A sheet metal bend allowance calculator
This bend allowance calculator gives the length of material used up in a sheet metal bend, and everything else you need to cut an accurate flat blank: the bend deduction, the outside and inside setbacks, the neutral-axis radius and the total flat length for an L-bracket, a U-channel or any part with several equal bends. It works in millimetres or inches and draws the part to scale.
You can calculate with a K-factor — the position of the neutral axis as a fraction of the thickness — or, if you do not have one, with the empirical formula from the US Federal Aviation Administration’s maintenance handbook, the usual answer to “bend allowance calculator without K factor”. A second mode works backwards from a test bend to find the K-factor for your material and press brake tooling.
How to use the Sheet Metal Bend Allowance Calculator
- Choose mm or inch and enter the thickness T, the inside radius R and the angle. Say whether the angle is the bend angle from flat or the included angle between flanges.
- Pick K-factor and type your value, or pick the FAA formula to work without one.
- Enter the flange lengths, separated by commas, and whether they are measured to the outside (mold lines) or to the inside surfaces.
- Read the bend allowance, bend deduction, setback and flat length, and check the scale drawing.
- To find K, switch to K-factor from a test bend, bend a strip of known length, and enter its two outside flange measurements.
Sheet Metal Bend Allowance Calculator features
BA, BD and setback
Bend allowance, bend deduction, outside and inside setback and neutral radius in one table.
With or without K
K-factor method, or the FAA empirical formula with the K-factor it implies.
Find your K-factor
Works back from a measured test bend.
Multi-bend parts
Any number of flanges with the same bend, measured outside or inside.
Scale drawing
The bent profile is drawn from your numbers, so impossible flanges stand out.
Bend allowance and bend deduction formulas
With the bend angle θ in radians, the neutral axis sits at R + K·T, so the bend allowance is BA = θ × (R + K × T) (in degrees, BA = angle × π/180 × (R + K·T)). The outside setback is OSSB = tan(θ/2) × (R + T), the distance from the tangent line to the mold point. The bend deduction is BD = 2 × OSSB − BA, and the flat length is the sum of the outside flange lengths minus one BD for every bend.
Example in millimetres: T = 2, R = 2, a 90° bend and K = 0.40. BA = 1.5708 × (2 + 0.8) = 4.398 mm; OSSB = 1 × (2 + 2) = 4 mm; BD = 8 − 4.398 = 3.602 mm. An L-bracket with outside flanges of 50 and 30 mm needs a blank of 80 − 3.602 = 76.398 mm.
The FAA Aviation Maintenance Technician Handbook – Airframe (FAA-H-8083-31A, chapter 4, read 24 October 2026) gives BA = (0.01743 R + 0.0078 T) × N, with N the bend in degrees; its example, R = 0.16 in, T = 0.040 in and 90°, gives 0.279 in. The ratio 0.0078 ÷ 0.01743 means this formula uses K ≈ 0.45. Note that the handbook’s “K chart” is a setback factor, tan(θ/2), not the neutral-axis K-factor.
Choosing a K-factor
K is never more than 0.5 for a plain bend: the neutral axis moves towards the inside as the radius gets tighter. Common rules of thumb, which the tool shows as a hint, are about 0.33 when R is less than T, about 0.40 for R between one and three times T, and 0.50 for large radii. These are typical starting values only — the real K depends on the alloy, temper, grain direction, die opening and bending method (air bending, bottoming or coining).
- For production parts, bend a test strip with the actual tooling and use the K-factor mode.
- Measure flanges to the outside mold lines — the virtual sharp corner — when you use bend deduction.
- For hems and very tight bends near zero radius, the simple formulas are less reliable; follow your press brake’s bend tables.
Related calculators
Estimate blank weight with the sheet metal weight calculator, set spindle speeds with the cutting speed calculator, and convert units with the length converter and angle converter.
Sheet Metal Bend Allowance Calculator FAQ
How do I calculate bend allowance for sheet metal?
Use BA = angle × π/180 × (R + K × T), where R is the inside radius, T the thickness and K the K-factor. For a 90° bend in 2 mm sheet with a 2 mm radius and K = 0.40, BA = 4.398 mm.
How do I calculate bend allowance without a K-factor?
Use the empirical formula from the FAA handbook: BA = (0.01743 × R + 0.0078 × T) × bend angle in degrees. It works in inches or millimetres and corresponds to a K-factor of about 0.45.
What is the difference between bend allowance and bend deduction?
Bend allowance is the arc length of the neutral axis through the bend; you add it to the flat portions. Bend deduction is what you subtract from the sum of the outside flange lengths. BD = 2 × outside setback − BA.
What is a typical K-factor?
Between about 0.3 and 0.5 for most metals: tighter radii give lower values. The bend allowance calculator suggests a rule-of-thumb value from your R/T ratio, but a test bend gives the right number for your shop.
Does the bend allowance calculator work in mm?
Yes. Choose mm or inch; switching converts the values you typed. The formulas are the same in any unit.
Should I enter the bend angle or the included angle?
Either — choose which in “Angle is the”. A flange bent up 60° from flat has an included angle of 120°; the formulas use the 60° bend angle.