Each veranda post is 150 x 150mm square.
They need to go into the ground about 800mm.
The holes need to be 450mm diameter.
However, the top 150mm of the hole will be 600mm diameter to act as a footing for the stone plinths that will be around each post.
600mm dia = 0.2827 area
600mm dia x 150mm deep = 0.04241 area
100 x 100 x 1m = 0.01 area
150 * 150 = 0.0225 area
450mm dia = 0.159 area
450mm dia x 850mm deep = 0.135 cubic meters
450mm dia x 850mm deep + 600mm dia x 150mm deep – 150x150x1m = 0.155
so 18 holes = 2.79 cubic meters concrete