For a slab or footing: Volume = Length × Width × Depth. For a round column: Volume = π × (Diameter ÷ 2)² × Height. Always add a 10% waste buffer for spillage, over-excavation, and uneven ground — especially for DIY pours.
A 10×10 foot slab (3.05 × 3.05 m) at a standard 4-inch (100 mm) thickness requires about 0.93 cubic metres (1.22 cubic yards) of concrete. That's roughly 93 bags of 20 kg pre-mix or 52 bags of 40 kg pre-mix. For projects this size, ordering ready-mix concrete delivered by truck is often cheaper and faster than mixing dozens of bags by hand. Add 10% for waste.
For small projects under 0.5 m³ (about 50 bags), pre-mixed bags are convenient — just add water and mix. For anything larger, ready-mix concrete delivered by truck saves enormous amounts of time and labour. A cubic metre of ready-mix costs roughly the same as buying bags but arrives already mixed and ready to pour. The breakeven point is usually around 1 m³ — above that, always go with ready-mix.
Concrete reaches about 70% of its full strength within 7 days and 99% within 28 days. During the first 48 hours, keep the surface moist to prevent cracking — cover it with plastic sheeting or spray water periodically. Avoid walking on a new slab for at least 24 hours and don't drive on a driveway slab for 7 days. In hot weather, concrete sets faster but is more prone to surface cracking, so curing is especially important.
For driveways, garage floors, and structural slabs, steel reinforcing mesh or rebar significantly reduces cracking and increases load-bearing capacity. A standard residential slab typically uses SL72 or SL82 mesh. Pathways and garden slabs can usually get away without reinforcement, but adding mesh is cheap insurance. Fibre-reinforced concrete (with synthetic fibres mixed in) is another option that reduces shrinkage cracking.
Multiply the length by the width by the depth (thickness) of the slab. This gives you the volume in cubic metres or cubic feet. For example, a 3m x 4m slab that is 0.1m thick = 1.2 cubic metres. Add 10% for waste and over-pour. This calculator handles the maths for slabs, footings, columns and steps.
Cement is an ingredient in concrete. Concrete is a mixture of cement, sand, gravel (aggregate), and water. The cement acts as the binder that holds the aggregate together as it cures. Ready-mix concrete is sold by the cubic metre or yard and delivered pre-mixed in a drum truck.
A standard 20kg bag of concrete mix typically makes about 10 litres (0.01 cubic metres) of concrete. For 1 cubic metre, you need approximately 100 bags. For 0.1 cubic metres (100 litres), you need 10 bags. This calculator shows bag counts for popular bag sizes.
For a standard residential slab (floors, driveways, patios): 100mm (4 inches) is typical. For heavy vehicle traffic: 125-150mm (5-6 inches). For pathways and garden slabs: 75-100mm (3-4 inches). Always check local building codes and consult a structural engineer for load-bearing slabs.
Concrete gains about 70% of its strength within 7 days and reaches full design strength at 28 days. During the first 24-48 hours it should be kept moist and protected from frost. You can typically walk on it after 24-48 hours, but avoid heavy loads until 7 days. Full curing continues for months.
A 10x10 foot slab at 4 inches thick needs about 1.22 cubic yards (0.93 cubic metres) of concrete. That is approximately 93 bags of 20kg pre-mix or 52 bags of 40kg pre-mix. Always add 10% for waste and uneven ground. For this volume, ready-mix delivery is usually more practical than mixing bags by hand.
Concrete does not technically dry — it cures through a chemical reaction with water. It is firm enough to walk on after 24 to 48 hours, reaches about 70% strength at 7 days, and achieves full design strength at 28 days. Keep the surface moist during the first few days to prevent cracking and ensure proper curing.
Light rain after the concrete has initially set (a few hours after pouring) is usually fine and can actually help curing. However, heavy rain during or immediately after pouring can wash away cement paste, weaken the surface, and cause pitting. If heavy rain is forecast, delay the pour or have plastic sheeting ready to cover the slab immediately after finishing.