How it is worked out
A hole is a cylinder: 3.1416 × radius × radius × depth. The post inside it is subtracted, as a square (width × width × depth) or a round (3.1416 × radius × radius × depth).
One cubic metre is 1,000 litres, which is why the per-hole figure is shown in litres.
This page works out quantity only. How deep and wide a post hole needs to be depends on the fence or structure, the soil and the wind, so follow the instructions for what you are building.
Worked example
The numbers this page opens with: number of holes 10 holes, hole diameter 300 mm, hole depth 600 mm, Post shape: square post, post width or diameter 90 mm, extra 10 %, 20 kg bags per cubic metre 108 bags.
The answer: concrete per hole 37.6 litres; bags per hole 4.1 bags; total with 10% extra 0.413 m³; 20 kg bags to buy 45 bags.
Questions
Does the post really make a difference?
For a slim steel post, hardly any. For a thick timber post in a narrow hole it can be a noticeable share of the hole, which is why you can switch it off and compare.