Use this sheet metal bend calculator to estimate bend allowance, bend deduction, outside setback, inside setback, and one-bend flat pattern length. Enter material thickness, inside bend radius, bend angle, and K-factor, then choose whether you want only bend values or a simple two-leg flat length calculation.
The calculator is intended for layout estimates, CAD flat pattern checks, and quoting support. Final production values should still be checked against your press brake tooling, material, bend method, and shop-approved bend tables.
Sheet Metal Bend Calculator
Contents
Estimate bend allowance, bend deduction, outside setback, inside setback, and one-bend flat length from material thickness, inside radius, bend angle, and K-factor.
What the Calculator Solves
Sheet metal flat patterns are affected by how the material stretches through the bend. The neutral axis does not usually sit exactly at the middle of the material thickness, so a K-factor is used to estimate where that neutral axis falls. From that, the calculator estimates how much material is consumed by the bend and how much should be deducted from outside flange dimensions.
| Output | What it means | Common use |
|---|---|---|
| Bend allowance | Arc length of the neutral axis through the bend. | Adding bend material to inside flange lengths. |
| Bend deduction | Amount removed from outside flange lengths to get the flat length. | Calculating flat length from outside dimensions. |
| Outside setback | Distance from the outside mold line to the tangent point. | Checking outside flange geometry. |
| Inside setback | Distance from the inside mold line to the tangent point. | Checking inside bend geometry. |
| Flat length | Estimated blank length for a single bend using two outside leg dimensions. | Quick flat pattern estimate before CAD or shop verification. |
Formulas Used
The calculator uses the bend angle in degrees and converts it to radians before calculating bend allowance and setbacks.
| Variable | Meaning |
|---|---|
| T | Material thickness |
| R | Inside bend radius |
| K | K-factor |
| A | Bend angle in radians |
| BA | Bend allowance |
| OSSB | Outside setback |
| ISSB | Inside setback |
| BD | Bend deduction |
Bend allowance: BA = A x (R + K x T)
Outside setback: OSSB = tan(A / 2) x (R + T)
Inside setback: ISSB = tan(A / 2) x R
Bend deduction: BD = 2 x OSSB – BA
One-bend flat length from outside legs: Flat length = Leg A + Leg B – BD
Worked Example
For a 90 degree bend using 5 mm material, a 1 mm inside radius, and a K-factor of 0.33, the calculator gives the following approximate values.
| Input or result | Value |
|---|---|
| Material thickness | 5 mm |
| Inside bend radius | 1 mm |
| Bend angle | 90 degrees |
| K-factor | 0.33 |
| Outside leg A | 100 mm |
| Outside leg B | 50 mm |
| Bend allowance | About 4.163 mm |
| Outside setback | About 6.000 mm |
| Bend deduction | About 7.837 mm |
| Estimated flat length | About 142.163 mm |
Choosing a K-Factor
K-factor depends on material, tooling, inside radius, bend angle, grain direction, and forming method. A value near 0.33 is often used as a starting estimate for common air bending, while higher values may be used when the neutral axis is estimated closer to the middle of the material thickness. For production work, use the K-factor or bend deduction values proven by your own tooling and material tests.
How to Measure the Legs
The one-bend flat length mode assumes outside leg dimensions measured to the outside apex of the bend. If your drawing uses inside dimensions, flange-to-tangent dimensions, or CAD-specific bend tables, make sure the input method matches the formula being used. Mixing inside and outside measurement conventions is one of the most common reasons a flat pattern estimate does not match the real part.
Frequently Asked Questions
Is bend allowance the same as bend deduction?
No. Bend allowance estimates material length along the neutral axis through the bend. Bend deduction is subtracted from outside flange dimensions to estimate the flat length.
Why does my CAD flat pattern differ from this calculator?
CAD systems may use a bend table, gauge table, custom bend deduction, material-specific K-factor, or shop-specific bend data. This calculator uses standard formulas, so it is best used as a transparent estimate and cross-check.
Should I use the bend angle or the opening angle?
The calculator can accept either. If you enter an opening angle, it converts it internally using bend angle = 180 – opening angle.
Can this replace a shop bend table?
No. A proven shop bend table is better for production because it reflects real material, tooling, and machine behavior. This calculator is useful for understanding the formulas and making quick layout estimates.
