You have a 2 mm bracket to bend, a drawing with outside dimensions, and a flat blank to cut. You do not want theory right now. You want a number.
This page gives you that number. Below you will find ready-to-use bend allowance and bend deduction charts for 90 degree bends, values for other common angles, the formula behind them, and a worked example you can copy for your own parts.
If you want the deeper explanation of why the neutral axis moves and how K-factor works, read our Bend Allowance: Sheet Metal Design Guide. This post is the quick reference that sits next to it.
Bend allowance (BA) is the length of the bend itself, measured along the neutral axis.
BA = \frac{\pi}{180} \times A \times (R + K \times T)Where A is the bend angle in degrees, R is the inside bend radius, T is the sheet thickness and K is the K-factor (where the neutral axis sits, as a fraction of thickness from the inside face).
Bend deduction (BD) is what you subtract from the outside dimensions to get the flat length. It is linked to BA through the outside setback (OSSB):
OSSB = \tan(A/2) \times (R + T) \qquad BD = 2 \times OSSB - BAFor a 90 degree bend, tan(45) is 1, so OSSB is simply R + T. That makes 90 degree bends easy to check by hand.
| Thickness T | R = 1.0 | R = 1.5 | R = 2.0 | R = 3.0 | R = 4.0 |
|---|---|---|---|---|---|
| 0.8 | 2.12 | 2.91 | 3.69 | 5.27 | 6.84 |
| 1.0 | 2.26 | 3.05 | 3.83 | 5.40 | 6.97 |
| 1.2 | 2.40 | 3.19 | 3.97 | 5.54 | 7.11 |
| 1.5 | 2.61 | 3.39 | 4.18 | 5.75 | 7.32 |
| 2.0 | 2.95 | 3.74 | 4.52 | 6.09 | 7.67 |
| 2.5 | 3.30 | 4.08 | 4.87 | 6.44 | 8.01 |
| 3.0 | 3.64 | 4.43 | 5.22 | 6.79 | 8.36 |
| Thickness T | R = 1.0 | R = 1.5 | R = 2.0 | R = 3.0 | R = 4.0 |
|---|---|---|---|---|---|
| 0.8 | 1.48 | 1.69 | 1.91 | 2.33 | 2.76 |
| 1.0 | 1.74 | 1.95 | 2.17 | 2.60 | 3.03 |
| 1.2 | 2.00 | 2.21 | 2.43 | 2.86 | 3.29 |
| 1.5 | 2.39 | 2.61 | 2.82 | 3.25 | 3.68 |
| 2.0 | 3.05 | 3.26 | 3.48 | 3.91 | 4.33 |
| 2.5 | 3.70 | 3.92 | 4.13 | 4.56 | 4.99 |
| 3.0 | 4.36 | 4.57 | 4.78 | 5.21 | 5.64 |
A quick sanity check: in the table above, the inside radius is never smaller than about 1 x thickness for the thicker sheets. Going tighter than the material allows risks cracking on the outside of the bend, especially across the grain.
Not every bend is 90 degrees. Here is how the numbers change for common angles on a 1.5 mm part.
| Bend angle | Bend allowance (mm) | Bend deduction (mm) |
|---|---|---|
| 30 degrees | 1.39 | 0.48 |
| 45 degrees | 2.09 | 0.81 |
| 60 degrees | 2.79 | 1.26 |
| 90 degrees | 4.18 | 2.82 |
The bend angle here is how far the flange is bent from flat, not the included angle between the flanges. A 45 degree bend leaves a 135 degree included angle. Mixing the two up is one of the most common reasons a calculated blank comes out wrong.
Say you need an L-bracket in 2 mm mild steel, with outside dimensions of 50 mm and 30 mm, and a 2 mm inside radius.
Method 1, bend deduction (the shop-floor way):
Method 2, bend allowance (the check):
Both methods give the same answer, which is exactly what you want. If they do not match on your own part, one of the inputs is inconsistent, usually an inside dimension used as an outside one.
More than most freshers expect. Here is a single 90 degree bend in 1.5 mm sheet at a 1.5 mm inside radius:
| K-factor | Bend allowance (mm) |
|---|---|
| 0.33 | 3.13 |
| 0.40 | 3.30 |
| 0.44 | 3.39 |
| 0.50 | 3.53 |
That is a 0.4 mm spread on one bend. Put six bends on a BIW bracket and the last mounting hole can be off by more than 2 mm. So treat this chart as a starting point:
It is a lookup table that gives the developed length of a bend for a given thickness, inside radius and angle. It saves you running the formula every time.
Bend allowance is added to the leg lengths measured to the tangent points. Bend deduction is subtracted from the outside dimensions. Both give the same flat length when used correctly.
For 90 degrees, BA = 1.5708 x (R + K x T), and BD = 2 x (R + T) – BA. Then subtract BD from the sum of the outside dimensions.
Around 0.40 to 0.45 is a common starting range for air bending at a moderate radius. Tighter radii push it lower, and coining or bottoming changes it again. Always confirm with your fabricator.
The formula is the same, but the K-factor and minimum radius change. Stainless needs a larger radius and springs back more; soft aluminium can take tighter radii. Recalculate with the right K or use your supplier’s table.
Charts help you calculate. Design judgement tells you which radius, relief and hole position will actually work on the press brake.
Navinyasa’s Sheet Metal & BIW Design Training in Bengaluru covers flat patterns, bend reliefs, flanges, hems, joggles and real automotive BIW parts, with live projects and mentor guidance.
Book a free demo or WhatsApp us at +91 990 113 6476
