An STL file, opened without installing anything.

Open it in your browser.

Open the STL viewer and drop the file on it. Binary and text STL files both open, of any size your device can hold, since nothing is uploaded.

It works the same on Windows, a Mac, a Chromebook and a phone. Microsoft's own 3D Viewer has been taken out of Windows, and the page it suggests instead does not list STL among the files it opens.

• Open an STL file
• Guide: a Microsoft 3D Viewer alternative

Its size, in millimetres.

An STL file holds numbers and no unit. Almost every 3D printer and CAD program means millimetres, so the viewer reads them that way and says the size of the box around the model: 52.9 × 26 × 55.2 mm, for example.

If the size looks a thousand or twenty-five times wrong, the file was made in metres or inches: choose the unit under Its numbers are in, and every size and the measure tool follow.

Press Measure, then two points on the model, for the distance between them; a press near a corner snaps to it.

• Open the 3D viewer
• Guide: open 3D files in your browser

Is it a closed shape?

A slicer needs a closed shape: every edge shared by exactly two triangles turned the same way. The viewer checks every edge and says so in words. When the shape is closed, it gives the volume, which a printer turns into material.

When it is not, it counts what is wrong: edges with a face on one side only (holes), edges shared by three faces or more, and faces turned the wrong way. Mending them comes in a later update.

Cut through the model to see inside it: a closed shape shows a flat fill where it is cut, and a missing wall shows as a gap.

• Guide: what a 3MF file is
• Open a 3MF file