STL units: fix scale before converting to CAD

The numbers need a unit interpretation

An STL contains coordinate values, but does not provide a standard unit definition that reliably tells every importer whether those values mean millimeters or inches. Treat the exporter's intended units as part of the handoff. A coordinate value of 25 can represent very different physical lengths when applications interpret it differently.

Changing the units displayed in a CAD document is not a substitute for checking the imported size. Verify the geometry against a known dimension. If you have no reference measurement, ask the source designer before assuming that a plausible-looking size is correct.

Import with an explicit unit choice

Fusion's Insert Mesh dialog lets you select how one source-file unit is interpreted, including millimeter, centimeter, meter, inch, and foot. Autodesk also documents cases where opening or uploading an STL produces an incorrect scale and recommends Insert Mesh with the correct unit selection.

Start a clean document, import a copy of the original, and choose the known source units. Measure a distinctive reference such as overall length or mounting-hole spacing. Document what you selected so that another person can reproduce the result instead of guessing how the model was scaled.

Use a measured ratio when correction is necessary

For a uniform size correction, divide the desired measurement by the current measurement. If a part should be 80 mm long but measures 800 mm, the factor is 80 / 800 = 0.1. If it should be 80 mm but measures 8 mm, the factor is 10. Keep those two measurements in the same unit before dividing.

For an inch-to-millimeter interpretation error, remember that one inch equals exactly 25.4 mm. Apply 25.4 or its reciprocal only after establishing which interpretation is wrong. Applying a familiar factor without a reference measurement can introduce a second mistake.

Uniform scaling changes every linear dimension together. It is appropriate for a unit mismatch, but not for changing only a hole diameter while preserving the bracket's outside size. That is a design modification and should be handled with an editing or rebuilding workflow.

Check more than the bounding box

A width, height, and depth check is useful, but those values depend on orientation. A diagonal part has a different axis-aligned bounding box from the same part lying flat. Use an identifiable distance on the part as a second check, and keep the orientation consistent when comparing overall dimensions.

For a mesh opening, do not mistake the distance between arbitrary triangle vertices for an authoritative design diameter. Supply a drawing or physical measurement when the exact fit matters. The measurement shown by your software and the nominal dimension intended by the designer may need to be reconciled.

After conversion or export, reopen the delivered file and repeat the reference measurement. Include the agreed units and dimension in the delivery note. This makes a small but consequential assumption visible throughout the workflow.

• Record source units and one known dimension.
• Select import units explicitly when available.
• Correct scale once, on a preserved copy.
• Recheck the final file in the destination application.