Sheet Metal Bend Calculator
Bend allowance BA = θ (π ÷ 180)(R + K T), outside setback, bend deduction BD = 2 × OSSB − BA and the flat blank length from two flange lengths, for any bend angle, thickness, inside radius and K-factor, in mm or inches.
Result…
How it's calculated
BA = θ × (π ÷ 180) × (R + K × T) OSSB = tan(θ ÷ 2) × (R + T) ISSB = tan(θ ÷ 2) × R BD = 2 × OSSB − BA Flat = L1 + L2 − BD (outside dimensions) Flat = L1 + L2 − 2 × ISSB + BA (inside dimensions)
When sheet metal bends, the inside is squeezed and the outside stretched; one layer, the neutral axis, keeps its length. It sits a fraction K of the thickness in from the inside face, so the length of the bend along it, the bend allowance, is the bend angle in radians times (R + K × T). The outside setback is how far each flange's outside line runs past the start of the bend to the mould line, and the bend deduction is what you subtract from the outside flange lengths to get the flat blank. These are the formulas given in the Wikipedia article on bending, which puts the K-factor typically between 0.3 and 0.5. 1 in = 25.4 mm exactly.
Your numbers, step by step
About the Sheet Metal Bend calculator
This sheet metal bend calculator works out the flat blank length you need to cut so that a bent part comes out at the right size. Enter the material thickness, the inside bend radius, the bend angle and the K-factor, then the two flange lengths, and it gives the bend allowance, the outside and inside setbacks, the bend deduction and the flat length. Flanges can be measured to the outside mould line, as most drawings show them, or along the inside. It works in millimetres or inches and shows every step with your numbers, so a fabricator, designer or student can check it by hand. The K-factor is the one figure you must supply: it sets where the neutral axis lies inside the sheet and changes with the material, thickness and tooling, so measure it from test bends or ask your supplier.
How to use it
- Pick the “Units” and enter the “Material thickness” and “Inside bend radius”.
- Enter the “Bend angle”, the angle the sheet is bent through (90 for a right angle), and the “K-factor”.
- Choose how the “Flange lengths measured”: “Outside (to the mould line)” or “Inside”.
- Enter the “Flange 1 length” and “Flange 2 length” from your drawing.
- Read the flat length to cut, with the bend allowance and bend deduction behind it.
Worked example
A 90° bend in 2 mm sheet with a 2 mm inside radius and a K-factor of 0.33, flanges of 50 mm and 30 mm to the outside. Bend allowance = 90 × π ÷ 180 × (2 + 0.33 × 2) = 4.178 mm. Outside setback = tan 45° × (2 + 2) = 4 mm. Bend deduction = 2 × 4 − 4.178 = 3.822 mm. Flat length = 50 + 30 − 3.822 = 76.18 mm (2.999 in).
Updated
Questions people ask
How do I calculate bend allowance?
Bend allowance = bend angle × π ÷ 180 × (inside radius + K-factor × thickness). It is the length of the neutral axis through the bend, as given in the Wikipedia article on bending.
What is the difference between bend allowance and bend deduction?
Bend allowance is the length of material in the bend itself, added to the straight parts. Bend deduction is what you take off the sum of the outside flange lengths: twice the outside setback minus the bend allowance.
What K-factor should I use for sheet metal bending?
The Wikipedia article on bending puts it typically between 0.3 and 0.5. The exact value depends on the material, thickness, radius and tooling, so the reliable way is to bend a test strip, measure it and work the K-factor back.
Is the bend angle the angle between the flanges?
No, and mixing them up is a common mistake. Enter the angle the sheet turns through: a part with flanges meeting at 60° has been bent through 120°.
Why is my bent part a little longer or shorter than planned?
Springback, a slightly different radius from the tooling, or a K-factor that does not match the material all shift the result. Adjust the K-factor from a measured test bend. Work out the part's mass with the metal weight calculator.
What does this bend calculator leave out?
It handles one bend between two flanges and does not model springback, bend relief, hems (180° folds) or the minimum radius a material can take. Your press brake operator or material supplier can confirm those.