Question about units in FEATool Multiphysics.

I would like to ask you about the units. My model is in mm, and then I supposed that the units for the young modulus would be MPa (N/mm^2) but I had to put it in KPa to try to match the results. The density I suppose that would be kg/mm^3.

To quote directly from the FEATool quickstart guide:

"FEATool Multiphysics does not prescribe or make users use a specific unit system, it is therefore up to the user define and use consistent units. If for example a geometry length unit is chosen as meters (m), then standard SI-units are recommended for the equation and boundary coefficients. Using another length unit such as millimeters (mm) or inches, will typically require corresponding rescaling of model coefficients to preserve unit consistency."

So in your case if you geometry/CAD model is in "mm" then yes your other coefficients and material parameters would have to be defined using [mm] as a base length scale.