Field note / site materials
Start with geometric volume.
For a rectangular pour, multiply length by width by thickness. This is the clean volume of the shape you have measured, before practical ordering judgement is applied.
Then show the allowance.
An allowance can cover measurement uncertainty, irregular edges, small losses, and the practical difference between calculated volume and the quantity that must be ordered. The important thing is not to hide it inside the geometric quantity.
What a simple estimate cannot know
- Falls, thickening, penetrations, construction joints, blinding, and irregular formwork.
- Supplier minimum loads, pump line losses, access constraints, washout, and rejected loads.
- Mix design, strength, durability, temperature, placing method, and curing requirements.