Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Bend allowance
0.1508
Bend setback
0.1375
Total flat pattern length
3.3758
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter bend angle (degrees).
- 2Enter inside bend radius (in).
- 3Enter material thickness (in).
- 4Enter k-factor.
- 5Enter flange 1 length (to bend tangent) (in).
- 6Enter flange 2 length (to bend tangent) (in).
- 7Read your bend allowance on the right — it updates as you type.
- 8Hit Share to keep the scenario or send it to someone.
About this calculator
When you bend sheet metal, the material stretches on the outside of the bend and compresses on the inside, and the neutral axis (where length doesn't change) shifts based on the K-factor for that material and bend radius. Bend allowance is the arc length of that neutral axis through the bend, and it's what you add to your two flange lengths to get the correct flat pattern for laser or punch cutting before forming. Get this wrong and parts come out over or under the drawn dimension after bending — a small error here compounds across multiple bends on the same part. K-factor typically runs 0.33-0.50 depending on material, thickness, and tooling; 0.4468 (equivalent to using half the material thickness as the bend line location) is a widely used default for air bending mild steel and aluminum. Always verify K-factor against your press brake setup and material with a first-article test bend before running production.
Worked example
Using the values the calculator loads with:
Inputs
- Bend angle: 90 degrees
- Inside bend radius: 0.0625 in
- Material thickness: 0.075 in
- K-factor: 0.4468
- Flange 1 length (to bend tangent): 2 in
- Flange 2 length (to bend tangent): 1.5 in
Results
- Bend allowance: 0.1508
- Bend setback: 0.1375
- Total flat pattern length: 3.3758
What each field means
Inputs
- Bend angle (degrees)
- The bend angle used in the calculation, measured in degrees. Starts at 90 degrees so you have a working example on load. Accepted range: 1–180 degrees.
- Inside bend radius (in)
- The inside bend radius used in the calculation, measured in in. Starts at 0.0625 in so you have a working example on load.
- Material thickness (in)
- The material thickness used in the calculation, measured in in. Starts at 0.075 in so you have a working example on load.
- K-factor
- The k-factor used in the calculation. Starts at 0.4468 so you have a working example on load. Accepted range: 0.1–0.5.
- Flange 1 length (to bend tangent) (in)
- The flange 1 length (to bend tangent) used in the calculation, measured in in. Starts at 2 in so you have a working example on load.
- Flange 2 length (to bend tangent) (in)
- The flange 2 length (to bend tangent) used in the calculation, measured in in. Starts at 1.5 in so you have a working example on load.
Results
- Bend allowance
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Bend setback
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Total flat pattern length
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
What K-factor should I use if I don't know my press brake's exact value?
0.4468 is the widely used default (equivalent to the neutral axis sitting at 50% of material thickness) and is a safe starting point for air bending steel and aluminum up to about 3/16 inch. For tighter tolerance work, run a test bend, measure the actual flat length needed, and back-calculate your shop's real K-factor.
Does K-factor change with material type?
Yes — softer, more ductile materials like aluminum tend toward slightly higher K-factors than harder materials like spring steel, and bottoming or coining dies shift the neutral axis differently than air bending. Thicker material and tighter bend radii also push K-factor around, which is why a single universal constant is only an approximation.
What's the difference between bend allowance and bend deduction?
Bend allowance is the arc length you add to your flange lengths measured to the bend tangent lines. Bend deduction is the amount you subtract from the sum of two flange lengths measured to their intersection point (mold line) instead. Both get you to the same flat pattern length — just from different starting reference points.
Why did my formed part come out short after bending?
Usually either the K-factor used didn't match the actual tooling/material combination, or the inside bend radius achieved on the brake differs from what was programmed (common with worn punches or incorrect die opening selection). Always validate with a first-article part before running a full batch.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Sheet Metal Bend Allowance Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/sheet-metal-bend-allowance
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/sheet-metal-bend-allowance" target="_blank" rel="noopener">Sheet Metal Bend Allowance Calculator — RevenueLab</a> (2026).</p>
Source: [Sheet Metal Bend Allowance Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/sheet-metal-bend-allowance) (2026).