Bend Allowance Calculator
Find bend allowance, outside setback, bend deduction and the flat blank length of a multi-bend sheet metal part from thickness, inside radius and K-factor.
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About this toolWhat is the Bend Allowance Calculator?
A bent part is always longer than its flat legs and shorter than the sum of its outside dimensions. The difference lives in the bend, where the outside of the sheet stretches, the inside compresses and one surface in between, the neutral axis, keeps its original length. Bend allowance is the length of that neutral arc. Get it right and the flanges come off the brake at the drawing size; get it wrong and every part in the batch is off by the same amount per bend.
Enter the sheet thickness, the inside bend radius and a K-factor, then list the part as a chain of flanges with the bend angle between each pair. The calculator returns the bend allowance, outside setback and bend deduction for each bend, the developed flat length for the whole part, and a to-scale strip of the blank showing where each bend zone falls. Flanges can be dimensioned to the outside mold lines, to the inside mold lines, or as flat legs between tangent lines, so you can type the numbers exactly as the drawing shows them.
If you do not have a measured K-factor, pick the forming method (air bending, bottoming or coining) and the material hardness and the tool reads a typical value from the generic K-factor table, choosing the row from your radius-to-thickness ratio.
- Bend allowance, outside and inside setback and bend deduction per bend
- Flat length for parts with up to 12 flanges and mixed bend angles
- Outside, inside or flat-leg dimensioning, and bend or included angle input
- Typical K-factor table for air bending, bottoming and coining by material hardness
- To-scale drawing of the flat blank, copy and share link
How to use the Bend Allowance Calculator
- Pick metric or inch units and enter the sheet thickness T and the inside bend radius R.
- Choose a K-factor: keep the typical table and select forming method and material, or switch to your own K from a test bend.
- Choose how the flanges are dimensioned on your drawing: outside mold lines, inside mold lines or flat legs.
- Type each flange length and the angle of the bend after it. Add or remove flanges for channels, hat sections and Z bends.
- Read the flat blank length, the per-bend allowance, setback and deduction, and copy the results or share the link with the shop.
Worked examples
Default U channel in 2 mm steel
Input
T 2 mm, R 2 mm, air bending mild steel (K 0.38), outside 50 + 30 + 50 mm, two 90° bends
Output
BA 4.335 mm, OSSB 4 mm, BD 3.665 mm, flat blank 122.67 mm
The sum of outside dimensions is 130 mm. Each bend takes off 3.665 mm, so the blank is 130 minus 7.33. The strip drawing shows flat legs of 46, 22 and 46 mm with the two bend zones between them.
Inch bracket in 16 gauge
Input
T 0.0598 in, R 0.0625 in, K 0.43, outside 2 in and 1.5 in, one 90° bend
Output
BA 0.1386 in, BD 0.1060 in, flat 3.394 in
When the radius is about equal to the thickness and K is near 0.43, a 90° bend deduction always lands close to 1.75 x T, a handy sanity check. Here 0.106 / 0.0598 = 1.77.
Open Z bend from included angles
Input
T 3 mm, R 3 mm, K 0.43, outside 40 + 25 + 40 mm, included angles 135° (bend angle 45°)
Output
BD 1.601 mm per bend, flat 101.80 mm
For a 45° bend the outside setback is tan 22.5° x 6 = 2.49 mm, far less than the 6 mm of a right-angle bend, so the deduction is small.
Background and insight
The formula BA = (π/180) x A x (R + K x T) is simply the arc length of a circle of radius R + K x T swept through the bend angle A. K is the fraction of the thickness from the inside surface to the neutral axis. Theory puts it at 0.5 for a gentle bend, but in a tight bend the inside compresses less than the outside stretches and the neutral axis migrates inward, so practical K-factors run from about 0.33 to 0.5. Machinery’s Handbook gives an empirical allowance of A x (0.01743 R + 0.0078 T) with A in degrees, which works out to a K of roughly 0.45, close to the 0.44 to 0.5 defaults that CAD sheet metal modules ship with.
Setback and deduction exist because drawings dimension to mold lines, the imaginary intersections of the flat faces extended past the bend. The outside setback, tan(A/2) x (R + T), is the distance from the tangent line where the bend starts to that outside intersection. Two setbacks minus the allowance gives the bend deduction, the amount you take off the sum of outside dimensions for each bend. For a 90° bend the setback is just R + T, which is why most shop charts are written for 90°.
Angle conventions trip people up. This tool works internally with the bend angle, the angle the sheet travels through (90° makes a right angle, 45° makes an open bend). Some drawings give the included angle between flanges instead, where a 135° included angle is a 45° bend. Switch the convention and the tool converts the numbers you already typed.
Common use cases
Flat patterns for laser and punch blanks
Get the developed length of brackets, channels and enclosures before cutting, without a CAD sheet metal module.
Checking CAD defaults
Compare the K-factor or bend table in SolidWorks, Inventor or Fusion with what your brake actually produces.
Calibrating a shop bend chart
Bend a test strip, measure it and back out the K-factor for each material, thickness and die combination.
Quoting and nesting
Estimate blank sizes for material take-offs and nesting before the flat pattern drawings exist.
Tips and limitations
- Use the inside radius the brake actually forms, not the punch tip radius. In air bending the radius comes mostly from the die opening, roughly 16% of V for mild steel.
- A K-factor is only valid for the combination of material, thickness, tooling and method it was measured on. Keep a small table per die.
- Dimension style matters: outside, inside and flat-leg dimensions of the same part give the same blank, but mixing them on one drawing gives the wrong answer.
- For bends past about 120° the setback grows rapidly. Check closed bends and hems with a test piece.
- Grain direction changes both springback and K slightly. Bend across the rolling direction where the part allows it.
Frequently asked questions
What is the bend allowance formula?
BA = (π/180) x A x (R + K x T), where A is the bend angle in degrees, R the inside radius, T the thickness and K the K-factor. It is the arc length of the neutral axis through the bend.
What is the difference between bend allowance and bend deduction?
Bend allowance is added to the flat legs (tangent to tangent) to get the blank. Bend deduction is subtracted from the sum of outside dimensions. They are linked by BD = 2 x OSSB − BA, where the outside setback OSSB = tan(A/2) x (R + T).
What K-factor should I use for sheet metal?
Without test data, about 0.33 to 0.40 for tight air bends (R up to T), 0.40 to 0.45 for radii of 1 to 3 x T and 0.5 for large radii. Softer materials sit at the low end. A test bend in your own tooling gives a better number.
How do I find the K-factor from a test bend?
Bend a strip of known flat length, measure the two outside flange lengths, and compute BD = sum of flanges − flat length. Then BA = 2 x OSSB − BD and K = (BA / (A x π/180) − R) / T. Enter that K as your own value.
Is the bend angle the same as the angle on the drawing?
Not always. The bend angle is how far the sheet turns, so a right-angle bend is 90°. Many drawings show the included angle between flanges; a 120° included angle is a 60° bend. The tool can take either.
Why is my flat pattern too long after bending?
Usually the inside radius is larger than assumed (air bending in a wide die) or the K-factor is too high. Measure the formed radius and recalculate, or derive K from a test piece.
Is there an in-depth guide to the Bend Allowance Calculator?
Yes. “Bend Allowance, Bend Deduction and the K-Factor Explained” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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