Bend Allowance Calculator.
Enter the material thickness, inside radius, bend angle and K factor. The calculator gives the bend allowance and the flat length of a sheet metal part with one bend, in inches or millimeters. It runs in your browser and sends nothing anywhere.
Ordering a bent part? You do not need this: send a STEP of the formed part and we make the flat. Also try the bend deduction calculator.
What bend allowance is.
Fold a strip of metal and the outside stretches while the inside compresses. The line in between, the neutral axis, keeps its length. Bend allowance is the length of that line through the bend, and it is the material the bend uses up.
BA = A × (π ÷ 180) × (R + K × T)
A is the bend angle in degrees, R the inside radius, K the K factor and T the thickness. The flat blank is the two straight legs plus the bend allowance: flat = leg 1 + leg 2 + BA, with each leg measured from the edge to where the bend starts.
A worked example
| Input or result | Value |
|---|---|
| Thickness, 16 gauge steel | 0.060″ |
| Inside radius | 0.060″ |
| Bend angle | 90° |
| K factor | 0.44 |
| Bend allowance | 0.1357″ |
| Two straight legs of 0.880″ | flat 1.8957″ |
The same part measured to its outside corners is two 1.000″ flanges; the bend deduction calculator gives the same 1.8957″ flat.
Choosing a K factor.
K factor puts the neutral axis inside the thickness, as a fraction from 0 to 0.5. A value of 0.5 would put it dead center; in real bends it sits lower, toward the inside of the bend.
The material, the thickness and how tight the radius is all move it, so no single K factor is right for every part. 0.44 is a common starting value. For an exact number, bend a test strip, measure it and solve the formula for K. Our K factor guide covers the why.
Getting good numbers
- Use the real thickness. 16 gauge steel is 0.0598″, which shops round to 0.060″. Our sheet metal gauge chart lists each gauge.
- Use the inside radius, not the outside one. The inside radius is the outside radius less the thickness.
- Enter the angle bent through. A 90° bend is 90; flanges opening to 135° between them are a 45° bend.
Send the formed part, we make the flat.
You do not have to solve this by hand to order from us. Send a STEP of the finished, formed shape. We produce the flat from that geometry and form it on our TRUMPF TruBend 7036 and 1150 press brakes, so the flat and the bends agree by construction.
The inside radius in your model is a target: the finished radius comes from the closest available tooling for your material and thickness. If a radius is critical, mark it on the drawing so it gets reviewed before we cut.
- Bends up to 96″ long, in 0.020″ to 0.500″ material
- Flange tolerance ±0.030″, bend angle ±1°
- Instant price for bent parts from a STEP file
Common questions
What is bend allowance?
Bend allowance is the length of the neutral axis through a bend: the material the bend itself uses up. Add it to the two straight legs and you get the length of the flat blank. The formula is BA = A × (π ÷ 180) × (R + K × T).
What is the difference between bend allowance and bend deduction?
Bend allowance is added to the straight legs, measured to where each bend starts. Bend deduction is taken off the outside flange lengths, measured to the outside corner. Both give the same flat length; use the one that matches how your drawing is dimensioned. Try the bend deduction calculator.
What bend angle do I enter?
The angle the metal is bent through, not the angle left between the flanges. A square 90° bend is 90. A part whose flanges open to 135° between them has a 45° bend.
What K factor should I use?
K factor is the position of the neutral axis as a fraction of the thickness, from 0 to 0.5, and real values sit below 0.5. Material, thickness and how tight the radius is all move it. 0.44 is a common starting value; a test bend on your material gives the exact number. Our K factor guide explains why.
Do I need to send ACP a flat pattern?
No. For a bent part, send a STEP of the formed shape. We make the flat from that geometry and form it on our TRUMPF press brakes, so the flat and the bends agree by construction. See sheet metal bending.
Have a bent part to make?
Upload the formed STEP for instant pricing and lead times. We make the flat, cut it and bend it in one shop.