How Much Concrete Do You Actually Need?
Concrete volume is simply length × width × thickness. The tricky part is unit conversion (thickness is usually in cm or inches while dimensions are in meters or feet) and the fact that real pours always need more than the theoretical volume.
- Always add 5-10% for waste, uneven subgrade, and edge loss
- For odd-shaped areas, break the shape into rectangles and add the volumes
- Circular slabs: volume = π × radius² × depth (enter diameter ÷ 2 as one dimension, same as diameter as the other, then divide by 4 — or just use this calc with approximate rectangle)
- Sloped surfaces: use average depth between highest and lowest points
Bags vs Ready-Mix Truck: When to Call the Truck
Pre-mixed bags from a hardware store are convenient for small projects. But mixing 40+ bags by hand or rented mixer is exhausting and time-consuming. Ready-mix trucks deliver exactly the right volume, already mixed to your specified strength.
- Under 0.5 m³ / 0.65 yd³: bags are fine, cheaper, no minimum order
- 0.5–1 m³: borderline — bags are doable but a truck is worth considering for quality
- Over 1 m³ / 1.3 yd³: truck almost always wins on price and finish quality
- Ready-mix minimum delivery: most companies have a 1 yd³ minimum; some charge a short-load fee under 3 yd³
- Truck timing: you have ~90 minutes from delivery to pour and screed before initial set
Standard Slab Thicknesses
Thickness is the biggest variable in load-bearing capacity and freeze-thaw durability. Under-thickness a slab and it cracks; over-thickness it and you waste money and add dead weight.
- Garden paths and patios (foot traffic only): 10 cm (4 in)
- Residential driveways (cars and light SUVs): 10–12 cm (4–5 in)
- Driveways with heavy trucks: 15 cm (6 in)
- Garage floors: 10–12 cm (4–5 in) with 4-inch gravel subbase
- Pool surrounds: 10 cm (4 in) with extra reinforcement at edges
Rebar and Reinforcement
Plain concrete handles compression well but cracks easily in tension. Rebar (steel reinforcement bars) or wire mesh significantly increases crack resistance. For any slab over 2 m × 2 m that will carry vehicle loads, rebar is worth the extra cost.
- Wire mesh: cheap, quick, good for small patios and paths
- Rebar grid: 10 mm (#3) bars on 30 cm centres for driveways
- Rebar should be placed at mid-depth, not at the bottom
- Control joints: saw-cut every 2-3 m in each direction to control where cracks form
- Fibre-reinforced concrete: an alternative to wire mesh — reduces surface cracking but not structural cracks