For Square and Rectangular Areas:
Measure area: Length x Width x Depth
ie: For an area 3mtrs×2mtr×100mm
=3×2×0.1 = 0.6mtrs³

Note: Allow 10% extra for wastage

For Triangular Areas:
Measure area: (Length x Width x Depth) ÷ 2

ie: A rectangle with the dimesions 2mtrs×3mtrs×100mm
=(2×3×0.1) ÷ 2 = 0.3mtrs³

Note: Allow 10% extra for wastage

For Post Holes:

Easiest way is to square the hole out.
ie: For a hole 300mm in diameter and 600mm deep
0.3 × 0.3 × 0.6 = 0.05mtrs³ per hole

For Block Wall Filling:

You require 0.8mtrs³ per 100 standard blocks (200mm high by 400mm long).

Points To Consider:

If you have an irregular shaped area, break this area down into smaller squares or rectangles and measure each as above. Then add all dimensions together to calculate your total concrete requirement.
Check measurements of formwork at three different points, if they vary, average these three dimensions. ie: If the three dimensions are as follows: 1.3m, 1.4m and 1.5m the average is 1.4m
To accurately measure depth, run a straight edge or a straight piece of timber across the formwork at three different points and take the depth measurements at each of these points then average as above.
Make sure you have suitable access through any openings or passage ways allowing clearance for a wheel barrow. Lay planks across any uneven or particularly soft ground.
Consider if you require reinforcement mesh and lay and position this before delivery.