Bend Allowance Calculator

Get the bend allowance for a sheet metal bend from material, thickness, inside radius and bend angle. Bend deduction and outside setback update with it. No signup.

Assumed K (starting value)

Your bend

mm
mm
°
90° is a right angle. Included angle = 180° − bend angle.
Auto K is an assumed starting value: R/T up to 1 gives 0.33, above 1 to 3 gives 0.40, above 3 gives 0.50 (Aluminum 5052 minus 0.02). Verify with a test bend.
mm
mm
Find K from a test bend

Bend one piece with the settings above, enter both outside legs above, then the flat blank length you started with.

mm

Results

Bend deduction3.822 mm
K-factor0.330
Bend allowance4.178 mm

Outside setback 4.000 mm · neutral axis 0.660 mm from the inside face

Flat length = leg A + leg B − bend deduction (3.822 mm). Add both outside legs to get the flat.

TROSSB90°
Neutral axisOutside lines to the virtual sharp (setback)

Formula estimate. Check it with a test bend.

What these numbers mean for your bend

At 2 mm thick with a 2 mm inside radius and a 90° bend (90° included), the neutral axis sits 0.660 mm from the inside face. The bend uses 4.178 mm of material, and each pair of outside dimensions needs 3.822 mm taken off.

Formulas

T is thickness, R is inside radius, A is the bend angle from flat, K is the K-factor.

Worked example

T = 2 mm, R = 2 mm, A = 90°, K = 0.33: bend allowance 4.178 mm, outside setback 4.000 mm, bend deduction 3.822 mm. With outside legs of 50 mm and 30 mm, the flat is 50 + 30 − 3.822 = 76.178 mm.

Questions

How is bend allowance calculated?

BA = (π/180) × A × (R + K × T). It is the arc length of the neutral axis, which sits K × T from the inside face.

What is bend allowance?

Bend allowance is the length of the neutral axis through the bend. Add it to the two straight inside lengths to get the flat blank.

What is bend deduction, and how is it different?

Bend deduction is what you subtract from the two outside leg dimensions to get the flat. Allowance is added to inside lengths; deduction is taken from outside lengths. They describe the same bend.

What is K-factor, and what is a good default?

K-factor is where the neutral axis sits, as a fraction of thickness from the inside face. Typical starting values run from about 0.33 for tight radii to 0.50 for large ones. This tool picks one from the radius-to-thickness ratio; you can type your own.

How do I find my real K-factor?

Bend a test piece, measure both outside legs and the flat blank you started with, and use 'Find K from a test bend' on this page. Use that K for the same material, thickness and tooling.

How do I use bend deduction to get the flat length?

Flat length = leg A + leg B − bend deduction, with both legs measured to the outside of the part.

Is the bend angle the included angle or the angle from flat?

It is the angle from flat: a right-angle bend is 90°. The included angle is 180° minus that.

Why do my real bends differ?

Material lot, grain direction, die opening and tooling all move the neutral axis. That is why a test bend beats any table.

Does this save my data?

Your unit and theme choice are kept in this browser only. Nothing is sent anywhere and there is no signup.

Related

Bend Deduction CalculatorSame calculator, deduction first.K-Factor CalculatorSame calculator, K-factor first.All toolsTools hub

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Results are formula-based estimates. Real bends vary with material lot, grain direction, die opening and tooling. Make a test bend before cutting production parts.