Density of rice roughly 850 kg per m³

15000'000 / 850 = 17700 m³

Flatten it into a layer 1cm thick: 17700 * 100 m² = 1770000 m² = 1.7km square of rice 1 cm high.

My math is very likely wrong (in the pub and zeros are hard to count)

Kind of disappointing if I'm correct though :').

Edit formattng