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Sheet Metal Flat Pattern Calculator
Enter the flange dimensions of the profile as outside or inside dimensions; with sheet thickness, inside radius and K-factor you get the flat length to cut on the laser, the value of each bend and the part weight. L, U, Z and hat profile templates included.
Sheet and bend settings
Assumed to be the same for all bends.
Suggested for R/T = 1.
Outside dimension: distance between the outer surfaces of the sheet.
Profile and flanges
Last flange
The bend angle is the angle bent from the flat sheet (included angle 120° → 60°). Each bend follows the flange in its row.
Weight (optional)
Length of the part along the bend line; if entered, area and weight are calculated.
7.85: mild steel and galvanized · 7.93: stainless steel · 2.70: aluminium. Whole sheets: sheet metal weight calculator
How was this calculated?
Bend 1 (90°)
BA = π ÷ 180 × 90 × (2 + 0.4 × 2) = 4.398; OSSB = tan(90° ÷ 2) × (2 + 2) = 4; BD = 2 × 4 − 4.398 = 3.602 mm (subtracted)
Bend 2 (90°)
BA = π ÷ 180 × 90 × (2 + 0.4 × 2) = 4.398; OSSB = tan(90° ÷ 2) × (2 + 2) = 4; BD = 2 × 4 − 4.398 = 3.602 mm (subtracted)
Flange total
50 + 100 + 50 = 200 mm
Flat length
200 − 3.602 − 3.602 = 192.8 mm
Track cutting and bending jobs in one place
LazerCRM is quoting and job-tracking software for laser cutting and sheet metal job shops; press brake bending jobs are tracked together with the cutting jobs. The interface is currently in Turkish. If you are interested, get in touch with us.
How do you calculate a sheet metal flat pattern?
The flat pattern (developed length) is the length of the blank before it is bent. If the flanges are dimensioned to the outside, the flat length is the sum of the flange dimensions minus the bend deduction (BD) of every bend. BD = 2 × tan(A ÷ 2) × (R + T) − BA, and the bend allowance is BA = π ÷ 180 × A × (R + K × T). If the flanges are dimensioned to the inside, 2 × tan(A ÷ 2) × R is subtracted and BA is added at each bend.
Example: a 2 mm U-channel with outside dimensions 50 + 100 + 50 mm, 2 mm inside radius, K = 0.40 and two 90° bends has a BD of about 3.602 mm per bend, so the flat length is 200 − 2 × 3.602 ≈ 192.80 mm. On a Z-profile the bends go in opposite directions, but the calculation is the same.
The result is an approximate starting value. The actual sheet thickness, the inside radius produced by the V-die, springback and the material batch change the flat length, and small errors add up as the number of bends grows. Verify it with a test bend before series production and record the value in your CAD bend table.
How to use the results
The results are technical estimates. For a binding quote, material price, scrap, labour and your own shop rules also have to be taken into account.
Frequently asked questions
How is the flat length of a sheet metal part calculated?
With outside dimensions: flat length = flange total − Σ BD. With inside dimensions: flat length = flange total − Σ (2 × tan(A ÷ 2) × R) + Σ BA. Here BA = π ÷ 180 × A × (R + K × T) and BD = 2 × tan(A ÷ 2) × (R + T) − BA.
What is the flat length of a 50 × 100 × 50 U-channel?
With outside dimensions, 2 mm sheet, 2 mm inside radius and K = 0.40, each 90° bend deducts BD ≈ 3.602 mm, so the flat length is 200 − 7.204 ≈ 192.80 mm. A different inside radius or K-factor changes the result.
Should I use outside or inside dimensions?
Use whichever surface the drawing is dimensioned to. Outside dimensions are measured between the outer surfaces of the sheet, inside dimensions between the inner surfaces. Choosing the wrong type causes an error of roughly two sheet thicknesses per 90° bend.
Is the flat pattern of Z-profiles and hat profiles calculated differently?
No. The direction of the bends does not change the flat length; what matters are the flange dimensions, bend angles, inside radius and K-factor. A Z-profile uses the same formula even though its two bends go in opposite directions.
What if I do not know the K-factor?
The calculator suggests a starting value from the R/T ratio: 0.33 below 1, 0.40 between 1 and 3, and 0.50 above 3. To determine your own K-factor, use the measured flat length mode of the bend allowance calculator.
Can I calculate the part weight as well?
Yes. Enter the part width and choose the density (mild steel and galvanized 7.85, stainless steel 7.93, aluminium 2.70 g/cm³); the part area and weight follow from flat length × width × thickness × density. For whole sheets, use the sheet metal weight calculator.
Related calculators
- Bend allowance & K-factor calculatorBend allowance (BA), bend deduction (BD) and flat length; K-factor from a measured flat length.
- Press brake tonnage calculatorBending force in tonnes and kN, inside radius and minimum flange length from thickness, length, Rm and die opening.
- Sheet metal weight calculatorApproximate sheet weight in kg from length, width, thickness and material density.
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