I have an area of 1000 sq metres, can you calculate how much stone i need if i'm putting 150mm of stone all over

Yeah that's not that hard,

150mm = 0.15m

You can imagine that it is a box of which you're trying to find the volume.

The formula for a box is length*width*heigth

You know length*width = 1000

and you know height = 0.15m

So the volume you need is 1000*0.15 = 150 m^3

Reinout