Enter bend and layout values
The form uses three columns on large screens, two on medium screens, and one on small screens.
Angle sweep for flat length and deduction
The graph shows how bend angle changes bend deduction and flat length while holding the other current values constant.
How the calculator works
BA = (π / 180) × Bend Angle × (Inside Radius + K × Thickness)
SB = tan(Bend Angle / 2) × (Inside Radius + Thickness)
BD = 2 × SB − BA
Flat Length = Flange A + Flange B − BD + Relief Adjustment
This model is useful for sheet metal trim, flashing, duct parts, edge caps, and similar construction fabrication work where outside flange lengths are known before cutting.
Using the calculator step by step
- Select one unit system and keep every dimension in that same unit.
- Choose a material preset, then apply the suggested K-factor if needed.
- Enter bend angle, inside radius, thickness, and the two outside flange lengths.
- Add any relief adjustment, set quantity, and include waste if you need total stock length.
- Press the calculate button to see bend allowance, setback, bend deduction, flat length, and the graph.
- Use the export buttons to save the result summary as CSV or PDF.
Worked example
| Item | Example Value |
|---|---|
| Unit | mm |
| Material | Mild Steel |
| Bend Angle | 90° |
| Inside Radius | 6.000 mm |
| Thickness | 1.200 mm |
| K-Factor | 0.420 |
| Flange A | 50.000 mm |
| Flange B | 40.000 mm |
| Bend Allowance | 10.216 mm |
| Setback | 7.200 mm |
| Bend Deduction | 4.184 mm |
| Flat Length | 85.816 mm |
Common questions
1. What is bend deduction?
Bend deduction is the value subtracted from the sum of outside flange lengths to get a flat blank length. It accounts for the material that forms the bend and keeps layout dimensions closer to the finished part.
2. How is bend deduction different from bend allowance?
Bend allowance is the arc length of the neutral axis through the bend. Bend deduction converts that information into a practical subtraction from outside dimensions. They are related, but they are not the same measure.
3. Why does the k-factor matter?
The k-factor places the neutral axis inside the material thickness. Small changes in k-factor can noticeably change bend allowance and flat length, especially when the bend radius is tight or the metal is thick.
4. Should flange lengths be inside or outside dimensions?
This calculator assumes outside flange lengths. If your shop drawings use inside dimensions or mold-line dimensions, convert them first so the bend deduction is applied to the right measurement basis.
5. Can I use inches instead of millimeters?
Yes. The equations are dimensionally consistent, so inches, centimeters, and millimeters all work. The main rule is to keep every entered dimension in the same unit system from start to finish.
6. Why did I get a very small or negative deduction?
Large inside radii, low bend angles, and higher k-factors can reduce bend deduction. A negative result is possible in some geometry cases, but it usually signals that you should double-check your bend assumptions and measurement method.
7. What does the waste factor do?
Waste factor increases the total required stock length for multiple parts. It helps when you want a safer purchasing estimate for field cuts, trim losses, test pieces, or small shop handling losses.
8. Is this useful for construction fabrication work?
Yes. It fits many construction tasks involving sheet metal bends, including flashing, custom trims, duct transitions, drip edges, and enclosure panels. Final production should still be verified with your shop standards and test bends.