Set the size and meaning of your model
Fix a model that came in at the wrong size, or say what its numbers mean. Assign a unit to unitless coordinates, convert units while keeping physical size, or resize with proportions locked. You see the resulting dimensions before anything is applied.
Set units or change the size of your model. Your file stays in your browser.
Three different operations
They are easy to confuse and they do not do the same thing.
Assigning a unit changes what the numbers mean and leaves them alone. A model whose coordinates read 25.4 is either 25.4 mm or 25.4 inches, and assigning the unit is you telling the file which.
Converting units keeps the physical size and changes the numbers. A part that is one inch long stays one inch long and its coordinates become 25.4.
Resizing changes the physical size. You give a factor, or a target for one dimension, and the other dimensions follow with the proportions locked.
Why this comes up so often
STL, OBJ, PLY and 3DS store no unit at all. The file has coordinates and nothing that says what they measure. A part designed in inches and exported with inch coordinates opens in a millimetre slicer at one twenty-fifth of its size, and nothing in the file can warn you.
The fix is to check one dimension you already know. Not the bounding box, which you have no independent memory of, but a hole spacing or a boss height you can measure on the drawing. The STL guide covers the trap in more detail.