Free Engineering Tool
Bend Allowance Calculator
Calculate bend allowance, bend deduction and flat length for sheet metal bending.
Enter material thickness, bend angle, inside radius, K-factor and straight leg lengths. The calculator estimates bend allowance, flat length and bend deduction — useful for flat pattern preparation and press brake setup.
Sheet Metal Bend Allowance Calculator
Enter your parameters to calculate bend allowance.
Sheet Metal Bend Allowance Calculator
Enter your material thickness, bend angle, inside radius, K-factor and the straight leg lengths on both sides of the bend. The calculator will estimate bend allowance, bend deduction and flat length for the part.
For many air-bending applications, inside radius is related to die opening and material thickness.
K-factor describes the position of the neutral axis as a fraction of thickness. Typical values for air bending carbon steel are often between 0.3 and 0.5. For a consistent bend, the inside radius should roughly match the die opening divided by 8 (e.g., V16 → r ≈ 2t).
Straight length from the inner edge of the sheet to the bend tangent point — equivalent to the 'inside leg' in flat pattern terminology.
Inside leg on the opposite side of the bend, also measured from the inner edge to the bend tangent.
Bend Calculation Result
Bend allowance (mm)
4.18
Flat length (mm)
104.18
Bend deduction (mm)
3.82
Bend deduction = outer legs − flat length. A positive value means the flat blank is shorter than the sum of outer leg dimensions; use this to trim your blank to the correct size.
How the calculation works
Bend allowance (BA) = (π / 180) × Bend Angle × (Inside Radius + K × Thickness). Flat length = Inside Leg A + Inside Leg B + BA. Bend deduction = (Inside Leg A + r + t) + (Inside Leg B + r + t) − Flat length.
For tonnage estimation and material planning, you can also use our Press Brake Tonnage Calculator and Sheet Metal Weight Calculator.
Enter your material thickness, bend angle, inside radius, K-factor and the straight leg lengths on both sides of the bend. The calculator will estimate bend allowance, bend deduction and flat length for the part.
What Is Bend Allowance in Sheet Metal Bending
Bend allowance is the length of the arc along the neutral axis through the bend region of a sheet metal part. When a sheet is bent, the outer surface is stretched and the inner surface is compressed. Somewhere between them is the neutral axis, which experiences almost no change in length.
Because of this stretching and compression, the flat pattern needed before bending is not simply the sum of the finished leg lengths. Bend allowance provides a way to quantify how much length is required in the bend region so that the final part matches the drawing after bending.
Calculating bend allowance helps designers and press brake operators predict flat lengths more accurately, reduce trial-and-error in the workshop and keep finished dimensions within tolerance.
How Bend Allowance Is Calculated
Based on the standard neutral axis arc length formula.
Neutral axis position
The K-factor describes where the neutral axis sits within the sheet thickness, expressed as a fraction of thickness. Typical values for air bending carbon steel are between 0.3 and 0.5.
Arc length formula
Bend allowance (BA) = (π / 180) × Bend Angle × (Inside Radius + K × Thickness). This approximates the arc length along the neutral axis through the bend.
Flat length derivation
Flat length = Leg A + Leg B + Bend Allowance. Bend deduction is then derived as (Leg A + Leg B) − Flat Length.
Factors That Affect Bend Allowance
Even with a standard formula, real-world conditions influence the result.
- 1
Material type
Different materials have different yield strengths and springback characteristics, which affect where the neutral axis sits in the thickness.
- 2
Thickness
Thicker sheets typically behave differently from thin sheets during bending. K-factor and bend allowance values may change as thickness increases.
- 3
Die opening
In air bending, die opening (V-opening) has a strong influence on inside radius and tonnage. A wider die opening usually produces a larger inside radius and different bend allowance.
- 4
Press brake setup
Tooling type, tool condition, crowning, backgauge support and operator technique all contribute to how a theoretical bend allowance matches real parts on a specific press brake.
Why Accurate Bend Allowance Matters
Benefits across the sheet metal fabrication workflow.
Improves bending accuracy
Correct bend allowance helps finished dimensions fall within specified tolerances, especially for angles and closed frames where errors accumulate.
Reduces material waste
Fewer trial parts and less rework mean lower material consumption and scrap cost.
Ensures correct flat pattern
Reliable bend allowance data allows designers and programmers to confidently generate flat patterns for laser cutting and punching.
From Flat Pattern to Machine Selection
Combine bend allowance data with tonnage and machine selection resources.
Related Guides
Related Machines
Bend Allowance Calculator FAQ
What is bend allowance in sheet metal?
Bend allowance is the length of the neutral axis through the bend region of a sheet metal part. It represents how much material is needed to form the bend so that the finished part matches the target dimensions after bending.
How is bend allowance calculated?
A common formula is BA = (π / 180) × Bend Angle × (Inside Radius + K × Thickness). This formula uses the bend angle in degrees, inside radius, material thickness and K-factor to approximate the length of the neutral axis.
What is a typical K-factor for sheet metal?
Typical K-factors for air bending mild steel often fall between 0.3 and 0.5, depending on thickness, tooling and inside radius. Many companies determine their own K-factors by making test bends, measuring flat lengths and back-calculating values.
Does bend radius affect flat length?
Yes. A larger inside bend radius generally increases bend allowance because the neutral axis travels along a longer arc. When designing parts and choosing tooling, it is good practice to keep a consistent relationship between material thickness, inside radius and die opening.
Why is bend allowance important for press brake bending?
Without correct bend allowance data, flat patterns may be off by several millimetres, especially when multiple bends accumulate errors. Accurate bend allowance helps ensure that parts produced on the press brake match the design.
Need a Press Brake for Accurate Sheet Metal Bending?
Share your material, thickness range, bending length and production requirements. We can help you match suitable press brake machines and tools so that bend allowance data, tonnage and flat patterns work together in a consistent process.