Bent sheet metal has a flat pattern. SolidWorks can unfold every edge flange, miter flange, and sketched bend into a precise blank geometry, and the SW-Flat Pattern Area cut-list property captures the result automatically.
Formed and stamped parts are a different story. A drawn cup, an embossed panel, a hydroformed bracket, a stamped louver grid — SolidWorks models these as solid bodies or as sheet metal with Form features, but there is no flat pattern feature, no unfolding algorithm, and no built-in blank area property. When a fabricator or a quoting tool asks for the blank size, you have to find it yourself.
This is a documented gap in SolidWorks that trips up engineers doing material estimation, nesting calculations, or BOM costing for formed components.
Why the obvious approaches give you wrong numbers
Two approaches feel intuitive but are both wrong for formed parts.
Mass Properties → Surface Area reports the total surface area of all faces on a solid body. For a simple drawn cup with uniform wall thickness, that includes the inside bottom, the outside bottom, the inside cylindrical wall, the outside cylindrical wall, and the top rim annulus. Sum all of that and you get roughly twice the blank area, not the actual blank.
Cut List Surface Area uses the same calculation — it pulls from the same Mass Properties engine. The value is not documented as “blank area” anywhere in the SolidWorks help, and it is not.
Bounding Box Area (SW-Bounding Box Area cut-list property, introduced in SolidWorks 2018) is the area of the smallest rectangle that contains the flat pattern. This property only populates for sheet metal parts with a flat pattern feature. For a formed solid body, it is blank or returns the bounding box of the formed geometry — neither is useful.
The correct approach: Mid-Surface + Mass Properties
The Mid-Surface feature (Insert → Surface → Mid-Surface) creates a surface body at the geometric midpoint between two opposing faces. For a part with uniform wall thickness, that midpoint is the neutral axis — the plane where material neither stretches nor compresses during forming.
The area of the mid-surface is the best available approximation of the original blank area, for the same reason the K-factor works for bends: the neutral axis is what you’re developing back to.
Step-by-step
Open the formed part. Make sure you’re working with a single body representing one blank (not an assembly component).
Go to Insert → Surface → Mid-Surface (or right-click the Surfaces CommandManager tab).
In the PropertyManager, select Face Pair 1: click the outer face, then the inner face of the same wall section. For a drawn cup, that’s the outer cylindrical wall and the inner cylindrical wall.
Add additional face pairs for each distinct wall section. SolidWorks stitches the individual mid-surface patches into one surface body. For a cup with a flat bottom and cylindrical walls, you need at least two pairs: bottom faces and wall faces.
Click the green checkmark. A surface body appears in the Surface Bodies folder in the FeatureManager.
Go to Evaluate → Mass Properties. In the dialog, expand the scope selector at the bottom and choose the mid-surface body specifically (not the solid body). The dialog shows Surface Area — this is the area of the mid-surface and your blank area estimate.
For a 50 mm diameter × 30 mm tall cup from 1.5 mm mild steel, the analytical blank area is approximately 5890 mm² (π × 25² + π × 50 × 30). The mid-surface method on the formed SolidWorks model typically returns within 0.5% of this, assuming the model wall thickness is uniform.
Limitations
Mid-surface blank area is an estimate under real forming conditions for two reasons:
Thinning. Deep drawing stretches material, particularly at the punch radius and wall. Wall thickness in a drawn steel cup can drop 10–20% from nominal at the punch nose. The thinner the wall, the larger the area it represents per unit of mass — so actual blank consumption is slightly larger than the mid-surface predicts.
Draw-in. Material from the blank flange flows inward during drawing. The blank blank area and the final part area are not the same thing because the forming process itself moves material. Mid-surface gives you the part area, not accounting for the scrap ring or holddown flange that was trimmed after drawing.
For quoting purposes, add 5–15% to the mid-surface area depending on draw severity and material. For die design and nesting, use dedicated forming simulation (AutoForm, DYNAFORM, PAM-STAMP) that tracks material flow.
For comparison with simple bent parts: SolidWorks flat pattern dimensions and K-factor explains how the neutral axis plays the same role in bending, and why K-factor errors in bends cause the same class of blank size discrepancy.
Analytical formulas for simple shapes
For symmetric formed shapes, closed-form surface area formulas are exact (in the absence of thinning). These are useful for cross-checking the mid-surface result.
Cylindrical cup (flat bottom, straight walls):
A = π × (D/2)² + π × D × H
where D = outer diameter of the cup body and H = cup height (outside). For a cup drawn from flat stock, the blank diameter that produces this cup (Erbich formula) is:
D_blank = √(D² + 4 × D × H)
Truncated cone (frustum):
A = π × (r₁ + r₂) × √((r₂ - r₁)² + H²) + π × r₁²
where r₁ = small radius, r₂ = large radius (including flange), H = height.
Hemisphere:
A = 2 × π × R²
For a full hemisphere of radius R, the blank diameter is 2R√2.
These formulas ignore corner radii at the punch and die nose. The actual blank area is slightly larger because the transition geometry adds surface area. For precision nesting, treat these as lower bounds.
Extracting mid-surface area via the SolidWorks API
When you’re processing multiple formed parts in a batch — for instance, pulling blank areas into a BOM export or checking every part in a vault — manual Mid-Surface creation per part is not practical. The API path is cleaner.
The approach: create the Mid-Surface feature programmatically, then read the resulting surface body’s area via IBody2.GetMassProperties.
using SolidWorks.Interop.sldworks;
using SolidWorks.Interop.swconst;
// Assumes swApp and model are already initialised
IPartDoc part = (IPartDoc)model;
// Get all surface bodies (includes Mid-Surface results)
object[] surfBodies = (object[])part.GetBodies2(
(int)swBodyType_e.swSheetBody, false);
if (surfBodies == null || surfBodies.Length == 0)
{
Console.WriteLine("No surface bodies found. Create a Mid-Surface feature first.");
return;
}
foreach (IBody2 body in surfBodies)
{
// GetMassProperties returns:
// [0] = volume (0 for surface bodies)
// [1] = surface area in m²
// [2] = mass (0 or based on material if assigned)
// [3..5] = centre of mass X, Y, Z
// [6..11] = moments of inertia
double[] props = (double[])body.GetMassProperties(0);
double areaMm2 = props[1] * 1e6; // Convert m² to mm²
Console.WriteLine($"Surface body '{body.Name}': {areaMm2:F2} mm²");
}
If the part has multiple surface bodies (perhaps you’ve built other surface features for modelling purposes), filter by name. Mid-Surface features create bodies named Surface-Mid-Surface1, Surface-Mid-Surface2, etc.
To filter programmatically:
foreach (IBody2 body in surfBodies)
{
if (body.Name.StartsWith("Surface-Mid-Surface"))
{
double[] props = (double[])body.GetMassProperties(0);
double areaMm2 = props[1] * 1e6;
// Write to custom property or export to BOM
}
}
If you need the area as a cut-list custom property (so it appears in a BOM or drawing annotation), the API path is IWeldmentCutListMember / ICustomPropertyManager on the cut-list folder — but note that formed solid bodies may not always generate a cut-list entry automatically. You may need to right-click the Solid Bodies folder and select “Create Cut List” first, or use IPartDoc.CreateCutList().
For uniform-thickness solid bodies that aren’t formed (simple laser-cut plates, extrusions), the simpler face-selection or volume/thickness approach avoids needing a surface feature entirely.
Bounding Box as a fallback (and why it overestimates)
If creating a Mid-Surface feature is impractical — maybe the geometry has thin walls or tangent-continuous faces that make face pair selection tedious — the bounding box dimensions give a rough upper bound on blank size.
For a formed cup, measure the outer diameter (bounding box XY) and add the wall areas analytically. The bounding box of the formed solid gives you the footprint of the part in its formed state, not the blank. For deep draws this overestimates blank area by 30–100% depending on draw ratio.
The SW-Bounding Box Area cut-list property (when it does populate for a solid body) gives the area of the bounding box rectangle, which for a circular cup is wider than the cup diameter × height. Do not use this for material quoting.
Worked example: stamped bracket with embossed ribs
Consider a 200 mm × 150 mm steel panel, 2 mm thick, with four embossed stiffening ribs that raise the surface by 8 mm. The panel has six mounting holes punched through it.
What’s the blank area?
- The flat base area (ignoring ribs and holes): 200 × 150 = 30,000 mm²
- Each rib adds embossed surface area. A trapezoidal rib 120 mm long × 20 mm wide × 8 mm deep has two sloped walls of approximately (8² + 10²)^0.5 × 120 = 1537 mm² each, plus the rib top face 0 × 120 = 0 (if fully formed). Adding walls: 4 × 3074 ≈ 12,300 mm² additional.
- Six holes of 10 mm diameter: −6 × π × 5² = −471 mm²
Estimated blank area ≈ 41,829 mm². Mid-Surface method on the SolidWorks model should return within a few percent of this number.
For a complete workflow — from blank area estimate to exporting the formed part geometry as DXF for the fabricator — the mid-surface gives you the material quantity while the formed solid itself is what gets shared with the press shop.
Where CadShift fits
CadShift’s flat pattern export operates on sheet metal parts with flat pattern features. Formed solid bodies are outside that scope — they need to be modelled as actual sheet metal (using a Form feature inside a sheet metal part file) before CadShift can batch-export them. If your formed parts start life as solid bodies and are converted to sheet metal later, the mid-surface blank area gives you what you need at the solid body stage.
For material estimation across a large assembly with a mix of bent sheet metal and formed solid bodies, the approaches described here complement the cut-list data CadShift exports for the bent parts.