The Onshape Mass properties tool is available in Part Studio and Assemblies for parts and assemblies. Find the Mass properties tool in the bottom right corner of the interface (desktop, shown below in a screenshot) or on the main toolbar (iOS and Android), the scales icon, when you have parts selected.

You can also use this tool to override properties and supply your own values for mass.

Example of location of Mass Properties tool

Properties are additive: for each additional part you select, its properties are added to the calculations in the dialog (desktop) or panel (iOS and Android). When you apply materials to parts, the density of the material is used in the calculations in the Mass properties dialog/panel. If a part has no material assigned, no figure for that part is used in the calculation (and a note is displayed in the dialog/panel to that effect).

Results of mass property calculations are approximate. The calculation of the properties may vary in accuracy, depending on the complexity of the geometry.

Enabling Show calculation variance displays the value and the difference between the lower and upper bound of the calculated value. If Show calculation variance is not enabled, the computed value without the bounds is displayed. The computations of the values are not affected by the state of the Show calculation variance checkbox.

Apply Materials to parts through the context menu on a part in the Parts list (or the graphics area).