Bend Allowance Calculator

What to calculate: The solve mode. It tells the calculator whether to find Bend allowance or K-factor.

Bend angle (degrees): The bend angle used in the formulas for one bend. The tool accepts values greater than 0 and less than 180.

Inside radius (selected unit): The radius on the inside face of the finished bend.

Material thickness (selected unit): The sheet thickness before bending.

K-factor: A unitless ratio that locates the neutral axis through the thickness. It is used in the bend allowance formula [2].

Known bend allowance (selected unit): A bend allowance you already know from a chart, test part, or shop rule, used to solve for K-factor.

Outside setback: The distance from the mold line to the bend tangent point on an outside leg.

Bend deduction: The amount subtracted from the sum of outside leg lengths to get flat length for one bend [1].

Flat length from outside legs: The unfolded length found from Leg 1 outside length (selected unit) plus Leg 2 outside length (selected unit) minus Bend deduction.

Neutral axis radius: The radius to the neutral axis, equal to inside radius plus K-factor times thickness.

Mistake: Entering the outside bend angle or a value of 0 or 180 in Bend angle (degrees) .
Fix: Enter the actual bend angle used by the formula, and keep Bend angle (degrees) between 0 and 180.

Mistake: Mixing inches and millimeters after setting Unit system .
Fix: Make sure Inside radius (selected unit) , Material thickness (selected unit) , Known bend allowance (selected unit) , Leg 1 outside length (selected unit) , and Leg 2 outside length (selected unit) all match the chosen Unit system .

Mistake: Typing a value into K-factor while using What to calculate = K-factor and expecting that number to control the result.
Fix: In K-factor mode, enter Known bend allowance (selected unit) instead. The calculator solves for K-factor from that measured bend allowance.

Mistake: Using an outside radius in Inside radius (selected unit) .
Fix: Enter the formed part's inside bend radius only. Using an outside value will change Bend allowance , Outside setback , and Bend deduction .

Mistake: Expecting Flat length from outside legs to appear when only one leg is entered.
Fix: Enter both Leg 1 outside length (selected unit) and Leg 2 outside length (selected unit) , or treat Flat length from outside legs as N/A.

Mistake: Assuming every shop should use the same K-factor .
Fix: If you have a tested or measured bend value, use What to calculate = K-factor to back-calculate a shop-specific K-factor and compare it with your current assumption.

• This calculator is for one bend only. It does not total multiple bends or build a full part flat pattern.
• Results depend heavily on the K-factor you use. Air bending, tooling, material, grain direction, and shop setup can shift real-world values.
• Flat length from outside legs is only calculated when both Leg 1 outside length (selected unit) and Leg 2 outside length (selected unit) are provided.
• The tool uses the standard single-bend formulas for bend allowance, outside setback, bend deduction, and flat length from outside dimensions [1][2]. If your shop uses a different convention, results may differ.
• Very small Bend angle (degrees) values can produce very small outputs that are sensitive to rounding and display format.
• Angles near 180 degrees make the tangent term in Outside setback grow rapidly, so the calculator blocks invalid or unstable edge values.
• K-factor is usually expected in a common engineering range such as 0 to 0.5, but unusual workflows may produce other values. A result outside your normal shop range should be checked against test bends.

Core formulas

The calculator uses the standard one-bend sheet metal relationships for bend allowance, K-factor, outside setback, bend deduction, flat length from outside dimensions, and neutral axis radius [1][1].

BA = (π/180) x A x (R + K x T)

K = (BA / ((π/180) x A) - R) / T

OSSB = tan((A/2) x (π/180)) x (R + T)

BD = 2 x OSSB - BA

FL = L1 + L2 - BD

Rna = R + K x T

What each symbol means

A is bend angle in degrees, R is inside radius, T is material thickness, K is K-factor, BA is bend allowance, OSSB is outside setback, BD is bend deduction, FL is flat length, and Rna is neutral axis radius.

Calculation flow

In Bend allowance mode, the tool first calculates Bend allowance from angle, inside radius, thickness, and K-factor. It then calculates Outside setback, Bend deduction, and Neutral axis radius. If both outside leg lengths are entered, it also calculates Flat length from outside legs.

In K-factor mode, the tool uses the known bend allowance to solve for K-factor first, then uses that solved value for the supporting outputs.

Mini example

Suppose Bend angle (degrees) = 90, Inside radius (selected unit) = 1.0 mm, Material thickness (selected unit) = 1.0 mm, and K-factor = 0.33.

Rna = 1.0 + 0.33 x 1.0 = 1.33 mm

BA = (π/180) x 90 x 1.33 = 2.089159... mm

OSSB = tan(45 x (π/180)) x (1.0 + 1.0) = 2.0 mm

BD = 2 x 2.0 - 2.089159... = 1.910841... mm

If the outside legs are 25 mm and 25 mm, then the flat length is found from outside dimensions by subtracting bend deduction from the leg sum.

FL = 25 + 25 - 1.910841... = 48.089159... mm

Display and validation notes

The selected unit system is used consistently for all length-based inputs and outputs. Auto display uses normal decimals for typical values and scientific notation only for very small or very large results. The calculator rejects blocked cases such as angle less than or equal to 0, angle greater than or equal to 180, negative inside radius, zero or negative thickness, and missing mode-required inputs.

Assumptions behind the math

These formulas assume standard single-bend geometry and a chosen K-factor. Real press brake results can differ because actual neutral axis position changes with material, tooling, bend method, and springback, so calibrated shop values often give the best match.