Concrete Calculator
Calculate how much concrete you need for slabs, columns, and stairs.
Pour dimensions
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How It Works
To estimate how much you need, calculate the volume of the area to be filled. For slabs, it is Length × Width × Depth. For columns, use the cylinder formula π × r² × h. Convert the result to cubic yards (divide cubic feet by 27) or cubic meters for ordering.
Premixed concrete comes in 40-lb, 60-lb, and 80-lb bags, yielding approximately 0.30, 0.45, and 0.60 cubic feet respectively. Always add a 10% waste factor to your calculation — uneven subgrades, spillage, and forms that are not perfectly level mean you will almost always need more than the exact mathematical volume.
Tips & Best Practices
Frequently Asked Questions
How much concrete do I need for a slab?
Calculate volume: Length × Width × Thickness (all in the same unit). For example, a 10 ft × 12 ft slab that is 4 inches thick needs 10 × 12 × 0.333 = 40 cubic feet, or about 1.48 cubic yards. Always add 5–10% extra for waste.
What is the difference between concrete and cement?
Cement is a binding ingredient (powder) made from limestone and clay. Concrete is the finished product made by mixing cement with water, sand, and gravel (aggregate). Cement makes up about 10–15% of the concrete mixture.
How do I calculate concrete for round columns?
Use the cylinder volume formula: V = π × r² × h, where r is the column radius and h is the height. For a 12-inch diameter column that is 8 feet tall: V = 3.14159 × 0.5² × 8 = 6.28 cubic feet.
How many bags of concrete do I need?
An 80-lb bag of premixed concrete yields approximately 0.6 cubic feet. Divide your total cubic footage by 0.6 to get the number of bags. For example, 40 cubic feet would need about 67 bags (40 / 0.6 = 66.7).
How thick should a concrete slab be?
For walkways and patios, 3.5 to 4 inches is standard. Driveways should be at least 4 inches, and garage floors 4 to 6 inches. Heavy-traffic or load-bearing slabs may need 6 inches or more with reinforcement.
Can I pour concrete in cold weather?
Concrete can be poured in cold weather but requires precautions. Below 40°F (4°C), use heated water and insulating blankets to prevent freezing during the critical first 24-48 hours. Never pour on frozen ground, and avoid pouring below 20°F (−7°C).
How long does concrete take to cure?
Concrete reaches about 70% of its strength in 7 days and full design strength (typically 4,000 psi for residential) in 28 days. You can walk on it after 24-48 hours, drive on it after 7 days, and place heavy loads after 28 days.
Should I use ready-mix or bags?
For small projects under 1 cubic yard (about 45 80-lb bags), premixed bags are practical. For larger pours, ready-mix truck delivery is more cost-effective, ensures consistent mixing, and allows continuous pouring for a better result.
What is rebar and when do I need it?
Rebar (reinforcing bar) is steel placed inside concrete to add tensile strength. It is recommended for driveways, structural slabs, retaining walls, and any concrete over 4 inches thick. For walkways and small patios, wire mesh may be sufficient.
Why do I need a waste factor?
A 5-10% waste factor accounts for spillage, uneven subgrade, forms that are not perfectly level, and concrete left in the mixer or wheelbarrow. Running short mid-pour creates a cold joint that weakens the slab, so it is always better to order slightly more.