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
If you know the density of the rectangular body you can find the volume and the plug it into this equation mass= (density)*(volume) If you have the means you can just submerge the body completely in water and find the volume of the water displaced that will be the mass as waters density is 1 g/cm^3
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.