Sheet Metal Flat Pattern Calculator – Flat Size from Finished Dimensions
Calculate an estimated flat pattern size from finished sheet metal dimensions. Account for bend allowance, bend deduction, K-factor, inside radius and multiple straight bends.
How flat pattern development works
- Finished dimensions describe the bent part you want to manufacture.
- Flange dimensions are entered using the outside-dimension convention when bend deduction is selected.
- Bend allowance represents the neutral-axis arc consumed by each bend.
- Bend deduction converts two outside flange dimensions into a developed flat length.
- K-factor estimates the neutral-axis location through the sheet thickness.
- Multiple bends require the bend allowance or deduction to be accounted for at every bend.
For a simple chain of straight flanges, the flat length can be estimated by adding the finished flange dimensions and applying the appropriate bend correction at each bend.
FLAT PATTERN → CUT → BEND → FINISHED PART
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Finished flange dimensions must use the same convention as the selected calculation method.
This calculator is an engineering estimate for simple straight bends. Actual flat patterns can differ because of material properties, tooling, bend radius, springback, forming method and shop-specific bend tables. Verify production dimensions with a test bend or established manufacturing data.
Bend correction formulas
Bend allowance
BA = θ × (R + K × T)
θ is the bend angle in radians, R is inside radius, T is material thickness and K is the K-factor.
Bend deduction
BD = 2 × (R + T) × tan(θ/2) − BA
This is a common outside-dimension convention. Your shop or CAD system may use a different bend convention.
Flat length using outside flange dimensions
Flat length = Σ finished flange dimensions − Σ bend deductions
Flat length using developed straight segments
Flat length = Σ straight developed segments + Σ bend allowances
Practical flat-pattern checks
- Confirm whether the drawing dimensions are inside, outside, tangent, or neutral-axis dimensions.
- Use the actual inside radius produced by the selected punch and die combination.
- Do not assume the same K-factor applies to every material and tooling combination.
- For multiple bends, check that the bend sequence does not cause flange or tooling interference.
- Allow for holes, slots, reliefs and notches separately; this calculator estimates developed linear dimensions.
- For high-precision production, use a validated bend table or measured test bend rather than relying solely on a generic K-factor.
Common flat-pattern applications
Simple covers
Multiple bends
Frequently Asked Questions
What is a sheet metal flat pattern?
It is the developed two-dimensional shape cut from sheet before it is bent into the finished part.
How is flat size calculated?
For outside finished dimensions, a common approach is to add the flange dimensions and subtract the applicable bend deductions. For developed neutral-axis dimensions, bend allowances are added instead.
Can this handle multiple bends?
Yes. The calculator supports one through six straight bends and applies the selected bend correction to each bend.
Why does K-factor matter?
K-factor determines the estimated neutral-axis position and therefore changes the calculated bend allowance.
Can I enter my own bend allowance?
Yes. The custom method lets you supply a known bend allowance per bend instead of calculating it from K-factor.
Is this suitable for production CNC cutting?
It can provide a useful starting calculation, but production dimensions should be verified against the actual material, tooling and validated bend data.