Concrete Calculator

For a 10×10 foot (3×3 meter) slab at 4 inches (100mm) thick, you need approximately 1.23 cubic yards or 0.94 cubic meters of concrete. The calculation: 10 × 10 × (4÷12) = 33.33 ft³ ÷ 27 = 1.23 yd³. Always add 10% for waste, spillage, and uneven subgrade, bringing your order to approximately 1.4 yd³ (1.07 m³). Most ready-mix suppliers sell in half-yard increments, so order 1.5 yd³ to ensure adequate coverage.

One cubic yard requires approximately 45 bags of 80-lb (36 kg) concrete mix, 60 bags of 60-lb (27 kg) mix, or 90 bags of 40-lb (18 kg) mix. For metric: one cubic meter needs about 59 bags of 36 kg mix. Each 80-lb bag yields roughly 0.6 ft³ (0.017 m³). For projects exceeding 1 yd³ (0.76 m³), ready-mix delivery is more economical, ensures consistent quality, and saves significant labor compared to hand-mixing dozens of bags.

Minimum practical thickness is 4 inches (100mm) for light residential use like patios and sidewalks. Driveways need 4-6 inches (100-150mm) for passenger vehicles; use 5-6 inches for trucks and SUVs. Garage floors require 4-6 inches depending on vehicle weight. Commercial floors need 6-8 inches (150-200mm) with reinforcement. Foundation slabs typically require 8-12 inches (200-300mm). Thinner slabs crack easily, have reduced load capacity, and significantly shorter service life.

For strip footings: multiply length × width × depth. Example: 50-foot (15m) long footing, 16 inches (400mm) wide, 8 inches (200mm) deep: 50 × (16÷12) × (8÷12) = 44.4 ft³ ÷ 27 = 1.65 yd³ (1.26 m³). For round pier footings, use π × radius² × depth. Standard residential footings are 16-24 inches (400-600mm) wide and 8-12 inches (200-300mm) deep. In cold climates, footings must extend below the frost line—typically 36-48 inches (900-1200mm) depth.

Concrete strength is measured in PSI (pounds per square inch) or MPa (megapascals). For residential: 2500-3000 PSI (17-21 MPa) suits sidewalks, patios, and interior slabs. Use 3500-4000 PSI (24-28 MPa) for driveways, garage floors, and exterior work in freeze-thaw climates. Commercial applications need 4500+ PSI (31+ MPa). Standard ready-mix is typically 3000 PSI (21 MPa). Higher strength concrete costs more but provides better durability, crack resistance, and longevity.

Concrete curing follows predictable stages: Initial set in 24-48 hours allows foot traffic. At 7 days, concrete reaches 70% design strength and can handle vehicle traffic. Full cure at 28 days achieves specified strength. Keep concrete moist during the first 7 days—cover with plastic, wet burlap, or apply curing compound. Ideal curing temperature is 50-75°F (10-24°C). Hot, dry, or windy conditions cause rapid moisture loss leading to surface cracking and reduced strength.

Ready-mix concrete typically costs $120-180 per cubic yard ($157-235 per m³) in North America, €90-150/m³ in Europe, and £90-130/m³ in the UK. Prices vary by location, strength specification, and additives. Delivery fees add $50-100 per load. Short-load fees apply for orders under 5-10 yards. Bagged concrete costs $4-6 per bag, making hand-mixing more expensive for projects over 0.5 yd³. Always get multiple quotes from local suppliers.

Reinforcement prevents cracks from widening and adds structural strength. For 4-inch residential slabs, welded wire mesh (6×6 W1.4×W1.4) is standard and economical. Driveways benefit from #3 rebar at 18-24 inch (450-600mm) spacing or fiber mesh. Structural slabs need #4 rebar at 12-18 inch (300-450mm) spacing. Footings require rebar for tensile strength. Place reinforcement at mid-depth, supported on chairs. Properly reinforced concrete dramatically outlasts unreinforced concrete in all applications.