Bend Allowance Calculator
This bend allowance calculator finds the bend allowance, bend deduction, and outside setback for a sheet metal bend from the material thickness, inside bend radius, bend angle, and K-factor. Add the two outside flange lengths and the bend allowance calculator also returns the flat blank length, so you can cut the sheet to size before it goes in the press brake.
Results
The Bend Allowance Calculator is part of our Dimensions collection. Browse more dimensions to size, convert, and check your work alongside this tool.
What the bend allowance calculator shows
When sheet metal bends, the outside stretches, the inside compresses, and one line in between, the neutral axis, keeps its length. Bend allowance is the length of that neutral axis through the bend. The bend allowance calculator works it out from the thickness, inside radius, angle, and K-factor, then derives the bend deduction, the amount you subtract from the outside flange lengths, and the outside setback. With both flanges entered, it gives the flat blank length you cut.
Bend allowance formula
The neutral axis sits a fraction K of the thickness in from the inside surface. Its arc length through the bend is the bend allowance:
A is the bend angle in degrees, R the inside radius, T the thickness, and OSSB the outside setback. The flat length of a single bend part is the sum of the two outside flanges minus one bend deduction.
Bend allowance calculator example
A 90° bend in 2 mm sheet with a 3 mm inside radius and a K-factor of 0.33, flanges of 50 mm and 50 mm to the outside:
Cut the blank at 95.75 mm and the bent part measures 50 by 50 to the outside.
How to use the bend allowance calculator
Enter the material thickness T, the inside bend radius R, the bend angle, and the K-factor. Use any unit you like, millimeters or inches, as long as every length uses the same one. To get the flat pattern length, add both outside flange dimensions, measured to the outside mold line. Leave the flanges at 0 if you only need the bend allowance and bend deduction.
Reading bend allowance and bend deduction
Use bend allowance when your drawing gives flange lengths to the tangent points of the bend: add the straight lengths and the bend allowance. Use bend deduction when the drawing gives outside dimensions, which is how most shop drawings are dimensioned: add the outside flanges and subtract the bend deduction. Both methods give the same flat length. The outside setback is the distance from the tangent point to the outside mold line, the virtual sharp corner.
When to use a sheet metal bend allowance calculator
- Cutting blanks for brackets, boxes, and enclosures
- Checking a CAD flat pattern before a production run
- Building a bend deduction chart for a press brake
- Finding the K-factor that matches a test bend
- Laying out bent parts by hand without CAD
Common bend allowance calculator mistakes
- Using the included angle. Enter how far the sheet bends from flat, not the angle between flanges.
- Mixing inside and outside radius. The formula uses the inside radius.
- Using one K-factor for every job. K changes with material, thickness, and die opening.
- Adding bend deduction. Bend deduction is always subtracted from outside dimensions.
Tips for accurate flat pattern length
- Bend a test strip on each new material and record the measured K-factor.
- Keep the inside radius at least equal to the thickness to avoid cracking.
- For parts with several bends, subtract one bend deduction per bend.
How to calculate bend allowance and the K-factor
The K-factor is the ratio of the neutral axis offset to the thickness. A K of 0.5 puts the neutral axis in the middle of the sheet, which only holds for gentle bends with a large radius. In a tight bend the inside compresses and the neutral axis shifts toward it, so K drops toward 0.33 or lower. That shift is why a fixed rule of thumb rarely matches the shop floor. To find your own K-factor, cut a strip of known length, bend it to 90° on your tooling, and measure both outside flanges. Bend deduction is the sum of the flanges minus the strip length; solve the formulas backward and you get BA, then K = (BA × 180 ÷ (π × A) − R) ÷ T. Record the value for each material, thickness, and die opening and your bend deduction chart will match the brake every time.
Bend deduction for angles other than 90°
For a 90° bend the outside setback equals R + T, because the tangent of 45° is 1. For other angles the tangent changes quickly: at a 135° bend in the example sheet, OSSB grows to 12.071 and the bend deduction to 15.518 while the bend allowance is 8.624. Always enter the bend angle, the amount the sheet turns from flat, not the included angle between the flanges. A part with an included angle of 45° has been bent through 135°. If you also need the weight of the formed part, the flat blank length times width times thickness gives the volume you need.
Bend allowance chart for 90° bends
Bend allowance and bend deduction for 90° bends at K = 0.33 and K = 0.44, in the same unit as the thickness:
| Thickness T | Inside radius R | BA (K 0.33) | BD (K 0.33) | BA (K 0.44) | BD (K 0.44) |
|---|---|---|---|---|---|
| 1 | 1 | 2.089 | 1.911 | 2.262 | 1.738 |
| 1.5 | 1.5 | 3.134 | 2.866 | 3.393 | 2.607 |
| 2 | 2 | 4.178 | 3.822 | 4.524 | 3.476 |
| 2 | 3 | 5.749 | 4.251 | 6.095 | 3.905 |
| 3 | 3 | 6.267 | 5.733 | 6.786 | 5.214 |
| 3 | 4.5 | 8.624 | 6.376 | 9.142 | 5.858 |
Bend Allowance Calculator FAQ
What is the bend allowance formula?
BA equals the bend angle times π divided by 180, times the inside radius plus the K-factor times the thickness. It is the arc length of the neutral axis through the bend.
What is the difference between bend allowance and bend deduction?
Bend allowance is added to the straight flange lengths. Bend deduction is subtracted from the outside flange dimensions. Both give the same flat pattern length.
What K-factor should I use?
A K-factor of 0.33 suits air bending when the inside radius is close to the thickness, and values from 0.40 to 0.50 suit larger radii. Bend a test strip and solve for K to match your own press brake.
Does the calculator work in inches?
Yes. Enter every length in inches and the results come out in inches. A 0.060 in sheet with a 0.060 in radius and K 0.44 gives a bend allowance of 0.136 in.