, and K-factor in practice.