Sheet Metal Bend Deduction Calculator
Calculate bend deduction from flange lengths, bend allowance, outside setback, inside radius, thickness, bend angle, K-factor, bend count, and press brake forming style.
Bend deduction results
Calculation breakdown
| Material | Typical K-factor | Minimum inside radius | Starting V-die | Springback cue |
|---|---|---|---|---|
| Aluminum 5052-H32 | 0.30 to 0.36 | 0.5T to 1.0T | 6T to 8T | 1 to 3 degrees |
| Aluminum 6061-T6 | 0.33 to 0.40 | 1.5T to 3.0T | 8T to 12T | 3 to 6 degrees |
| Cold rolled mild steel | 0.38 to 0.44 | 0.8T to 1.5T | 6T to 8T | 1 to 3 degrees |
| Galvanized mild steel | 0.36 to 0.42 | 1.0T to 2.0T | 8T to 10T | 2 to 4 degrees |
| 304 stainless steel | 0.38 to 0.45 | 1.0T to 2.5T | 8T to 12T | 3 to 7 degrees |
| Copper or brass | 0.34 to 0.42 | 0.5T to 1.5T | 6T to 10T | 1 to 3 degrees |
| Term | Formula | Inputs used | Practical meaning |
|---|---|---|---|
| Bend allowance (BA) | A radians × (R + K × T) | Angle, radius, K-factor, thickness | Developed arc length along the neutral axis. |
| Outside setback (OSSB) | tan(A / 2) × (R + T) | Angle, inside radius, thickness | Distance from tangent point to outside virtual sharp. |
| Bend deduction (BD) | 2 × OSSB - BA | Setback and bend allowance | Amount removed from outside flange totals. |
| Outside flat length | Flange A + Flange B - BD | Outside flange dimensions | Single-bend blank length when legs are outside dimensions. |
| Inside flat length | Flange A + Flange B + BA | Inside tangent dimensions | Single-bend blank length when legs are inside straight lengths. |
| Preset | Thickness | Inside radius | Angle | K-factor | Typical use |
|---|---|---|---|---|---|
| 5052-H32 0.063 in bracket | 0.063 in | 0.063 in | 90 degrees | 0.33 | Small aluminum angle brackets and panels. |
| 16 ga CRS shelf flange | 0.060 in | 0.047 in | 90 degrees | 0.42 | Cold rolled shelves, trays, and formed lips. |
| 18 ga galvanized duct | 0.048 in | 0.062 in | 90 degrees | 0.38 | Duct flanges and light sheet transitions. |
| 304 stainless cover bend | 0.075 in | 0.094 in | 90 degrees | 0.40 | Stainless covers with higher springback. |
| 1/8 in mild steel angle | 0.125 in | 0.125 in | 90 degrees | 0.42 | Brackets and heavier machine guards. |
| 0.040 in copper flashing | 0.040 in | 0.032 in | 135 degrees | 0.36 | Open hems, trim bends, and flashing. |
| Material thickness | Air bend V-die start | Likely inside radius | Minimum outside flange | Notes |
|---|---|---|---|---|
| 0.030 in / 0.8 mm | 0.236 in / 6 mm | 0.030 to 0.040 in | About 0.165 in | Thin sheet needs accurate backgauge support. |
| 0.048 in / 1.2 mm | 0.375 in / 10 mm | 0.045 to 0.060 in | About 0.260 in | Common HVAC and light sheet range. |
| 0.063 in / 1.6 mm | 0.500 in / 12 mm | 0.060 to 0.080 in | About 0.350 in | Works well with 6T to 8T V openings. |
| 0.075 in / 2.0 mm | 0.630 in / 16 mm | 0.075 to 0.100 in | About 0.440 in | Stainless may need wider V and overbend. |
| 0.125 in / 3.2 mm | 1.000 in / 25 mm | 0.120 to 0.160 in | About 0.700 in | Confirm tonnage and punch nose rating. |
Brake inputs and Flange outputs including flat length, neutral axis, outside setback, bend deduction (or allowance), and tooling checks. Use this sheet metal bend deduction calculator to determine these values. Whether or not a part fits depend heavily on getting the flat pattern correct.
How do we deal with the changes in geometry caused by bending metal? Enter bend deduction. As a piece of metal is bent, it compress on one side and stretches on the other. In effect, the finished legs are smaller then the total of the flat blank. Without accounting for this change, your flanges might not fit together or your brackets will pull in. Feed in accurate brake information, and the calculator takes care of the rest. However, knowing what the inputs mean avoid mistakes.
Understanding Bend Deduction Inputs
The material’s behavior is where you should of begin. Cold-rolled steel, 304 stainless steel, aluminum 5052 … none are alike. The K-factor, that neutral-axis location expressed as a decimal between 0.3 and 0.45, captures most of the difference. When the K-factor is low, it shift the neutral axis toward the inside face (typically resulting in less deduction). So that 90-degree bend on 0.063-inch aluminum needs to have a different flat length compared to the same geometry bent from mild steel. Every other calculation then skews based off an incorrect selection for this K-factor.
The inside radius also matter. The inside radius isn’t just a cosmetic choice; it really matters. If your radius on the inside is too small, like when bending spring steel or 6061 aluminum, it will crack. A radius that is too large cause your bend allowance to increase, which decreases the size of your outside dimension. Rather than using the theoretical radius your tooling should make, you have the option to input the actual radius your tooling creates with this calculator. That one measurement will often alter the deduction by twenty or thirty thousandths of an inch. Two or three tenths can make a perfect fit become a gap.
Then there is bend angle and spring back. Rarely will the angle of the print be the same as the part coming off the brake. Stainless wants extra overbend, while copper relaxes less. Springback provides a field in the tool where you can choose to calculate the flat length using either the finished angle or the overbent angle. For production most flats is made with the finished angle. Making an adjustment on test coupons is beneficial. It will help eliminate scrap.
The length of flanges is another measurement choice. It’s either inside to tangent or outside to outside. In one case, this flips the equation. Generally prints are done outside because that can be checked easy with calipers. In outside mode, the sum of the legs get reduced by a bend deduction. In inside mode, it gets increased by bend allowance. Either way the calculator will go along whichever way you select. Just realize how the print depicts it.
The numbers is one thing, the press brake is another. The formed radius will be influenced by die opening, no matter what you think. Grain direction can affects your K-factor by a couple of points. Springback varies lot to lot on coated sheet such as galvanized sheet due to coating thickness variances. That’s why even today the most proficient shops cut a test blank, form it, see how much they got and then back-calculate their actual K-factor for that specific job. The beauty of the calculator is it gets us there faster, but it doesn’t take the place of the physical coupon.
Look for minimal flange warning. This means that if your leg is shorter than roughly six times the material thickness in air bending, then the material won’t completely fill out and the bend radius cannot be predicted. Your leg), then the material won’t completely fill out and the bend radius cannot be predicted. That’s why it warns of this and lets you know to increase your leg length or spread your die a bit before you bend the metal.
The purpose of bend deduction is to develop solid habits rather than obsess over decimal precision. Don’t rely on gauge charts; measure what you get. Write down the exact radius created by your die and punch combination. Document the grain direction and lot number of your material. Plug that information into the calculator, read off the flat length, cut a test piece, tweak if necessary. Do it enough and the numbers starts to feel less like guesswork and more like experience.
The next time an expensive piece of material and a tight tolerance print come across your workbench, think about how the metal will react to the interface of the die and punch. If you can get the deduction correct, the rest of the task becomes easier.
