Free Fence Post Count calculator with clear step-by-step results.
A straight run needs one more post than it has bays, and every corner and gate adds posts of its own - gates need a post each side. Concrete volume comes from the hole geometry, reduced slightly because the post itself occupies part of the hole.
Line posts
Posts = ceil(fence length / post spacing) + 1
Concrete volume
Volume = pi x (hole diameter / 2)^2 x depth x posts x 0.85
Around 2.4 m suits standard panel fencing; 1.8-2.0 m is better for close-boarded fencing in exposed positions where wind load is high.
A common rule is one third of the above-ground height, with a minimum of 600 mm. In soft or waterlogged ground, go deeper and widen the hole.