Free Engineering Tool
Compute the developed (flat) length of round and square tube parts. Build the bend sequence — straights and bends in order — enter the tube OD, wall thickness, bend angles and centerline radii; the calculator returns the total blank length using the neutral-axis method BA = θ × (R + K×T).
The unfold length is what you cut before bending. Getting it right avoids scrapped blanks and rework on every part. This is the same method our tube-bending team uses when programming rotary draw and CNC benders — straight sections measured between bend tangent points, minus the tangent offsets, plus the neutral-axis arc of every bend.
All bends within recommended radius limits for this material.
Bend allowance uses the neutral-axis method BA = θ × (R + K×T). Tangent offsets R × tan(θ/2) are removed from adjacent straights. Confirm with a trial bend — springback and mandrel choice affect the final result.
The developed length of a bent tube is the sum of its net straight sections and its bend allowances. Each bend is measured on the neutral axis — the position inside the wall that neither stretches nor compresses.
Units: dimensions in mm or inches (auto-converted), angles in degrees, θ converted to radians internally. The K-factor defaults to the selected material and tube profile; override it with your shop's measured value.
| Material | K (round) | K (square) | Min CLR round | Min CLR square |
|---|---|---|---|---|
| Mild Steel | 0.44 | 0.38 | 2× OD | 2.5× OD |
| 4130 Chromoly | 0.43 | 0.37 | 2.5× OD | 3× OD |
| DOM Steel | 0.43 | 0.37 | 2× OD | 2.5× OD |
| ERW Steel | 0.44 | 0.38 | 2.5× OD | 3× OD |
| Stainless 304 | 0.43 | 0.37 | 3× OD | 3.5× OD |
| Stainless 316 | 0.43 | 0.37 | 3× OD | 3.5× OD |
| Aluminium 6061 | 0.41 | 0.35 | 3× OD | 4× OD |
| Aluminium 7075 | 0.40 | 0.34 | 3.5× OD | 4.5× OD |
| Titanium | 0.42 | 0.36 | 3.5× OD | 4× OD |
| Copper | 0.42 | 0.36 | 2.5× OD | 3× OD |
| Brass | 0.42 | 0.36 | 2.5× OD | 3× OD |
K-factors are typical values for rotary-draw bending without mandrel. Minimum CLR is a safe starting point; tight radii may need a mandrel or wiper die.
25.4 mm OD mild-steel tube (K = 0.44), 1.65 mm wall, bent 90° with a 76.2 mm CLR between two straights of 200 mm and 150 mm. Bend allowance BA = (π/180 × 90) × (76.2 + 0.44 × 1.65) ≈ 120.8 mm. Tangent offset J = 76.2 × tan(45°) = 76.2 mm. Net straights: 200 − 76.2 = 123.8 mm and 150 − 76.2 = 73.8 mm. Total developed length = 123.8 + 73.8 + 120.8 = 318.4 mm — this is the blank length to cut before bending.
The developed (unfold) length is the sum of the net straight sections plus the bend allowances. Each bend consumes material along the neutral axis: BA = θ × (R + K×T), where θ is the bend angle in radians, R the centerline radius (CLR), K the material K-factor and T the wall thickness. Because the bend arc replaces part of each adjacent straight, the tangent offset R × tan(θ/2) is subtracted from the straights on either side of every bend.
The K-factor positions the neutral axis through the bend as a fraction of wall thickness. Typical round-tube values are 0.40–0.44 (mild steel 0.44, stainless 0.43, aluminium 6061 0.41, copper/brass 0.42); square tube runs about 0.05 lower (0.34–0.38). Values vary with bending method, mandrel use and wall thickness — confirm with a trial bend for critical parts.
It depends on the material and whether the tube is round or square. Common rules of thumb (as multiples of tube OD): mild steel 2× (round) / 2.5× (square), 4130 & ERW 2.5× / 3×, stainless 304/316 3× / 3.5×, aluminium 6061 3× / 4×, titanium 3.5× / 4×. Bending below the minimum risks cracking, collapsing or excessive wall thinning.
The diameter-to-wall-thickness ratio tells you how thin-walled the tube is. Above roughly 25:1 for round tube (20:1 for square), the tube is prone to collapsing or wrinkling on the inside of the bend. In that range, use a mandrel bender or specify a thicker wall.
Yes. All tubes spring back after bending, opening the angle slightly and enlarging the radius. The unfold calculation is purely geometric — it gives the correct blank length — but the press/rotary draw bender still needs overbend compensation (typically a few degrees, more for stainless and aluminium) and sometimes radius compensation. Confirm with a trial part.
A mandrel supports the inside of the bend to prevent collapse and produces a cleaner, more consistent radius. It does not change the neutral-axis length itself, but it lets you bend to tighter radii and thinner walls than free bending, which is why the unfold result stays valid while the achievable radius range widens.
The developed length is the first step of material costing and directly determines tube consumption. For a production quote, send your drawing to Balford at shawn@balford.net — our engineers confirm bend sequence, tooling, tolerances and pricing, typically within 24 hours.
Send your drawing to shawn@balford.net — our engineers will confirm the bend sequence, developed lengths and pricing within 24 hours.