Sheet Metal Bend Allowance Calculator
Calculate bend allowance, outside setback, and bend deduction for a sheet metal bend from bend angle, inside radius, thickness, and K-factor — plus, optionally, the flat blank length from the two outside-mold-line leg lengths.
Input
The angle the material is bent through, measured from flat — 90° for a right-angle bend.
The radius at the inner surface of the bend, not the outer.
Locates the neutral axis through the thickness as a fraction from the inside surface (0) to the outside (1). Varies with material, tooling, bend radius and bending method — 0.33 is a common air-bending rule of thumb; validate against press brake trials or vendor bend tables for production.
Leave both legs blank to skip the flat blank length calculation.
Output
Bend Result
| Metric | Value |
|---|---|
| No data yet | |
Guides
Work out how much material a sheet metal bend consumes — the bend allowance, the outside setback, and the bend deduction — from the bend angle, inside radius, thickness, and K-factor, and optionally carry it all the way to the flat blank length.
How to use it
- Enter the bend angle — how far the material is bent from flat (90° for a right-angle bend).
- Enter the inside bend radius and material thickness, both in the same unit.
- Enter the K-factor — the fraction of the way through the thickness where the neutral axis sits (0 at the inside surface, 1 at the outside). 0.33 is a common air-bending rule of thumb; your shop's own bend tables or press brake trials are more accurate for a specific material and setup.
- Read the Bend Result for the bend allowance, outside setback, and bend deduction.
- Optionally, enter the two outside-mold-line leg lengths — the flange lengths you'd measure on the finished part, from the bend apex to each end — to get the flat blank length:
Leg 1 + Leg 2 − Bend Deduction.
The three core results
- Bend Allowance — the arc length of material along the neutral axis consumed by the bend:
θ(rad) × (R + K×T). - Outside Setback — how far the outside mold line (where the two flat legs would meet if extended past the bend) sits back from the bend apex:
(R + T) × tan(θ/2). - Bend Deduction — the amount to subtract from the sum of the two outside-mold-line leg lengths to get the flat blank length:
2 × Outside Setback − Bend Allowance.
K-factor
K-factor locates the neutral axis — the layer of material that neither stretches nor compresses during the bend — as a fraction of the thickness from the inside surface. It isn't a material constant: it shifts with the bend radius-to-thickness ratio, the bending method (air bending vs. bottoming vs. coining), the tooling, and even grain direction. Air bending on mild steel commonly lands around 0.33; tighter radii and bottoming/coining push it higher, toward 0.4–0.5. Treat the default as a starting estimate, not a substitute for your own shop's bend tables.
Limits
This is an estimation tool, not a replacement for shop-specific bend tables or press brake trials. It doesn't account for springback, tool radius (as opposed to material inside radius), material batch variation, thickness tolerance, or machine-specific setup differences — all of which shift the real result. Validate against a test bend before committing a production flat pattern.
Privacy
Everything runs in your browser — no dimensions leave your machine.
Related tools
For a chain of dimensions across multiple features, see the Tolerance Stackup Calculator. For the weight of the plate before it's bent, use the Plate Weight Calculator.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/sheet-metal-bend-allowance-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"bendAngle": "90",
"insideRadius": "3",
"thickness": "2",
"kFactor": "0.33",
"leg1Length": "",
"leg2Length": ""
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `sheet-metal-bend-allowance-calculator` tool (Sheet Metal Bend Allowance Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.