You measure it, using a ruler of another measuring device.
the three dimensions needed to find the area of a rectangular solid object are: Height, Length and Width.
If it is a cube (basically a square) or a rectangular prism multiply the top, the length, and the base.
Volume = Length * Width * Height
False on two counts. A rectangular shape is 2-dimensional and so can have no mass. If it is rectangular but has length, width and height then it is a cuboid object. Then, multiplying the length width and height will give the volume, not the mass.
A rectangular prism has a volume that can be found by multiplying the object's length width and height. To obtain the mass one must also know the density of the object and multiply that by the volume.
length, width and height (:
Volume of any prism = cross-section area times length
To find the volume of a rectangular object, you multiply its length, width, and height. To find the mass, you would then multiply the volume by the object's density, which is typically provided in grams per cubic centimeter.
width x height x depth or length x width x height
The equation to find the volume of a rectangular object is (a) volume equals length times width times height.
length times width
The same as for any other three dimensional rectangular object: Length X Width X Depth.