Area Calculator

For a 20 ft × 20 ft slab at 4 inches thick, the volume is 20 × 20 × (4/12) = 133.33 cubic feet. Divide by 27 to get 4.94 cubic yards. Ready-mix companies typically sell in quarter-yard increments, so you'd order 5 cubic yards. Adding a 5% waste factor brings it to 5.19 cubic yards, so 5.25 or 5.5 yards is safe. If you're mixing bags, an 80 lb bag yields 0.6 cubic feet, so you'd need 223 bags—impractical for a slab this size. For smaller projects like a 10x10 slab at 4 inches, you'd need 1.23 cubic yards, or about 56 bags of 80 lb concrete. Use our concrete calculator for precise bag counts and to compare ready-mix versus bagged concrete costs.

A standard concrete driveway for passenger vehicles should be at least 4 inches thick. However, if you park heavy vehicles like RVs or trucks, use 5 to 6 inches. For example, a 24 ft × 24 ft driveway at 4 inches requires 7.11 cubic yards, while at 6 inches it requires 10.67 cubic yards. The extra depth increases cost but prevents cracking. Always add a 4- to 6-inch compacted gravel base and consider rebar or wire mesh for reinforcement. If you're pouring a driveway that will experience freeze-thaw cycles, a 5-inch thickness with air-entrained concrete is recommended. For a typical two-car driveway (20 ft × 20 ft), 4 inches is sufficient. Check local building codes for minimum requirements. Our area calculator can help you adjust depth and see the volume difference instantly.

To convert square feet to cubic yards, multiply the area by the depth in feet, then divide by 27. For example, a 500 square foot patio at 4 inches deep: first convert 4 inches to feet (4 ÷ 12 = 0.333 ft). Then 500 × 0.333 = 166.67 cubic feet. Finally, 166.67 ÷ 27 = 6.17 cubic yards. If you order 6.5 cubic yards, you'll have a small buffer. Remember that depth must be in feet for this formula. If you have depth in inches, divide by 12 first. This conversion is crucial because concrete is ordered in cubic yards, not square feet. Many people mistakenly order based on square footage and end up short. For a 1,000 square foot slab at 6 inches, you'd need 18.52 cubic yards. Use our area calculator to avoid manual errors and get instant results for any dimensions.

One cubic yard equals 27 cubic feet. An 80 lb bag of concrete mix yields approximately 0.6 cubic feet. Therefore, 27 ÷ 0.6 = 45 bags per cubic yard. For a 10 ft × 10 ft slab at 4 inches thick, volume is 33.33 cubic feet, requiring 33.33 ÷ 0.6 = 55.56, so 56 bags. If you use 60 lb bags, each yields about 0.45 cubic feet, so you'd need 27 ÷ 0.45 = 60 bags per cubic yard. For the same slab, 33.33 ÷ 0.45 = 74.07, so 75 bags. Bagged concrete is practical for jobs under 1 cubic yard (45 bags of 80 lb). Above that, ready-mix is more economical and less labor-intensive. For a 4 ft × 8 ft raised garden bed at 6 inches deep, volume is 16 cubic feet, requiring 27 bags of 80 lb. Our calculator automatically computes bag counts for both sizes.

The formula is: Volume (cubic feet) = Length (ft) × Width (ft) × Depth (ft). Depth must be in feet, so if you have inches, divide by 12. For example, a 12 ft × 12 ft slab at 4 inches: depth = 4 ÷ 12 = 0.333 ft. Volume = 12 × 12 × 0.333 = 48 cubic feet. To convert to cubic yards, divide by 27: 48 ÷ 27 = 1.78 cubic yards. If you're using metric, the formula is Length (m) × Width (m) × Depth (m) = cubic meters. For a 3 m × 4 m slab at 0.1 m thick, volume = 1.2 cubic meters. Always add 5-10% for waste. For a 20 ft × 30 ft slab at 5 inches, volume = 20 × 30 × (5/12) = 250 cubic feet = 9.26 cubic yards. Order 10 cubic yards to be safe. This formula works for any rectangular pour. For circular slabs, use π × radius² × depth.

Ready-mix concrete typically costs between $125 and $175 per cubic yard in the US, depending on region and mix design. A 4-inch slab for a 20 ft × 20 ft area (4.94 cubic yards) would cost roughly $618 to $865 for the concrete alone. Add $50 to $100 per cubic yard for delivery fees if you're outside the standard radius. For smaller projects, bagged concrete costs about $4 to $6 per 80 lb bag. A 10 ft × 10 ft slab at 4 inches needs 56 bags, costing $224 to $336—often more expensive than ready-mix for the same volume. However, bagged concrete avoids delivery minimums, which can be 1 to 3 cubic yards. For a 4 ft × 4 ft footing at 12 inches deep (0.59 cubic yards), you'd need 27 bags of 80 lb, costing $108 to $162. Always get quotes from local suppliers. Our concrete calculator helps you compare costs by showing bag counts and volume. For gravel or topsoil, use our gravel calculator or topsoil calculator.