Bend Allowance Calculator
Calculate sheet metal bend allowance, bend deduction, setback, neutral-axis geometry, flat pattern length, or reverse K-factor from a measured bend.
Calculator is for informational purposes only. Terms and Conditions
K-factor locates the neutral axis through the sheet thickness; production flat patterns should be verified with your material, tooling, and shop bend data.
Choose what to calculate
Use the normal bend allowance mode for layout work, or reverse the equation to estimate K-factor from a measured bend allowance.
Enter the bend geometry
A 90° bend angle means the sheet turns 90° from flat. Length fields may be entered in inches or millimeters.
The K-factor is an empirical neutral-axis position, not a universal material constant. Use measured shop data for tight-tolerance production work.
Result
Primary answer first, followed by layout checks, warnings, and transparent calculation steps.
Fabrication details
- Outside setback—
Show calculation steps Review angle conversion, neutral-axis geometry, bend allowance, deduction, flat length, and checks
- Enter valid values to see the complete calculation.
Bend geometry
Schematic cross-section showing the inside radius, sheet thickness, neutral axis, and bend angle. Geometry is illustrative, not a fabrication drawing.
Method, Sources, and Assumptions
Calculation basis, authoritative references, active assumptions, limitations, and production verification requirements.
Bend allowance is calculated as the arc length of the neutral axis using BA = π(R + KT)A/180. Bend deduction and outside setback are geometric derivatives used for outside-dimension flat patterns.
- K-factor is entered by the user and should be verified for the actual material, radius, tooling, and forming process. The loaded K = 0.44 example is illustrative, not a universal recommendation.
- The bend angle is the turn from flat unless Included Angle is selected.
- Flat length uses either outside / virtual-sharp dimensions or straight tangent lengths according to the selected flange convention.
Calculator guide
How the Bend Allowance Calculator Works
The Bend Allowance Calculator determines the developed length consumed by a sheet-metal bend from material thickness, finished inside bend radius, bend angle, and K-factor. It also reports the neutral-axis radius, outside setback, bend deduction, radius-to-thickness ratio, and—when both flange dimensions are supplied—flat pattern length.
Bend allowance is the arc length measured along the bend’s neutral axis. The equation itself is straightforward; the practical accuracy of a flat pattern depends mainly on whether the entered radius, angle convention, flange-dimension convention, and K-factor match the actual part and forming process.
- Minimum inputs
- Thickness, inside radius, bend angle, and K-factor.
- Primary output
- Bend allowance in inches or millimeters.
- Best production check
- Calibrate K-factor from a representative test bend when dimensional accuracy matters.
How to Use the Calculator Correctly
Start with the finished bend geometry you actually need, then choose the dimension convention that matches the drawing. The calculator updates automatically as valid values change, and the U.S./metric selector converts physical lengths rather than simply changing labels.
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Enter actual sheet thickness and finished inside radius
Use a dimensional thickness, not a gauge number. For the radius, use the finished inside bend radius when known rather than assuming the punch-tip radius and finished part radius are identical.
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Enter the correct angle convention
By default, the angle is the amount the sheet turns from flat: a right-angle bend is 90°. If the drawing instead gives the included angle between the flanges, select Included Angle; the calculator converts it with \(A_\mathrm{bend}=180^\circ-A_\mathrm{included}\).
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Use a defensible K-factor
K-factor locates the neutral axis through the material thickness. The example value loaded by the calculator is illustrative, not a material specification. For production work, use a shop bend table, validated CAD rule, or K-factor derived from a representative test bend.
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Choose the flange dimension method before using flat length
Use Outside / Virtual-Sharp Dimensions when the legs are dimensioned to the theoretical outside corner; that workflow subtracts bend deduction. Use Straight Tangent Lengths when the legs stop at bend tangency points; that workflow adds bend allowance.
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Use reverse K-factor mode for calibration
Select K-Factor from Measured Bend Allowance when you have a known bend allowance from a test bend or validated flat pattern. The calculator rearranges the same neutral-axis equation and flags physically unusual results for review.
Bend Allowance Formula, K-Factor, and Flat Length
The calculator uses the K-factor neutral-axis method documented by SOLIDWORKS. In plain language, bend allowance is arc length: convert the bend angle to radians and multiply by the radius of the neutral axis.
Bend allowance formula
Plain language: bend allowance equals the bend angle in radians multiplied by the neutral-axis radius \(R+KT\).
SOLIDWORKS defines \(A\) as the angle through which the material is bent, \(R\) as inside bend radius, \(T\) as material thickness, and \(K=t/T\), where \(t\) is the distance from the inside face to the neutral sheet.
Reverse K-factor formula
If bend allowance has been measured or established from a real part, the equation can be rearranged to estimate the effective K-factor for that bend geometry.
Outside setback and bend deduction
Outside setback measures from the tangent point to the virtual sharp. Bend deduction is the amount removed from the sum of outside / outside / virtual-sharp flange dimensions.
Flat pattern length
These are not competing formulas. They are two dimensioning workflows for the same physical bend: subtract bend deduction from virtual-sharp dimensions, or add bend allowance between straight tangent lengths.
- \(BA\)
- Bend allowance Developed arc length of the bend measured along the neutral axis.
- \(A\)
- Bend angle Angle through which the sheet is bent from flat after any included-angle conversion.
- \(R\)
- Inside bend radius Finished radius measured from the bend center to the inside surface.
- \(T\)
- Material thickness Actual sheet thickness used in the neutral-axis and outside-radius geometry.
- \(K\)
- K-factor Ratio locating the neutral axis through the sheet thickness, defined as \(t/T\).
- \(OSSB\)
- Outside setback Distance from the bend tangent point to the theoretical outside corner along a flange.
- \(BD\)
- Bend deduction Amount subtracted from summed outside / virtual-sharp flange dimensions to obtain flat length.
90° Bend Allowance Formula
For a 90° bend, the angle term simplifies because \(90^\circ\pi/180=\pi/2\):
A 90° bend does not have one universal allowance. The result still depends on the finished inside radius, material thickness, and K-factor.
Bend Allowance vs. Bend Deduction
Bend allowance is the developed length of the bend along the neutral axis. Bend deduction is the amount subtracted from the sum of outside / virtual-sharp flange dimensions. Use BA with straight tangent lengths; use BD with outside / virtual-sharp dimensions.
Worked Bend Allowance Example
Use the calculator’s default geometry as a reproducible 90° example: 0.0625 in material, 0.0625 in inside radius, and K = 0.44. Two 2.000 in outside / virtual-sharp flanges are added so the same example also checks bend deduction and flat pattern length.
Find the neutral-axis radius
Calculate bend allowance
Calculate setback and bend deduction
Calculate the flat pattern
Result
BA = 0.14137 in; BD = 0.10863 in; flat length = 3.89137 in
The bend consumes about 0.14137 in of developed length along the neutral axis. Because the flange dimensions are measured to the virtual sharp, bend deduction—not direct BA addition—is used to obtain the flat blank.
How to Interpret Bend Allowance Results
Treat bend allowance as the developed length of the curved region—not as extra material added to every dimension. Whether BA is added directly or converted to bend deduction depends on where the drawing’s straight-leg dimensions begin and end.
What the number means
The primary BA result is arc length along the neutral axis. The neutral-axis radius shown by the calculator is \(R+KT\), which makes the underlying geometry easy to verify manually.
K-factor sensitivity
Holding angle and thickness constant, \(\partial BA/\partial K=\theta T\). For a 90° bend in 0.0625-in sheet, changing K by 0.05 changes BA by about 0.00491 in per bend. Six bends with the same systematic K error could accumulate about 0.0295 in of developed-length difference.
Fast sanity check
For a 90° bend, \(BA=(\pi/2)(R+KT)\). If the neutral-axis radius is 0.090 in, BA should be a little over 1.57 times that radius, or about 0.141 in.
Use R/T as a bend-severity clue, not a universal K-factor table
The calculator reports \(R/T\) because radius relative to thickness is more informative than radius alone. A 1 mm radius is a relatively large bend on 0.25 mm sheet but a tight bend on 3 mm sheet. Do not convert R/T directly to a universal K-factor unless the relationship comes from validated data for the actual process.
Watch for suspicious reverse K-factor results
If reverse K-factor mode produces a value far from the expected neutral-axis region, first recheck the measured bend allowance, finished radius, actual thickness, and angle convention. A mathematically consistent back-calculation can still be based on inconsistent measurements or the wrong dimension convention.
Example Bend Allowance Chart
This chart is a controlled example, not a universal lookup table. It holds thickness, inside radius, and K-factor constant so you can see how bend angle alone changes bend allowance.
| Bend angle | Bend allowance |
|---|---|
| 30° | 0.733 mm |
| 45° | 1.100 mm |
| 60° | 1.466 mm |
| 90° | 2.199 mm |
| 120° | 2.932 mm |
| 135° | 3.299 mm |
| 150° | 3.665 mm |
Because \(BA= heta(R+KT)\), bend allowance scales linearly with bend angle when radius, thickness, and K-factor are held constant. Use the calculator above for your actual geometry.
What Changes Bend Allowance in Real Fabrication?
The calculator’s geometry is deterministic for the values entered, but the values that describe a real press-brake bend can move with material behavior and process setup. The most important production question is whether the entered radius and K-factor represent the bend the machine actually makes.
Material and thickness
Autodesk’s 2026 Inventor guidance identifies bend angle, material, and thickness as factors that affect K-factor. Use the actual sheet thickness rather than assuming a gauge designation directly supplies the correct dimensional value.
Inside radius and tooling
Changing the formed inside radius changes the neutral-axis radius \(R+KT\) directly. In production, confirm the radius generated by the actual punch, die, opening, material, and forming setup rather than treating tooling labels as guaranteed finished geometry.
Air bending, bottoming, and coining
Different bend operations produce different material flow and tooling contact conditions. ANSYS specifically lists bend-operation type and tools among the physical factors that a purely geometric K-factor does not capture automatically.
Springback
Springback affects the relationship between the loaded tooling position and the final released angle. The calculator expects the bend geometry you want to evaluate; it does not calculate overbend or springback compensation.
Grain direction and formability
Grain direction can affect bendability and minimum-radius requirements. Follow the material producer’s guidance for the actual alloy, grade, and temper; for example, SSAB notes grain-direction considerations in its own press-brake guidance for applicable products.
CAD bend tables
SOLIDWORKS supports bend allowance, bend deduction, K-factor, and bend tables. For a repeatable shop process, a validated bend table can preserve empirical values by thickness, radius, and angle instead of relying on one constant K-factor for every bend.
Common Bend Allowance Mistakes
Most large bend-development errors come from using the right equation with the wrong geometry, dimension convention, or empirical input. Check these items before blaming rounding or unit conversion.
Using included angle as bend angle
A drawing that shows 150° between the flanges corresponds to a 30° bend from flat, not a 150° bend. Use the calculator’s Included Angle option when that is the convention shown.
Treating K-factor as a universal material constant
K-factor represents neutral-axis position for a bend condition. Generic values can be useful examples, but production values should be based on validated process data when dimensional accuracy matters.
Mixing tangent and outside / virtual-sharp flange dimensions
Adding BA to outside / virtual-sharp dimensions or subtracting BD from tangent lengths mixes two different bookkeeping systems. Identify the drawing convention first, then use the matching flat-length relationship.
Entering gauge instead of thickness
Gauge is material-dependent. Convert the gauge designation to a real thickness for the selected material before entering \(T\).
Assuming punch radius equals finished inside radius
The BA equation needs the bend radius that describes the finished geometry. If actual parts do not match the model, measure the formed radius and recalibrate the bend rule.
Expecting BA to include springback or tonnage
Bend allowance is a flat-development relationship. It does not determine press-brake force, machine capacity, minimum flange, springback compensation, or tooling suitability.
Assumptions, Accuracy, and Production Limits
This calculator is best treated as a closed-form sheet-metal development model plus an empirical K-factor input. The arithmetic can be exact for the entered values while the real blank still differs if those values do not represent the actual bend process.
Single-bend development
The optional flat-pattern calculation handles one bend between two flange dimensions. For a multi-bend part, apply the correct allowance or deduction to each bend using a consistent dimension convention.
Constant entered K-factor
Direct BA mode uses the K-factor you enter for that bend. It does not automatically infer K from alloy, temper, tooling, radius ratio, or bend method.
Finished geometry basis
The model uses the entered finished inside radius and effective bend angle. It does not simulate elastic-plastic forming, residual stress, springback, or tool penetration.
No formability or tooling check
A valid BA result does not prove that the bend radius is safe for the material or that the press brake, punch, die, and tooling load limits are adequate.
Physical K-factor interpretation
For conventional sheet-metal neutral-axis interpretation, K-factor is normally bounded at or below 0.50, corresponding to the neutral axis at or inside mid-thickness. A value above 0.50 should be treated as a custom CAD/process parameter or a sign that the measurements and model assumptions need review.
Sources and Calculation Verification
The governing K-factor relationship and CAD workflow were checked against current official documentation. The worked example was recomputed directly, then verified by reverse-solving K-factor and by independent inch-to-millimeter conversion.
- SOLIDWORKS Design Help 2026 — K-Factor — defines K-factor as neutral-sheet position relative to thickness and gives \(BA=\pi(R+KT)A/180\), with bend angle defined as the angle through which the material is bent.
- SOLIDWORKS Design Help 2026 — Bend Table — documents bend allowance, bend deduction, and K-factor bend tables for sheet-metal flat development.
- SOLIDWORKS Design Help 2026 — Bend Calculation Tables — documents virtual-sharp versus tangent-length handling in developed-length calculations.
- Autodesk Support 2026 — How to Calculate K-Factor for Inventor Sheet Metal — identifies bend angle, material, and thickness as K-factor influences and recommends measuring a bent sample when unfolded length does not match production.
- ANSYS Discovery — Calculating Bend Allowances — defines \(K=t/T\), explains neutral-axis compression/tension geometry, and emphasizes reverse calculation from an actual bend for process-specific K-factor.
- SSAB — Press Brake Setup and Preparation — supports the need to check tooling, bendability, grain direction, bend allowance, and press-brake setup for the applicable material/product before production bending.
How the calculation was checked: the default 90° example gives \(R_n=0.0900\) in and \(BA=(\pi/2)(0.0900)=0.1413717\) in. Reverse-solving that value returns \(K=0.44\), and converting \(0.1413717\) in with 25.4 mm/in gives 3.59084 mm.
Bend Allowance FAQ
These questions address common sheet-metal flat-pattern decisions that are not obvious from the formula alone.
What is bend allowance?
Bend allowance is the developed arc length of a sheet-metal bend measured along the neutral axis. It represents the length of flat material occupied by the bend region.
What is the bend allowance formula?
For the K-factor method used here, \(BA=\frac{\pi A}{180}(R+KT)\), where \(A\) is bend angle in degrees, \(R\) is inside radius, \(T\) is thickness, and \(K\) locates the neutral axis through the thickness.
How do you calculate bend allowance for a 90-degree bend?
At \(A=90^\circ\), the formula simplifies to \(BA=\frac{\pi}{2}(R+KT)\). There is no single 90° bend-allowance value because the answer still depends on radius, thickness, and K-factor.
What is the difference between bend allowance and bend deduction?
Bend allowance is the neutral-axis arc length through the bend. Bend deduction is the amount subtracted from flange dimensions measured to the virtual sharp. Tangent-length development adds BA; virtual-sharp development subtracts BD.
What K-factor should I use for sheet metal?
Use a K-factor validated for the actual material, thickness, radius, tooling, and bend process when possible. Generic values are planning inputs, not universal material constants. In the conventional physical neutral-axis interpretation, K at 0.50 corresponds to mid-thickness; values above that deserve special review rather than being treated as ordinary production defaults.
Is bend angle the same as included angle?
Not always. This calculator’s bend angle is the amount the sheet turns from flat. If a drawing shows the included angle between flanges, convert it with \(A_\mathrm{bend}=180^\circ-A_\mathrm{included}\), or select the calculator’s Included Angle option.
Does bend allowance account for springback?
No separate springback model is included. The calculator develops the entered finished geometry. Tool overbend, elastic recovery, and machine/process compensation must be established separately for the real forming setup.
Can I enter sheet-metal gauge instead of thickness?
No. The calculator requires dimensional thickness. Gauge is material-dependent, so convert the specific material’s gauge designation to inches or millimeters before entering \(T\).
Why does my CAD flat pattern not match the bent part?
First compare actual thickness, finished radius, bend angle, and dimension convention with the CAD model. If those match, calibrate the K-factor or bend table from a representative bent sample made with the real material and tooling, as current Autodesk and ANSYS guidance recommends.