Concrete Volume Calculator
Calculate exact cubic yards of concrete needed for slabs, walls, columns, and footings. Enter values for instant results with step-by-step formulas.
Formula
Volume (cu ft) = Length × Width × Depth ÷ 12 → Cubic Yards = Volume ÷ 27
For rectangular slabs, multiply length by width (in feet) by depth (converted from inches to feet by dividing by 12). Divide the cubic feet result by 27 to get cubic yards. For cylindrical columns, use π × radius² × height. One cubic yard equals 27 cubic feet or 0.7646 cubic meters.
Worked Examples
Example 1: Standard Driveway Slab
Problem: Calculate concrete needed for a 20 ft × 10 ft driveway that is 5 inches thick.
Solution: Volume = 20 × 10 × (5/12) = 83.33 cu ft\nCubic yards = 83.33 / 27 = 3.09 cu yd\n80lb bags = 83.33 / 0.6 = 139 bags
Result: 3.09 cubic yards or 139 bags of 80lb concrete
Example 2: Concrete Column Footing
Problem: Calculate concrete for a cylindrical column 2 ft in diameter and 4 ft tall.
Solution: Volume = π × (1)² × 4 = 12.57 cu ft\nCubic yards = 12.57 / 27 = 0.47 cu yd\n80lb bags = 12.57 / 0.6 = 21 bags
Result: 0.47 cubic yards or 21 bags of 80lb concrete
Frequently Asked Questions
How do I calculate the volume of concrete needed for a slab?
Multiply the length by the width by the depth (thickness) of the slab. If measuring in feet with depth in inches, convert the depth to feet first by dividing by 12. The formula is: Volume (cu ft) = Length (ft) × Width (ft) × Depth (in) ÷ 12. Then divide by 27 to convert to cubic yards, which is how ready-mix concrete is sold. Always add 5-10% extra for waste and spillage.
How many bags of concrete do I need for a small project?
An 80-pound bag of premixed concrete yields approximately 0.6 cubic feet of cured concrete. A 60-pound bag yields approximately 0.45 cubic feet. For a 4-inch thick, 4×4 foot slab, you need about 5.33 cubic feet, which equals roughly 9 bags of 80-pound mix or 12 bags of 60-pound mix. For larger projects (over 1 cubic yard), ordering ready-mix delivery is usually more cost-effective.
What is the standard thickness for a concrete slab?
Residential concrete slabs are typically 4 inches thick for patios, walkways, and garage floors. Driveways should be 5-6 inches thick to support vehicle weight. Footings for walls or posts are usually 8-12 inches deep and extend below the frost line. Commercial and industrial floors may be 6-8 inches or more depending on load requirements. Thicker slabs require more reinforcement but provide greater structural capacity.
Should I add extra concrete for waste and overage?
Yes, always order 5-10% more concrete than your calculated volume. Concrete is lost to uneven subgrade, form variations, spillage, and testing samples. For irregular shapes or sloped surfaces, add 10-15%. It is far more costly to have a second truck deliver a small makeup load than to slightly over-order. Excess concrete can be used for small side projects like stepping stones or mailbox posts.
How do I calculate the amount of concrete needed for a project?
Calculate volume in cubic feet (length x width x depth), then divide by 27 to convert to cubic yards. Add 5-10% for waste and spillage. One cubic yard of concrete covers 81 square feet at 4 inches thick.
What are the standard concrete mix ratios?
Common ratios by volume are 1:2:3 (cement:sand:gravel) for general purpose, 1:1.5:3 for structural work, and 1:2:4 for foundations. The water-to-cement ratio should be 0.45-0.55 for optimal strength. Lower water content produces stronger concrete.