Need the broader workflow first? Read how to check if a 3D model is ready to print.
Choosing between STL, 3MF and other formats? Read the best file format for 3D printing guide.
For a closer look at thin-area findings, read the 3D Print Wall Thickness Guide.
For a complete check, repair and verification workflow, read How to Make an STL File Printable.
After slicing, inspect the final file with the G-code Preflight Checker.
How this check works
Local preflight and the server Quality Checker are different methods. This browser screen joins exactly matching vertices and counts connected shells through vertex connectivity; the loose-part removal tool uses full shared edges. The server applies its own edge-based topology checks and keeps independent 3MF build items separate. A component count from one workflow should not silently replace another's interpretation.
The report parses the STL once in your browser, counts topology findings, calculates bounds and tests practical right-angle orientations against your saved custom printer profile. Without a saved profile, fit is skipped. It samples up to 2,000 triangle locations for wall thickness, so narrow features can be missed and sampled values are approximate.
STL does not declare units. Confirm whether the coordinates represent mm, cm, metres or inches before trusting physical measurements. A clean report cannot guarantee a successful print: calibration, material, orientation, supports and slicer settings still matter.
For holes, pins and mating features, use the Tolerance & Clearance Calculator to record your intended fit and calibration adjustment before the final export.
Example workflow
Inspect each finding before changing the file. An opening may be intended, and disconnected pieces can be a valid assembly. Confirm the source units and intended geometry, choose a supported correction only when needed, then reopen the saved copy and compare the affected region. Review scale, fit and the actual slicer layers separately.
Preflight FAQ
Can a preflight check tell me an STL is printable?
No. It identifies specific mesh, size and sampled-thickness concerns before slicing; it cannot account for your printer setup or use case.
Why are STL units ambiguous?
STL stores coordinates but no reliable unit declaration, so the same numbers can represent very different physical sizes.
Why does a closed mesh matter for volume?
A solid-volume calculation needs a closed, manifold surface. Holes and non-manifold edges make that result unreliable.
Why is wall thickness sampled?
Sampling keeps the browser tool responsive, but it is not an exhaustive measurement of every point on the model.