K-Factor Calculator
Solve sheet metal K-factor from a measured bend allowance, or calculate bend allowance and deduction from a known K-factor.
CALCULATION MODE
Use a measured bend allowance from a test part or shop data.
MATERIAL & BEND GEOMETRY
Enter the angle through which the sheet turns, from 0° to 180°.
MEASURED BEND ALLOWANCE
Measure along a test bend using a consistent bend-angle convention.
Your bend calculation
Neutral axis, allowance, and deduction
SOLVED K-FACTOR
0.132 in
Bend allowance
Arc length on neutral axis
0.108 in
Bend deduction
For outside flange dimensions
0.4006
K-factor
Neutral-axis ratio
0.024 in
Neutral-axis offset
Measured from inside face
Validate before production. Material, tooling, bend method, and springback affect the result. Confirm the value with a test bend or a shop-specific bend table.
ALLOWANCE AT OTHER K-FACTORS
K = 0.3
0.1225 in
K = 0.4
0.1319 in
K = 0.5
0.1414 in
Comparison only; use a verified K-factor for your process.
Step-by-Step Solution
Here's exactly how this answer was calculated, one step at a time.
- 1
Convert the bend angle to radians
θ = 90° × π ÷ 180 = 1.5708 rad
The bend allowance equation uses radians for its arc calculation.
- 2
Rearrange the bend allowance equation
K = (BA ÷ θ − R) ÷ T = (0.132 ÷ θ − 0.06) ÷ 0.06 = 0.4006
A measured allowance, inside radius, thickness, and bend angle determine the inferred K-factor.
- 3
Find the neutral-axis offset
t = K × T = 0.4006 × 0.06 = 0.024 in
This is the estimated distance from the inside face to the neutral axis.
K-Factor Calculator: Work Out Sheet Metal Bend Allowance
A K-factor helps describe where the neutral axis sits inside a sheet metal bend. That position affects how much material is used in the curved section and therefore how long a flat blank needs to be before bending. This K-factor calculator works in both directions: enter material thickness, inside bend radius, angle, and a measured bend allowance to solve for K; or enter a known K-factor to calculate bend allowance, bend deduction, and neutral-axis distance.
The tool accepts inches or millimeters, supports bend angles in degrees, and lets you calculate one bend or a batch of identical bends. It shows the formula step by step and gives a small comparison of bend allowance at several K-factor values. Use the solve mode to turn a test coupon or shop measurement into a K-factor you can enter into a CAD or sheet-metal software rule. Use the known-K mode to estimate flat-pattern numbers for a part when your material and tooling data already provide the K-factor.
K-factor is not a universal constant for a metal type. It can change with material, thickness, bend radius, grain direction, tooling, air bending or bottoming method, and the way the bend allowance is measured. Treat the result as a starting calculation. For production work, validate it with a test bend and measured dimensions, then use the validated shop value or manufacturer data in your flat-pattern rule.
What Is K-Factor in Sheet Metal?
When sheet metal bends, the outside surface stretches and the inside surface compresses. Somewhere between those faces is a layer whose length changes very little during the bend. This is called the neutral axis. K-factor expresses how far that axis is from the inside face as a fraction of the material thickness. If the thickness is T and the neutral axis lies a distance t from the inside, then K = t ÷ T.
A K-factor of 0.40 means the neutral axis is estimated at 40% of the thickness measured from the inside surface. For a 2 mm sheet, that corresponds to a neutral-axis offset of 0.8 mm. The ratio itself has no unit, so the same K-factor is used whether dimensions are entered in inches or millimeters. The calculated neutral-axis distance does have a unit because it is a length.
The K-factor is one way to model sheet metal bend allowance. CAD programs may also use bend tables, bend deduction tables, or measured bend calculations. Each method is a way to predict the developed flat length from the formed geometry. Use the method and settings that match your design system, tooling, and shop process rather than mixing values from different conventions.
Bend Allowance Formula
For an angle entered in degrees, bend allowance is calculated as BA = π × (R + K × T) × A ÷ 180. In this equation BA is the arc length along the neutral axis, R is the inside bend radius, K is the K-factor, T is the sheet thickness, and A is the bend angle in degrees. The angle is converted to radians within the formula, so all length inputs must use the same unit.
To solve for K from a measured bend allowance, rearrange the equation: K = ((BA ÷ θ) − R) ÷ T, where θ is the bend angle in radians. The calculator makes that conversion for you. Enter a positive thickness, an inside radius, the bend angle, and the bend allowance measured from your test part or trusted shop data. It then reports the inferred K-factor and the neutral-axis offset.
For example, with a 1 mm sheet, a 1 mm inside radius, a 90-degree bend, and a measured allowance of about 3.14 mm, the equation gives a K-factor near 0.50. Small measurement errors can change the answer, especially with a short bend allowance or a small angle. Repeat measurements and use a consistent bend-angle convention when setting a production value.
How to Use the K-Factor Calculator
Choose Solve K-Factor when you know a measured bend allowance. Enter the thickness and inside radius from your actual setup. Enter the angle the sheet turns through, such as 90 degrees for a right-angle bend, and enter bend allowance in the same unit as the other dimensions. Select the number of identical bends if you want a total allowance for a repeated part or batch. The individual bend results remain visible so you can compare them with the total.
Choose Calculate Bend Allowance when you already have a K-factor from a qualified bend table, a validated shop test, or your CAD setup. Enter the K-factor as a decimal between 0 and 1. The calculator estimates bend allowance and bend deduction for the geometry you specify. The K-factor value should represent your material, thickness, radius, and forming process as closely as possible.
Switch between Imperial and Metric whenever needed. The length inputs convert together, while the K-factor remains the same because it is a ratio. The result cards show neutral-axis position, allowance, deduction, and batch totals. Copy or download a CSV for a setup note, save the current figures in your browser, or print the calculation for a job traveler. Keep material and tooling details with the saved result so it can be repeated accurately.
Bend Deduction and Flat Length
Bend allowance describes the length of the curved section measured along the neutral axis. Bend deduction is the amount subtracted from the sum of outside flange dimensions to estimate the flat blank length. For a simple single bend, outside setback on one side is (R + T) × tan(A ÷ 2). Bend deduction is twice that setback minus bend allowance: BD = 2 × setback − BA.
If the two outside flange dimensions are measured to the virtual sharp, the flat length for a single bend is approximately flange one + flange two − bend deduction. For a part with several bends, each bend and its reference geometry must be handled consistently. This calculator reports bend deduction per bend and the total for repeated identical bends, but it does not calculate a complete multi-bend blank from a finished part drawing.
Different CAD packages and shop drawings can define bend angle, flange dimensions, and bend deduction in different ways. Confirm that the included angle or bend angle matches your software's convention. Do not combine inside dimensions with an outside-dimension bend deduction. A clear sketch with measurement points prevents sign mistakes and reduces scrap during setup.
Why K-Factor Changes Between Bending Jobs
Material affects how a bend behaves. Mild steel, stainless steel, and aluminum do not all stretch and compress the same way, and different grades can behave differently within the same family. Thickness, temper, work hardening, grain direction, and surface condition can also influence the formed radius and measured allowance. A value from one material batch is not automatically a reliable value for another.
Tooling and forming method matter as well. The punch radius, die opening, press-brake setup, air-bending method, coining, and bottoming can change the resulting inside radius and material flow. If the actual inside radius differs from the value used in the calculation, the bend allowance changes even if the K-factor is held constant. Measure the formed part rather than relying only on a nominal tool label.
A shop often develops bend data by making test bends from the same stock and setup used in production. Measure thickness, inside radius, bend angle, and the relevant flat or formed dimensions carefully. Then derive the bend allowance or deduction using the shop's convention. Record the material, thickness, grain direction, tooling, and machine so the value can be repeated later.
How to Measure a Test Bend
Start with a clean test coupon cut from the material you expect to form. Measure its thickness at several points with a suitable gauge, and note the direction of the grain if the sheet has one. Form a bend using the planned punch, die, machine settings, and angle. Allow the part to spring back and settle before taking final measurements, because the formed angle and radius may change after the load is removed.
Measure the inside radius and the bend angle using a consistent method. A radius gauge, inspection tool, or measurement fixture can help. The bend angle is the amount of direction change through the bend, not always the smaller included angle shown between the flanges. Write down the convention used by the drawing or CAD package before entering an angle in the calculator.
Measure the flat length or flange dimensions needed to determine bend allowance. Make sure you know whether the values are inside dimensions, outside dimensions, or dimensions to the virtual sharp. If the bend allowance is not directly available, use the appropriate geometric relation for the drawing convention and verify it against another test piece. Repeat the measurement to catch small reading errors.
Understanding the Results
The solved K-factor is a ratio. A result near the center of the material thickness means the calculated neutral axis is roughly centered, but that does not make the number correct for every job. A result below zero or above one usually signals that an input or measurement convention needs review, or that the simplified model does not fit the forming process. Check angle, thickness, radius, and bend-allowance measurements before using an unusual result.
Bend allowance is shown per bend and as a total for your selected number of identical bends. The total is useful for a batch estimate, but the same bend count does not mean a finished part's whole flat length has been calculated. Bend deduction uses the outside setback model and depends on consistent outside dimensions. The neutral-axis offset is K multiplied by sheet thickness, measured from the inside face.
The comparison table shows how allowance changes when K changes while the entered radius, thickness, and angle remain fixed. This is helpful for seeing why a small K-factor adjustment can affect flat pattern length. It is not a material lookup table or a recommendation. Replace comparison values with a validated shop value before releasing production drawings.
Common Sheet Metal Bend Calculation Mistakes
A frequent mistake is mixing units, such as entering thickness in millimeters and bend radius in inches. The calculator has a unit switch, but after changing it, check each displayed measurement before calculating. Another mistake is entering the inside included angle where the formula expects the amount of bend. A 90-degree turn is straightforward, but acute and obtuse bends deserve a quick sketch.
Using a generic K-factor copied from an unrelated material or software example can produce a flat pattern that is consistently too long or too short. The K-factor is affected by process and setup. Use a bend table or test result that matches your material and tooling. If you change thickness, radius, die opening, or forming method, check whether your existing shop value still applies.
Finally, do not assume bend deduction can be applied to any two dimensions without checking the drawing reference. The outside setback formula uses outside geometry. Inside flange measurements, virtual sharp dimensions, and tangent dimensions are not interchangeable. Label the dimensions clearly, perform a trial bend, and inspect the first article before running a full batch.
Frequently Asked Questions
What is the K-factor formula for sheet metal?
K is the distance from the inside face to the neutral axis divided by sheet thickness. Bend allowance is BA = θ × (R + K × T), where θ is in radians, R is inside radius, and T is thickness.
How do I calculate K-factor from bend allowance?
Rearrange the bend allowance equation: K = (BA ÷ θ − R) ÷ T. Use the bend angle in radians and consistent units for the length measurements.
What is a typical sheet metal K-factor?
There is no single value that fits every material and forming setup. Use a bend table or measured value that matches your material, thickness, tooling, and process.
What is the difference between bend allowance and bend deduction?
Bend allowance is the arc length along the neutral axis. Bend deduction is subtracted from outside flange dimensions to estimate the flat blank length.
Does K-factor change with material thickness?
It can. Material, thickness, inside radius, tooling, and forming method affect the bend and the value that fits a measured part.
Can I use this result in CAD software?
Yes, as an input to a sheet-metal rule after checking that the software and drawing use the same angle and bend conventions. Validate the rule with your material and tooling.
Why is my calculated K-factor negative or above 1?
Check the bend angle, thickness, radius, and measured allowance, and confirm all measurements use the same units and bend convention. The simplified model may not fit some forming processes.
Does this calculator choose press-brake tooling?
No. It calculates bend geometry from the values you enter. It does not select tooling, machine force, die opening, or forming settings.