Bending Guide
How to Calculate Flat Pattern Length
Turning a multi-bend drawing into a single flat blank dimension — plus where each bend line needs to land.
Every folded sheet metal part starts life as a flat blank. Get its length right and the part folds up exactly to the drawing; get it wrong and you're either trimming a part that's too long or scrapping one that's too short. The flat pattern calculation is really just bookkeeping — sum the flanges, subtract what each bend "costs" you, and you're left with the number to cut.
Skip the running totals
Add as many bends as your part needs and get the flat length, per-bend deductions and bend line positions instantly, with a live diagram.
The flat pattern formula
A part with n bends always has n+1 flanges — a simple right-angle bracket has one bend and two flanges. To find the flat length:
Each bend's own deduction comes from bend allowance and bend deduction: BD = 2 × OSSB − BA, calculated separately for every bend since angle and radius can differ bend to bend.
Worked example: a 3-flange, 2-bend bracket
Take a bracket with three flanges of 50mm, 40mm and 50mm, both bends at 90°, 2mm material, 2mm inside radius, K-factor 0.44:
Step by step
- Sum of flanges: 50 + 40 + 50 = 140mm
- Bend allowance per bend: BA = (π/180) × 90 × (2 + 0.44×2) ≈ 4.52mm
- Outside setback per bend: OSSB = tan(45°) × (2+2) = 4.00mm
- Bend deduction per bend: BD = 2×4.00 − 4.52 ≈ 3.48mm
- Total deduction (2 bends): 3.48 × 2 = 6.96mm
- Flat length: 140 − 6.96 ≈ 133.0mm
That 133mm is the flat blank to cut before folding. Change either bend's angle or radius and you'd calculate its deduction separately — mixed geometry per bend is common and the maths handles it the same way, bend by bend.
Locating the bend lines
Once you know the flat length, the next practical question is where to mark or score each bend line on the blank. Working from one end, the position of each bend line is the running total of flange lengths up to that point, minus the deductions already applied before it, minus half of that bend's own deduction — since the deduction is effectively shared between the two flanges either side of it. This is exactly what the calculator's "Bend line positions" output gives you directly, along with a live diagram of the folded shape as you adjust each input.
Common mistakes
- Forgetting a part has one more flange than bends. A 2-bend part needs 3 flange measurements, not 2.
- Using one bend deduction for a part with mixed angles. Every bend with a different angle or radius needs its own BD calculated separately.
- Mixing up inside and outside flange dimensioning. The standard flat-length formula assumes flanges are measured to the outside of the part — make sure that matches how your drawing is actually dimensioned.
- Using a generic K-factor on a tight-tolerance part. See K-factor explained for how to measure your own instead.
Add your part's real bends
Enter each flange and angle, add or remove bends freely, and watch the flat length and diagram update live.
Frequently asked questions
How do you calculate flat pattern length for sheet metal?
Add up all the flange lengths, calculate the bend deduction for each bend, add those deductions together, then subtract the total deduction from the total flange length.
How many flanges does a part with 3 bends have?
Four. A part always has exactly one more flange than it has bends — each bend sits between two flanges.
How do I find where a bend line falls on the flat blank?
Run a cumulative total of flange lengths up to that bend, subtract the deductions already applied before it, then subtract half of that bend's own deduction, since it's shared between the two adjacent flanges.