I had assumed that it would be the average of the top and bottom areas multiplied by the height, but this does not seem to give an accurate answer (about 1% over estimated). Is my google sketchup wrong, or my maths?
eg. suppose, top edge length is et= 6.25, bottom edge eb= 5, and height h=15. My formula 0.75*sqrt(3)*h*(et^2+eb^2)
gives 1248.29 whilst google gives 1238.15
See attached diagram showing the exploded object.