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3DAI
Free · runs in your browser · files stay on your device

STL Cross Section Viewer: Cut Through Any 3D Model

Drop a model, pick the X, Y or Z axis and drag the cut. The viewer shows the part sliced open in 3D and the exact 2D outline of the cut, with its area, perimeter, size and holes. It also finds enclosed voids, and you can save the section as SVG, DXF or PNG.

Loading the cross section viewer...
Demo with a sample file, recorded in this tool.

How to view a cross section of an STL file

  1. 1

    Drop your model

    STL, OBJ, 3MF or GLB up to 100 MB. It is read in your browser and never uploaded. Or open one of the three samples.

  2. 2

    Pick an axis and cut

    X, Y or Z, then drag the slider or type a position in millimetres. Step one layer at a time with the arrows, or jump to 25, 50 or 75%.

  3. 3

    Read the section

    The cut face is orange in 3D. The 2D view shows the outline with a mm grid, and the panel gives the area, perimeter, size, islands and holes.

  4. 4

    Find problems and save

    Click the area chart to jump to a narrow spot, check the void count, and download the section as SVG, DXF or PNG, or the area profile as CSV.

What the colours mean

The part on one side of the plane is hidden so you can look straight into the model. Whatever the plane passes through is drawn as a flat face, and the same cut is drawn in 2D below the viewer.

Orange faceThe solid material where the plane cuts the model. In the 2D view it is the filled area.
Cyan outlineA hole in the section: an opening through the part or a hollow inside it at this height. Click a hole in the list to ring it in the 2D view.
Red dashed lineA contour that does not close because the mesh has a gap there. Area and perimeter skip it. Repair the model to get a closed section.
Yellow line on the chartWhere the cut is now. The chart shows the section area at every position along the axis, so dips are narrow spots and gaps are empty layers.

What the numbers mean

Section numbers are measured on the exact outline of the cut, not on pixels. Model numbers are measured on the whole closed mesh once, when the file is opened.

AreaThe solid area of the cut in mm², holes subtracted. For a print, this is how much material one layer at this height holds.
PerimeterThe total length of every outline and hole edge in the cut. For FDM, it is roughly the length of one wall loop at this height.
Width x heightThe size of the box around the section, measured in the plane of the cut.
Islands and holesIslands are separate solid pieces in this cut. Holes are openings inside them. A hole you did not design is often a hidden cavity.
Volume and voidsThe model volume, and the number and volume of enclosed voids: empty spaces fully sealed inside the part, such as trapped air or uncured resin pockets.
Open edgesMesh edges that have only one triangle, so the surface has a gap. Sections through those spots come out as open (red) lines.

Found a problem inside? Fix it or make a new part

Close the gaps that leave open contours, check that the walls you just cut through are thick enough to print, or turn a photo into a new 3D model with AI. The AI generator uses credits; this viewer does not.

Image to 3D with AI →Repair an STLCheck wall thickness

More free 3D printing tools

Wall thickness checkerSTL repairCompare STL filesLaser cut slicerSTL to DXFSTL viewer

STL cross section viewer FAQ

Drop the STL on this page, pick the X, Y or Z axis and drag the slider. The model is cut open in the 3D view with the cut face in orange, and the 2D panel shows the exact outline of the section with its area, perimeter, size and holes. Nothing is installed or uploaded.
Cut through the middle along each axis. A solid part shows one filled shape; a hollow part shows an outline with a hole inside it. The model card also counts enclosed voids, which are sealed empty spaces inside the mesh, and gives their volume, so a hidden cavity shows up even if you do not cut through it.
Yes. Set the layer height (0.2 mm by default) and use the previous and next buttons or the arrow keys on the slider to move one layer at a time. The viewer shows the true outline of the model at each height; it does not add infill, walls or supports like a slicer does.
The solid area of the cut at every position along the chosen axis, from one end of the model to the other. Low points are narrow necks that may be weak or break, sudden drops are overhangs or hollows, and gaps are heights with no material. Click anywhere on the chart to move the cut there.
Yes. The current section downloads as an SVG for design and laser tools, a DXF for CAD, or a PNG image with the numbers on it. Coordinates are in millimetres. The area profile along the axis downloads as a CSV for a spreadsheet.
They are contours that do not close, because the mesh has a gap (open edges) where the plane crosses it. The area and perimeter leave them out. Run the model through a repair tool, such as the free STL repair page, and the section closes.
STL and OBJ files do not store units. The page assumes millimetres; change Model units to centimetres, inches or metres and every number and export is converted to millimetres for you. If the part looks ten or a thousand times too big or small, that is almost always the reason.
STL (binary and ASCII), OBJ, 3MF and GLB up to 100 MB and 1.5 million triangles. Several parts in one file are cut together. The file is read and cut in your browser, in a background worker, and never leaves your device. The tool is free with no signup.

Sections and volumes are computed in your browser from the triangles in the file. They show the shape of the model itself, not the toolpaths a slicer would print, and are a guide, not a certified inspection report.