What is bend allowance?
Bend allowance is the arc length of the neutral axis through a bend in sheet metal. When metal is bent, the inside compresses and the outside stretches. Somewhere between those two surfaces is a layer that does not change length much, called the neutral axis. Bend allowance tells you how much of your flat blank is consumed by that curved region.
If you make cabinets, brackets, enclosures, HVAC fittings, or precision fabricated parts, bend allowance is essential for accurate flat pattern development. Even a small error can stack up quickly and push final dimensions out of tolerance.
Bend allowance formula
The calculator above uses the standard equation:
BA = (π / 180) × A × (R + K × T)
- BA = Bend allowance
- A = Bend angle in degrees
- R = Inside bend radius
- T = Material thickness
- K = K-factor (location of neutral axis as a fraction of thickness)
This value stays in the same unit system as your inputs. If you enter millimeters, the result is millimeters. If you enter inches, the result is inches.
Related values you also get
The tool also computes helpful fabrication values:
- Outside Setback (OSSB): (R + T) × tan(A / 2)
- Bend Deduction (BD): 2 × OSSB − BA
- Flat Length (if two flanges are entered): L1 + L2 − BD
How to use this bend allowance calculator
1) Enter the bend geometry
Fill in bend angle, thickness, and inside radius. Use the same unit system for all dimensions.
2) Set the K-factor
Use your shop’s known K-factor if available. If you are starting out, 0.33 is a common estimate for air bending mild steel, but your actual value can vary.
3) Optional: enter two flange lengths
Enter flange lengths if you want immediate flat length output for a single bend part.
4) Click Calculate
Review BA, OSSB, BD, and flat length results. For production work, validate values against test coupons and your press brake setup.
Choosing the right K-factor
K-factor is often the biggest source of error in bend calculations. It is not a universal constant. It changes with:
- Material type (mild steel, stainless steel, aluminum, etc.)
- Material temper and hardness
- Tooling style and V-die opening
- Bending method (air bend, bottoming, coining)
- Inside radius relative to thickness
Best practice: build a bend chart from real samples in your shop. Measure formed parts, back-calculate effective K-factor, then standardize by material and thickness.
Quick example
Suppose you need a 90° bend in 1.5 mm steel with an inside radius of 2.0 mm and K-factor 0.33.
- BA = (π/180) × 90 × (2.0 + 0.33×1.5)
- BA ≈ 3.92 mm
If flange lengths are 50 mm and 40 mm, you can use BD to estimate flat length. This avoids trial-and-error cutting and improves first-part accuracy.
Common mistakes to avoid
- Mixing unit systems (for example, radius in inches and thickness in mm)
- Using an arbitrary K-factor across every material and die setup
- Confusing inside bend radius with punch tip radius assumptions
- Forgetting that springback can affect achieved angle and final dimensions
- Ignoring grain direction for sensitive materials
Final thoughts
A good bend allowance calculator saves scrap, setup time, and rework. Use this tool for quick estimates, then refine with shop-specific bend data to get reliable, repeatable results across your full fabrication workflow.