Concrete quantities for slabs, footings and terraces
Calculate geometric concrete volume from measured dimensions, then state any ordering allowance separately. A quantity calculation does not determine structural slab thickness, reinforcement, foundation depth or concrete strength.
Reviewed:
Calculator
Slab or terrace
V [m³] = length [m] × width [m] × thickness [m]
An illustrative 5 m × 4 m terrace at 0.15 m thickness contains 3.00 m³. A stated 5% ordering allowance gives 3.15 m³. Convert 150 mm to 0.15 m before multiplying. The 150 mm and 5% inputs are arithmetic examples, not structural or waste recommendations. Check changes in thickness, edge beams, openings and the actual support level.
Footings and separate pours
A rectangular footing 0.8 m × 0.8 m × 0.3 m has volume 0.192 m³; four identical footings total 0.768 m³ before an allowance. Sum different shapes individually. For a cylindrical element, V = π × D² / 4 × height. Use dimensions from the design; quantity formulas do not check soil bearing capacity or reinforcement. Avoid counting a footing twice where it overlaps a slab.
Before ordering concrete
Check net dimensions, measurement units, pour sequence and the supplier's delivery increments. Record the source of the allowance and separate it from calculated net volume. Bag yield comes from the product documentation and cannot be inferred from bag mass alone. The required mix, exposure class, curing and structural thickness belong to the project specification.
Concrete thickness comes from the design
Choose the pour type first: ground-supported slab, suspended slab, pad, footing or walkway. Structural thickness depends on loads, span or ground support, reinforcement, joints, exposure and the specified concrete. A geometric volume calculator cannot determine that thickness. Use the dimension from the approved design, measure changes in level and edge beams, then enter it in millimetres. The 150 mm slab example on this page is an arithmetic input, not a universal minimum. Footings need width and depth from the foundation design.
Worked examples — illustrative inputs
| L [m] | W [m] | d [m] | V [m³] |
|---|---|---|---|
| 5 | 4 | 0.15 | 3.000 |
| 0.8 | 0.8 | 0.30 | 0.192 |