How to calculate a sheet metal flat pattern
When you bend sheet metal, the outside of the bend stretches and the inside compresses. Between them lies the neutral axis — a line whose length doesn't change during bending. To cut a blank that ends up the right size after folding, you need to know how much material the bends consume. That's what bend allowance, bend deduction and the flat pattern calculation tell you.
A part always has one more flange than it has bends — a simple right-angle bracket has two flanges and one bend. Cut the blank too long and the part is oversized; too short and it's scrap. Calculating the flat length before cutting saves material and avoids rework.
Bend allowance
Bend allowance (BA) is the arc length of the neutral axis through the bend — the material "used up" by the bend. It depends on bend angle, inside radius, thickness and K-factor:
Bend deduction
Bend deduction (BD) is subtracted from the sum of the outside flange dimensions to give the flat length. It comes from the outside setback (OSSB) and the bend allowance:
Flat pattern length
Add the flange lengths measured to the bend lines, then subtract the bend deduction for every bend:
The calculator handles any number of bends, shows where each bend line falls along the flat blank, and draws a live diagram of the folded part as you type.
Finding your exact K-factor from a test bend
Generic K-factors get you close, but the most accurate flat patterns come from measuring your own material on your own machine. Bend a test coupon, measure the finished flanges and the flat length you started with, then work the K-factor backwards from the real bend deduction. Switch the calculator to Solve for K-factor mode, enter your measured flat length and the test bend, and it returns the exact K value to use on production parts.
Frequently asked questions
What is bend allowance?
Bend allowance is the arc length of the neutral axis through a bend — the amount of material consumed by the bend, used to work out the flat pattern of a sheet metal part.
What's the difference between bend allowance and bend deduction?
Bend allowance is the developed length of material within the bend. Bend deduction is the amount removed from the sum of the outside flange dimensions to reach the correct flat length. They describe the same bend from different reference points.
What K-factor should I use?
It usually falls between 0.3 and 0.5 — commonly 0.44 general, 0.33 for steel, 0.38 for aluminium. For accuracy, measure it from a test bend using the solve-for-K mode.
How do I find my exact K-factor?
Bend a test coupon, measure the finished flanges and the flat blank you started with, then back-calculate K from the actual bend deduction. The solve-for-K mode does this for you.