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How to Calculate a Sheet Metal Flat Length
Manesh Jayawardhana
CIO & Co-founder
Cut a 90 mm blank, bend it at 90 degrees in the middle, and the two legs measure 44 mm and 44 mm rather than 45 and 45.
Nothing went wrong. Metal stretches on the outside of a bend and compresses on the inside, and the material has redistributed itself around the corner.
The neutral axis and the K-factor
In any bend, the outer surface is in tension and stretches while the inner surface is in compression and shortens. Somewhere between them is a layer that does neither — the neutral axis.
The neutral axis is not at the middle of the material. It sits closer to the inside of the bend, and how much closer is expressed as the K-factor: the neutral axis position as a fraction of material thickness.
bend allowance = angle(radians) × (inside radius + K × thickness)
flat length = sum of flange lengths + bend allowance
K typically falls between about 0.33 and 0.5. Where it falls depends on the material, its thickness, the bend radius relative to thickness, and the forming method — and that range is wide enough to change a flat length by a millimetre or more on a single bend.
| Variable | Effect on K |
|---|---|
| Softer material | Lower K |
| Tight radius relative to thickness | Lower K |
| Large radius | Approaches 0.5 |
| Air bending vs bottoming | Different values |
Air bending complicates the radius
The formula needs the inside bend radius, and in air bending you do not directly set it.
In air bending the punch pushes the material into a V-die without pressing it fully into the corner. The resulting radius is determined largely by the die opening, not by the punch nose radius. A wider die produces a larger radius from the same tooling.
That means the radius to put into the calculation is the one the process actually produces, which depends on the die you are using and the material. Assuming the punch radius is a common error and it propagates straight into the flat length.
Bottoming and coining press the material into the die and produce a radius much closer to the punch, which is more predictable and requires considerably more tonnage.
Springback
Metal is elastic before it is plastic. Release the punch and the part springs back slightly toward flat.
The amount depends on material strength, thickness and radius — higher strength materials spring back more, which is why the same tooling and program produce a different angle in mild steel and stainless.
Compensating means over-bending by the springback amount, and the amount is established empirically for each material and thickness combination. There is no reliable way to calculate it from first principles for a given shop and press.
The test piece is not optional
Between K-factor uncertainty, radius variation in air bending, springback and batch-to-batch material variation, the calculated flat length gets you close.
A test piece establishes the actual value for this material, this tooling and this machine. Once established, it is recorded and reused for that combination — so it is a one-off cost per setup rather than per job.
Skipping it on a production run is how a batch of parts comes out consistently a millimetre short.
Common mistakes to avoid
- Using a generic K-factor for every material and thickness.
- Assuming the punch radius is the bend radius in air bending.
- Ignoring springback, or assuming it is the same across materials.
- Calculating from the outside dimensions rather than the inside ones without adjusting.
- Running a batch without a test piece for a new material and tooling combination.
How to do it with Bend Allowance Calculator
The Bend Allowance Calculator computes the flat length.
- Enter thickness, inside bend radius, angle and flange lengths.
- Use a K-factor appropriate to the material and forming method.
- Read the bend allowance and total flat length.
- Cut a test piece and record the actual K-factor for that setup.
Other engineering calculators are in the tools directory.
Frequently asked questions
What is the K-factor?
The position of the neutral axis as a fraction of material thickness. It varies with material, thickness, radius and forming method, and it is the largest source of error in a calculated flat length.
Why does my part come out the wrong size?
Usually the K-factor, an inside radius different from the one assumed, or springback. In air bending the radius is set by the die opening rather than the punch, which catches people out.
Do I still need a test piece?
Yes. Calculation gets you close; material batch variation, tooling wear and springback account for the rest. One test piece per setup is always cheaper than a wrong batch.
Final thought
Record the K-factor you measure for each material and tooling combination. The calculation is the same every time; the constant is what you are actually building up.