If the dimensions are cubic feet then you are probably calculating a volume. The formula you want depends on the shape of the object. If it is a rectangular slab or box then the volume is given by
volume = length width height
If the shape is a right circular cylinder like a tin can then
volume = r2 h
where r is the radius and h is the height.
For other shapes you need other formulas. The formula depends on the shape but an important aspect is that if you want the volume in cubic feet then all the dimensions you use must be feet. In the two expressions above length, width, height, r and h must be measured in feet.