From folded part to flat blank
A flat pattern is the shape you cut before any forming happens. Bend the blank and the corners eat material, so the flat length is always shorter than the sum of the finished outside dimensions. This calculator unfolds a multi-bend part: add up the outside flange lengths, subtract the deduction for every bend, and you have the strip length to shear or nest.
The flat pattern formula
Flat length = (sum of outside dimensions) – (number of bends x bend deduction per bend). Each bend deduction comes from BD = 2 x OSSB – BA, where OSSB = (R + T) x tan(A/2) and BA = (A x pi/180) x (R + K x T).
How to measure the outside dimensions
Lay the part out flat in your mind, flange by flange, and measure each leg to the outside mold line at the corners. Add them into one total and enter it above as the total outside length. The tool then removes one bend deduction for each bend.
Parts with different bend angles
This calculator applies one bend angle to every bend, which covers the common case of an all-90-degree box or bracket. If your part mixes angles, total each bend separately with the bend deduction calculator and subtract the sum from your outside total.
Dial in K first
The whole flat pattern rides on the K-factor. If your blanks come out consistently long or short, measure your real K with the K-factor calculator using a test bend, then plug it in here for blanks that form to size the first time.
Worked example
A bracket with 300 mm of total outside length and three 90-degree bends in 2 mm steel, R = 3 mm, K = 0.33: each bend deducts 4.25 mm, so the flat blank is 300 – 3 x 4.25 = 287.25 mm.
FAQ
Why is my flat blank shorter than the finished part?
Because the outside corners gain material as the metal wraps the radius. The bend deduction removes that gain so the formed part lands on its target dimensions.
Does the radius matter much?
Yes. A larger inside radius increases both setback and deduction, shortening the blank. Always use the actual formed radius, not the punch nose radius, for sharp bends.