How to calculate concrete volume by hand
Concrete is a volume calculation. Length and width alone are not enough: depth makes the quantity change quickly.
Rectangular slabs and trenches
Convert all measurements to the same unit before multiplying. In metres, a 3 m × 2 m slab at 0.1 m deep is 0.6 m³, or 600 L. For a trench, use the same relationship with the trench width and depth.
Prism volume
volume = length × width × depth100 mm is 0.1 m. A 125 mm slab is 25% more volume than a 100 mm slab across the same footprint.
Round holes and piers
For a round hole, use π × radius² × depth, then multiply by the number of identical holes. Measure the actual hole diameter and depth; a widened hand-dug hole can use noticeably more concrete than the post size suggests.
Move from volume to bags
Divide the allowance-adjusted volume by the yield printed on the bag, then round up. If you are comparing ready-mix, retain the cubic-metre figure and check the supplier’s ordering requirements.