Calculate the exact amount of concrete needed for your slab project. Results in cubic yards (for ready-mix delivery) and number of bags (for DIY projects).
Engineering Formulas & Specifications
Volume Calculation Formula
Volume (yd³) = [Length (ft) × Width (ft) × Thickness (in) ÷ 12 × (1 + Waste%)] ÷ 27
Step-by-step calculation:
- Convert thickness to feet: Thickness (in) ÷ 12 = Thickness (ft)
- Calculate volume in cubic feet: Length × Width × Thickness (ft)
- Apply waste factor: Volume × (1 + Waste% ÷ 100)
- Convert to cubic yards: Volume ÷ 27 (1 yd³ = 27 ft³)
- Bag calculation: Total ft³ × 150 lbs/ft³ ÷ Bag weight (lbs)
Standard Concrete Weights:
- Normal weight concrete: 150 lbs per cubic foot
- Lightweight concrete: 110-115 lbs per cubic foot
- Heavyweight concrete: 200-250 lbs per cubic foot (radiation shielding)
Minimum Thickness by Application
- Residential sidewalks: 4 inches
- Patios & walkways: 4-5 inches
- Driveways (passenger cars): 5-6 inches
- Driveways (trucks/RVs): 6-8 inches
- Garage floors: 5-6 inches
- Foundation walls: 8-10 inches
Concrete Strength Ratings
- 2500-3000 psi: Residential slabs, walkways
- 3500 psi: Driveways, patios (standard)
- 4000 psi: Garage floors, commercial
- 5000+ psi: Heavy industrial, structural
- Air-entrained: For freeze-thaw climates
Bag Coverage Estimates
| Bag Size | Cubic Feet | 4″ Thickness |
|---|---|---|
| 40 lb | 0.30 ft³ | 0.90 sq ft |
| 60 lb | 0.45 ft³ | 1.35 sq ft |
| 80 lb | 0.60 ft³ | 1.80 sq ft |
| 90 lb | 0.68 ft³ | 2.04 sq ft |
Rebar Specifications
- #3 rebar (3/8″): Light duty, 4″ slabs
- #4 rebar (1/2″): Standard slabs, driveways
- Spacing: 12-18″ on center each way
- Cover: 2″ from bottom, 1″ from edges
- Alternative: 6×6 W2.9 welded wire mesh
Frequently Asked Questions
How much does a yard of concrete cover at different thicknesses?
One cubic yard of concrete (27 cubic feet) covers:
| Thickness | Coverage Area | Common Application |
|---|---|---|
| 2 inches | 162 sq ft | Topping layers, overlays |
| 3 inches | 108 sq ft | Garden walks, shed floors |
| 4 inches | 81 sq ft | Patios, sidewalks, garage floors |
| 5 inches | 65 sq ft | Residential driveways |
| 6 inches | 54 sq ft | Heavy-duty driveways, slabs |
| 8 inches | 40.5 sq ft | Foundation walls, heavy industrial |
Note: These are theoretical coverages. Actual yield is 2-5% less due to consolidation and uneven subgrade.
Should I use ready-mix concrete or bags for my project?
Ready-Mix Concrete (Truck Delivery):
- Best for: Projects over 1 cubic yard (27 bags or more)
- Advantages: Consistent mix quality, faster placement, professional finish
- Minimum order: Typically 3-4 cubic yards (varies by supplier)
- Cost: $120-$200 per yard plus delivery fee ($75-$150)
Bagged Concrete (DIY):
- Best for: Projects under 1 cubic yard, repairs, small projects
- Advantages: No minimum order, easier for small jobs, can mix as needed
- Disadvantages: Labor intensive, inconsistent mixing, higher cost per yard
- Cost: $4-$8 per 60lb bag = $200-$400 per cubic yard
Break-even point: Typically around 1-1.5 cubic yards. For larger projects, ready-mix is more economical.
How do I prepare the subgrade for a concrete slab?
Proper subgrade preparation is critical for slab longevity:
- Excavation: Dig 6-8 inches deeper than final slab thickness
- Compaction: Use a plate compactor to achieve 95% compaction
- Base material: Add 4-6 inches of compacted gravel (3/4″ minus)
- Grading: Slope base away from structures (1/4″ per foot minimum)
- Moisture control: Dampen base before concrete placement (not wet)
- Vapor barrier: Install 6-mil poly sheeting for interior slabs
Testing: Perform “proctor test” or simple footprint test – no visible depression when walking on compacted base.
What’s the difference between concrete and cement?
These terms are often confused but refer to different materials:
Cement:
- Powdered binding agent (Portland cement)
- Typically 10-15% of concrete mix by volume
- Never used alone for structural applications
- Types: Type I (general purpose), Type II (moderate sulfate resistance), Type III (high early strength), Type IV (low heat), Type V (high sulfate resistance)
Concrete:
- Composite material made of cement, aggregates, and water
- Typical mix: 10-15% cement, 60-75% aggregates (sand & gravel), 15-20% water
- May include admixtures (air-entrainers, retarders, accelerators)
- Gains strength through hydration chemical reaction
Mortar: Cement + sand + water (for masonry work)
Grout: Cement + sand + water (higher water content for flowability)
How long does concrete take to cure and when can I use it?
Concrete curing is a chemical process, not just drying:
| Time After Pour | Curing Stage | What You Can Do |
|---|---|---|
| 2-4 hours | Initial set | Finish surface (float, trowel) |
| 24 hours | Walking strength | Walk carefully, remove forms |
| 3-7 days | 50-70% strength | Light foot traffic only |
| 7 days | 70% design strength | Light vehicle traffic (cars) |
| 28 days | Full design strength | Full use, heavy vehicles |
| 90 days | Maximum strength | Complete curing |
Curing Best Practices:
- Water curing: Keep surface moist for 7 days (spray, cover with plastic)
- Curing compounds: Membrane-forming sprays for large projects
- Temperature: Ideal range 50-85°F. Below 40°F requires insulation/heat
- Never let concrete freeze during first 24 hours
What reinforcement should I use for my concrete slab?
Proper reinforcement prevents cracking and increases strength:
Rebar (Reinforcing Bar):
- #3 (3/8″): 4″ slabs, light duty (0.376 lbs/ft)
- #4 (1/2″): Standard for 4-6″ slabs (0.668 lbs/ft)
- #5 (5/8″): Heavy duty, driveways (1.043 lbs/ft)
- Spacing: 12-18″ on center both directions
- Placement: Mid-depth or slightly above center
- Chairs: Use rebar chairs to maintain position
Welded Wire Mesh (WWM):
- 6×6 W1.4/W2.0: Light residential (0.05/0.07 lbs/sq ft)
- 6×6 W2.9: Standard for 4″ slabs (0.10 lbs/sq ft)
- 4×4 W2.9/W4.0: Heavy duty (0.15/0.20 lbs/sq ft)
- Placement: Pull up to upper 1/3 during pour
Fiber Reinforcement:
- Micro-synthetic fibers: Reduce plastic shrinkage cracks
- Macro-synthetic fibers: Structural reinforcement
- Steel fibers: Industrial floors, heavy-duty
- Dosage: 0.5-1.5 lbs per cubic yard
Rule of thumb: For residential slabs, use #4 rebar at 16″ on center OR 6×6 W2.9 mesh.