Bend Allowance Calculator — BA, BD & K-Factor Explained
This bend allowance calculator helps you find the exact flat-blank length needed before a sheet metal bend. Enter material thickness, inside radius, bend angle, and K-factor to instantly get the bend allowance (BA), bend deduction (BD), and outside setback. It works as a bend deduction calculator too — both values are calculated simultaneously.
What Is Bend Allowance?
When sheet metal is bent, the outer surface stretches while the inner surface compresses. Somewhere in between lies the neutral axis — the layer that neither stretches nor compresses. The bend allowance is the arc length of the neutral axis through the bend zone:
BA = (π ÷ 180) × θ × (Ri + K × t)
Where:
- θ — bend angle in degrees
- Ri — inside bend radius
- K — K-factor (position of the neutral axis)
- t — material thickness
The bend allowance represents the actual material consumed by the bend. You add it to the flat leg lengths to get the total blank (flat pattern) length.
K-Factor
The K-factor (0 to 1) defines where the neutral axis sits within the material cross-section. A K-factor of 0 places the neutral axis at the inner surface; a K-factor of 0.5 places it at the geometric center. Typical values:
- 0.3 — very sharp bends, soft materials (aluminium)
- 0.33 — soft materials, radius < material thickness
- 0.40 — general-purpose steel (most common default)
- 0.45–0.50 — large radii, hard materials
Your material supplier's bend allowance chart will list the correct K-factor for each metal type and thickness.
Bend Deduction (BD)
The bend deduction is an alternative way to express the same information using the outside mold line (OML) method. Instead of adding BA to flat legs, you subtract BD from the extended flat legs:
OSSB = tan(θ ÷ 2) × (Ri + t)
BD = 2 × OSSB − BA
Where OSSB is the Outside SetBack — the distance from the mold line to the tangent point of the bend. Both BA and BD describe the same geometry; which you use depends on your CAD system or shop practice.
Sheet Metal Bending — Step-by-Step Example
Suppose you need to bend a 2 mm thick steel plate to 90° with an inside radius of 3 mm (K = 0.40):
- Bend Allowance: BA = (π/180) × 90 × (3 + 0.40 × 2) = 1.5708 × 3.8 ≈ 5.969 mm
- Outside Setback: OSSB = tan(45°) × (3 + 2) = 1 × 5 = 5 mm
- Bend Deduction: BD = 2 × 5 − 5.969 ≈ 4.031 mm
If your two flat legs are 50 mm and 30 mm (measured to the outside mold lines), the flat-blank length is: 50 + 30 − BD = 80 − 4.031 = 75.969 mm.
Units — Metric & Imperial
The calculator supports both metric (mm) and imperial (inches) units. The bend angle is always entered in degrees regardless of the unit system. Switch between systems with the toggle button in the form — the formulas remain identical, only the unit labels change.
Frequently Asked Questions
What is the difference between bend allowance and bend deduction?
Bend allowance (BA) is added to the flat leg lengths measured to the tangent points of the bend. Bend deduction (BD) is subtracted from the extended flat leg lengths measured to the outside mold line corners. Both give the same total blank length — they are just different measurement conventions.
How do I choose the right K-factor?
Use the K-factor from your material supplier's data sheet or bend allowance chart. In the absence of specific data, use 0.40 for mild steel (the most common value for general fabrication) and 0.33 for aluminium with tight radii.
Does bend angle affect the K-factor?
No. The K-factor is a material and process property — it depends on the material type, grain direction, tooling, and lubrication, not on the bend angle. However, the neutral axis position does shift slightly at very acute angles, so K-factor tables often list values per radius-to-thickness ratio.
Can I use this calculator for CNC press brake programming?
Yes. The BA and BD values this calculator produces are the same values used in flat-pattern development in CAD software and CNC press brake controllers. Verify that your controller uses the same K-factor convention (some use the Y-factor instead).