>>17414000

Note that pennies, when stacking side-to-side, waste some space. You can calculate the space it will take altogether by noting that hexagons pack perfectly and each hexagon contains 3 pennies (center + 120 angles).

Each hexagon has the diameter of a penny as an edge, so 19,05mm according to wikipedia. This adds up to 942,848mm2 in area per hexagon, or 314,283mm2 per penny.

Furthermore, a penny is 1,52mm thick, so each penny takes roughly 477,7 mm3 of space. You multiply this by one billion and then divide it by one billion to convert to m3, so you end up with 477,7 cubic meters to store all these pennies, which is even less than I had highballed.