STL has no units: how to check a model's size

The coordinates inside an STL file are plain numbers. Nothing in the file says whether they mean millimetres, centimetres or inches. Your slicer assumes a unit, usually millimetres, and if the model was drawn in another unit it comes out the wrong size. (STL format overview)

How a unit mistake looks

• A model designed in inches and read as millimetres comes out 25.4 times too small. A 2-inch part appears as a 2 mm speck.
• A model drawn in centimetres and read as millimetres is 10 times too small.
• A model exported from a tool that uses metres can be a thousand times too small, or a thousand times too large when it goes the other way.

Spotting it early is easy. Seeing it after hours of printing is not.

Read the bounding box first

The bounding box is the smallest box that fits the model, given as an X, Y and Z size. Check it against what you expect:

1. Open the file in a viewer that shows the size. MeshView shows X, Y and Z in millimetres together with the triangle count.
2. Compare each value to the real part. A phone case near 150 mm and a reading of 5.9 suggests the file is in inches.
3. If the numbers are off by 25.4, 10 or 1000, you have found a unit problem. Scale the model in the slicer by that factor.

A known reference

3DBenchy, the popular test boat, is published at 60 × 31 × 48 mm. If you open its STL and see those figures, the viewer is reading the file correctly. That makes it a handy way to confirm a viewer before you trust it with an unfamiliar file.

Why 3MF avoids the problem

A 3MF file states its unit, and the default is millimetres. That is one more reason to prefer it for anything that matters. See STL vs 3MF.

Size is not the only number to check

• Triangle count. A very low count on a model that should be detailed suggests the export was too coarse. A very high count makes slicing slow.
• Volume. It lets you estimate material use, but only for a watertight mesh. A reading of “not watertight” means you should check the model first. See how to check an STL before you print it.

Take a minute to read these numbers before slicing. It is the cheapest print test you will ever run.