Bend Allowance Calculator & Flat Pattern Results
Calculate sheet metal flat length from formed flange dimensions, or determine formed dimensions from a known blank length. Compare bend allowance and bend deduction, use a preliminary K factor or measured shop data, and review the dimensional results.
Sheet Metal Flat Pattern Calculator: Input Dimensions
Use this sheet metal bend allowance calculator and sheet metal bending calculator for a single bend. Outside flange dimensions refer to the intersection of the extended, straight outside faces (virtual sharp).
Bend angle means rotation from the original flat sheet: 90° for a typical right-angle bend. It does not mean the internal opening angle.
Bend Allowance Calculator: Choose Calculation Basis
0.33 is an example assumption, not a validated material-specific constant. Confirm results against a trial bend or factory bend table.
Sheet Metal Flat Length Calculator: Two Directions
A flat length alone cannot determine both flange lengths. Supply one outside leg or select the equal-leg assumption.
Calculated Bend Allowance, Deduction and Flat Length
This is a one-bend geometric estimate, not an approved DXF, tool-clearance check, minimum flange guarantee or production-ready tolerance prediction.
Bend Allowance Formula and Bend Deduction Formula
| Calculation | Equation for one bend | Purpose |
|---|---|---|
| Bend allowance | BA = θ × π / 180 × (R + K × t) | Neutral-axis arc length through the bend. |
| Outside setback | OSSB = (R + t) × tan(θ / 2) | Offset from outside virtual sharp to bend tangency. |
| Bend deduction | BD = 2 × OSSB − BA | Deducted from the sum of outside virtual-sharp leg dimensions. |
| Flat pattern | Flat = A + B − BD | Developed blank length of a single-bend L-section. |
| Reverse formed length | B = Flat − A + BD | One flange length from a known flat length and other flange. |
| Back-calculate K factor | K = (BA ÷ θrad − R) ÷ t | Uses measured BA, or BA inferred from measured BD. |
| Inside bend radius calculator | R = BA ÷ θrad − K × t | Uses measured BA and a known K; validates R ≥ 0. |
For inverse formulas, θrad = θ × π / 180. Here θ is the bend rotation from flat, A and B are outside virtual-sharp dimensions, t is thickness, R is the inside radius. The expressions apply to one constant-radius bend, not an arbitrary multi-bend profile.
Bend Allowance vs Bend Deduction: Which Result Should You Use?
The bend allowance formula measures neutral-axis material length within the formed bend. The bend deduction formula converts the sum of outside flange dimensions into the blank length. Use bend deduction when your drawing gives both outside virtual-sharp dimensions, or bend allowance when building a flat profile from its straight tangent segments. Both methods should agree for the same bend geometry.
For a bend deduction calculator, a shop-measured bend deduction can improve estimates for a specific material, thickness, inside radius, bend angle and tooling setup. A k factor calculator sheet metal workflow is supported here by solving K from measured bend allowance or bend deduction. In the bend radius calculator sheet metal mode, the tool solves R from measured BA and a known K. Changing material in the selector does not silently change K or R.
Shop Calibration, Material and Flat Pattern Assumptions
For preliminary design, use an assumed K factor and verify the result with a sample bend. For production, use a measured bend deduction or bend allowance from the same bend angle, material grade, thickness, die, punch and bend process. An inside bend radius calculator can back-solve the radius only if an independent K value and the actual bend allowance are known.
The sheet metal flat length calculator models one bend, so a multiple-bend part needs a separate calculation for each bend and a consistent dimension chain. For complex folds, overlap, corner relief, grain direction, springback, bend proximity and minimum flange length, review the result with the fabricator before cutting a production blank. Learn more about sheet metal fabrication services.
Sheet Metal Bending Calculator: Common Questions
How to calculate bend allowance?
Convert the bend rotation angle to radians, then multiply by the neutral-axis radius: BA = θrad × (inside radius + K × sheet thickness). Confirm the K factor and radius for the actual bend setup.
How to calculate sheet metal flat pattern?
For one bend with outside virtual-sharp dimensions A and B, calculate outside setback and bend allowance, then bend deduction BD = 2 × OSSB − BA. The flat blank length is A + B − BD. Multiple bends require a consistent dimension chain.
What is K factor in sheet metal?
K factor is the distance from the inside face to the neutral axis divided by sheet thickness. It varies with material and forming conditions; the example value 0.33 is not a certified parameter. This calculator can infer K from measured bend allowance or bend deduction.
How to calculate bend deduction?
First calculate OSSB = (R + t) × tan(θ / 2) and BA = θrad × (R + K × t). Then calculate BD = 2 × OSSB − BA. Shop measurements may replace the theoretical BD for equivalent forming conditions.
How to calculate sheet metal bend length?
For a single L-shaped bend, if you mean the length of material through the bend, calculate the neutral-axis bend allowance. If you mean total flat blank length, use A + B − bend deduction. These are different dimensions.
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