How Much Concrete Do I Need? A Practical Guide
Concrete is one of the few construction materials where ordering the wrong amount has immediate and expensive consequences. Too little means the truck leaves before your pour is complete, and the cold joint where you resume weakens the structure. Too much means you are paying for excess material that sets in the truck or gets dumped. Accurate volume estimation before ordering is not optional. It is the difference between a smooth pour day and a costly disaster.
The Basic Formula
For any rectangular slab or footing, the concrete volume is length times width times thickness. A driveway that is 6 metres long, 3 metres wide and 0.1 metres thick (100mm) requires 1.8 cubic metres of concrete. A sidewalk that is 15 metres long, 1 metre wide and 0.1 metres thick needs 1.5 cubic metres. The formula is simple, but the measurements must be accurate. Even a 10mm error in thickness across a large slab can change the volume by several percent.
Common Project Types
Different projects have different typical dimensions:
- Garage slab: typically 100-150mm thick, depending on vehicle weight expectations
- Driveway: 100mm for cars, 150mm if heavy vehicles will use it
- Sidewalk or path: 75-100mm thick
- Patio: 100mm is standard for residential
- Footings: depth and width determined by local building codes, typically 300-450mm wide and 200-300mm deep
- Post holes: cylindrical, calculated as pi times radius squared times depth
- Retaining wall footing: width is typically 1.5 to 2 times the wall thickness
For post holes and cylindrical piers, use the cylinder formula. A post hole 300mm in diameter and 900mm deep requires about 0.064 cubic metres of concrete. That seems small, but ten post holes for a deck total 0.64 cubic metres, nearly a full cubic metre when you add the waste factor.
The Waste Factor
Theoretical volume calculations assume perfect forms, uniform thickness and zero spillage. Reality is different. Ground surfaces are uneven, forms flex slightly under the weight of wet concrete, and some material is always lost during transport and pouring. The standard industry practice is to add 5 to 10 percent to your calculated volume. For a project requiring 5 cubic metres of concrete, order 5.25 to 5.5 cubic metres.
For projects with irregular ground, such as a slab on sloped terrain where the thickness varies, increase the waste factor to 10 to 15 percent. The uneven substrate absorbs more concrete than flat, prepared ground.
Bagged vs Ready-Mix
Small projects can use pre-mixed bags rather than ordering a delivery truck. A standard 20-kilogram bag of concrete mix yields approximately 0.009 cubic metres when mixed. A 40-kilogram bag yields about 0.018 cubic metres. For a small patio slab of 2 cubic metres, you would need roughly 111 bags of 40-kilogram mix. At that quantity, a ready-mix truck delivery is almost certainly more economical and practical.
The breakeven point between bagged and ready-mix varies by region, but most contractors switch to ready-mix at around 0.5 to 1 cubic metre. Below that, bags are convenient. Above it, the labor of mixing individual bags becomes prohibitive.
Reinforcement Does Not Change Volume
Rebar, wire mesh and fiber reinforcement occupy space within the concrete, but the volume they displace is negligible for ordering purposes. A rebar grid in a 100mm slab displaces less than 1 percent of the total volume. Do not subtract reinforcement volume from your concrete order. The waste factor more than compensates for any displacement.
Double-Check Before Ordering
Measure twice. Calculate once. Verify with a tool. Concrete cannot be returned once the truck arrives, and ready-mix companies charge for full or partial loads regardless of how much you actually use. Plugging your dimensions into a concrete calculator takes less time than a phone call to the supplier and can save hundreds of dollars in wasted material.