Sheet Metal Calculation Formulas: Layout, Weight, Area and Tooling Force
Most sheet metal shops use the same handful of formulas every day: flat pattern length from bend allowance or deduction, blank area and weight for quoting, and punching force for press selection. This calculator puts them together for a single bend, with a chart showing how weight moves with thickness.
Your numbers
Outside dimension of the first flange, measured to the virtual sharp.
Outside dimension of the second flange.
Angle the sheet turns through. A right angle flange is 90.
Your results
Planning formulas for a single bend. K-factor and bend deduction depend on material, tooling and bend method; verify with a test bend or your shop's bend table before cutting production parts.
Blank weight as thickness changes
Sweeps Material thickness from 0.5 to 6, holding your other inputs fixed.
Connect formulas to your quoting data
Optional: we review how weights, flat lengths and costs could be pulled from your ERP into quotes.
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How to use the calculator
Enter the outside leg dimensions to the virtual sharp, the part width and the thickness. Choose the material, the bend angle (the angle the sheet turns through), the inside radius and a K-factor. The flat blank length, area and weight follow. For punching, enter the hole diameter and the material shear strength to get force per hole; multiply by the number of holes struck together. Export the inputs and results to PDF for the job file.
- Measure legs to the outside virtual sharp
- Use 90 for a right angle flange
- Multiply punching force by holes struck at once
The formulas
Bend allowance is the bend angle in radians multiplied by the sum of the inside radius and the K-factor times thickness. Outside setback is tangent of half the bend angle times radius plus thickness. Bend deduction is twice the outside setback minus the bend allowance. Flat length is the sum of the outside legs minus the bend deduction. Weight is flat area times thickness times density. Punching force is the hole perimeter times thickness times shear strength.
- BA = angle (rad) x (R + K x T)
- OSSB = tan(angle / 2) x (R + T)
- BD = 2 x OSSB - BA
- Flat = A + B - BD
Worked example
A 2 mm mild steel bracket with legs of 60 and 40 mm, one 90 degree bend, 2 mm inside radius and K of 0.4 has a bend allowance of about 4.4 mm and an outside setback of 4 mm, so the bend deduction is about 3.6 mm. The flat length is 96.4 mm. At 100 mm wide the blank is about 0.0096 square meters and weighs roughly 0.15 kg. A 10 mm hole in the same steel needs about 22 kN, or about 2.2 metric tons.
Common mistakes
The usual errors are mixing inside and outside dimensions, entering the included angle instead of the deflection angle, using one K-factor for every material and tool, and ignoring grain direction near tight radii. Weight from the calculator excludes holes and scrap, so use your nesting yield for material cost. Always confirm layout with a test bend, because press brake tooling and springback shift real results.
- Do not mix inside and outside legs
- Deflection angle is 180 minus the included angle
- Verify with a sample bend before production
Frequently Asked Questions
What is the sheet metal bend allowance formula?
Bend allowance equals the bend angle in radians multiplied by the sum of the inside radius and the K-factor times the material thickness. It gives the arc length of the neutral axis through the bend and is added to the flat legs.
How do I calculate sheet metal weight?
Multiply the blank area by the thickness to get volume, then by the material density. For mild steel use about 7850 kilograms per cubic meter. Subtract holes and cutouts if you want net weight, and use nesting yield for purchased weight.
What K-factor should I use?
0.33 to 0.5 is the usual range, with 0.4 a common default. The right value depends on the material, thickness, radius and bend method, so refine it from test bends and keep a bend table by material.
How is punching force calculated?
Multiply the cut perimeter by the material thickness and the shear strength. For a round hole the perimeter is pi times the diameter. Add a margin when choosing a press, and consider shear angle or staggered punches to reduce force.
If flat lengths, weights and material costs are rebuilt in spreadsheets for every quote, we can help you connect them to item and routing data in your ERP with a private assistant on your own servers.
Related Tools
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Keep exploring
Free tools
- Sheet Metal Flat Blank Length CalculatorTurn outside leg dimensions and bend data into the flat pattern length, using outside setback, bend allowance and bend deduction.
- Sheet Metal Bend Allowance CalculatorEnter thickness, inside radius, K-factor and bend angle to get bend allowance, bend deduction and the flat blank length for a single bend.
- Punching Force Calculator for Sheet MetalEstimate the tonnage to punch round holes in sheet metal, including stripping force, die clearance and a machine capacity check.
Guides
- ERP for Metal Fabricators: What the System Must HandleERP for metal fabricators and sheet metal shops: nested material, multi-level BOMs, heat number traceability, outside processing, and shop floor data.
- Sheet Metal Quoting Software for FabricatorsHow to choose sheet metal quoting software: DXF import, nest-aware material cost, bend and weld time, outside finishing, and a clean handoff to your ERP.
- AI for Sheet Metal Fabrication Shops: What Works TodayWhere AI helps a fabrication shop today: reading RFQs and prints, checking quotes against past jobs, answering ERP questions, and flagging late jobs.
AI on your ERP
- Epicor Kinetic AI for Metal Fabrication: Jobs, Heat Numbers and Outside Processing in Plain EnglishPrivate AI on Epicor Kinetic for metal fabricators: ask about jobs, nests, heat numbers and outside processing in plain English, grounded in live data.
- AI for Dynamics 365 Business Central in Metal FabricationPrivate AI for Business Central metal fabricators: quote-to-job costing, heat-lot traceability, outside processing and remnant stock grounded in your data.