Notch & Relief Calculator
Calculate corner relief dimensions for sheet metal bend clearance including round, rectangular, and slot relief types.
About this calculator
When sheet metal is bent to an edge that meets another bend or a flange, the material at the corner has nowhere to stretch -- the bend deformation from one flange runs directly into the material of the adjacent one, and without relief the sheet tears or cracks right at the corner as the brake forms the bend. A relief cut (also called a corner notch) removes the material at that intersection before bending, so each flange can form independently without fighting the other. This calculator sizes that relief cut for three common approaches: a round relief (a drilled or punched hole at the corner, which spreads stress around a smooth radius rather than concentrating it at a sharp corner), a rectangular notch (a squared-off cut, simpler to program on a laser or punch but leaves sharper internal corners that can themselves become new crack-initiation points), and an oblong slot (elongated along the bend line, useful when the relief needs to clear a longer stretch of material).
The sizing logic follows a widely used sheet-metal fabrication rule of thumb: minimum relief width and depth should be at least the sum of the inside bend radius and the material thickness, since that's roughly the zone the bend's stretched material occupies at the corner, with a working margin (this calculator adds 20-25%) added for real-world tolerance stack-up in tooling and material variation. Because tearing risk scales with how much the material has to stretch, thicker material and larger bend radii both call for larger relief cuts, and round reliefs are generally considered the most crack-resistant option for ductile materials because a smooth radius avoids the stress concentration a sharp internal corner creates. This calculator does not evaluate a specific alloy's ductility or grain direction, both of which affect real-world tear risk beyond geometry alone.
Inputs
Results
Recommended Relief Width
4.38 mm
How to Use This Calculator
- Enter material thickness, inside bend radius, and bend angle.
- Select relief type (round drilled hole, rectangular square notch, or oblong slot).
- Review relief size, minimum relief width, and relief position relative to the bend line.
- Relief cuts prevent tearing at bend ends -- always required when bending to the edge of a flange.
- Round relief holes reduce stress concentration better than rectangular slots for ductile materials.
How the result changes with Inside Bend Radius
| Inside Bend Radius | Recommended Relief Width |
|---|---|
| 1 | 3.13 mm |
| 1.5 | 3.75 mm |
| 3 | 5.63 mm |
| 5 | 8.13 mm |
What each input means
- Material Thickness
- Sheet metal thickness.
- Inside Bend Radius
- Inside radius of the bend at the corner.
- Bend Angle
- Bend angle in degrees.
- Relief Type
- Shape of the corner relief cut.
What each result means
- Round Relief Diameter
- Shown only for round relief type; 0 when a different relief type is selected.
- Rectangular Notch Width
- Shown only for rectangular relief type; 0 when a different relief type is selected.
- Oblong Slot Width
- Shown only for oblong (slot) relief type; 0 when a different relief type is selected.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersMaterial Thickness = 1.5, Inside Bend Radius = 2, Bend Angle = 90, Relief Type = 1 = 4 input(s) provided
- Calculate Recommended Relief WidthRecommended Relief Width4.38 = 4.38
- Calculate Recommended Relief DepthRecommended Relief Depth4.2 = 4.2
- Calculate Round Relief DiameterRound Relief Diameter7 = 7
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does relief size depend on both material thickness and bend radius, not just one?
Because both dimensions contribute to how much material at the corner has to stretch and move during the bend. Thicker material has more volume to displace as it forms the bend radius, and a larger inside bend radius means the bend itself sweeps across more material. The rule of thumb this calculator applies -- minimum relief size equal to bend radius plus material thickness -- captures both effects together rather than sizing off either dimension alone, which is why increasing either input on its own increases the recommended relief size.
Which relief type should I use for a ductile material like mild steel or aluminum?
Round relief holes are generally considered the most crack-resistant option because a smooth circular cut spreads bending stress around a continuous radius rather than concentrating it at a sharp internal corner. Rectangular notches and oblong slots are simpler to program on some cutting equipment and use less material, but their sharp internal corners can themselves become new stress-concentration points where a crack starts -- a particular concern in less ductile materials or tighter bend radii.
Does the bend angle affect the relief size this calculator recommends?
No -- this calculator's relief sizing depends on material thickness and inside bend radius, not on how many degrees the flange is bent through. The relief cut has to be large enough to let the corner material move without tearing at any point during the bend, and that clearance requirement is governed by thickness and radius geometry rather than the final bend angle itself. Bend Angle is still recorded as one of the input parameters in the calculation steps so it appears alongside the part's other dimensions, but no output on this page changes when you adjust it.
Is 'minimum' relief width safe to use, or should I always use the recommended size?
Treat Minimum Relief Width as the theoretical floor, not a safe working target. It represents the smallest cut that clears the bend geometry with zero margin for real shop conditions -- tooling wear, material thickness variation, and alignment tolerance all eat into that margin. Recommended Relief Width adds roughly 20-25% on top of the minimum specifically to absorb that variation, which is why it's the value this calculator highlights as the primary result.
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