Concrete Calculator for Slabs & Post Holes
Calculate exactly how many bags of Quikrete or Sakrete you need for slabs, patios, or fence posts with 5% waste margin included.
Diagram
Typical driveway: 20-40 ft
Standard width: 10-12 ft
4" walkways, 6" driveways
Scroll down for your answers. For mobile users only.
Quick Tips
Add 5-10% extra for spillage and uneven ground.
Work quickly - concrete begins to set in 45-60 minutes.
Ideal temperature: 50°F to 90°F for pouring.
Calculation Results
Walkway/Sidewalk • 10ft × 10ft × 4in
Bag Requirements (Including 5% Waste)
Consider Ready-Mix Delivery
For 1.3 cubic yards, ready-mix concrete delivery may be more cost-effective and easier than mixing bags.
Complete Concrete Guide
Understanding Concrete Volume Calculations
Calculating concrete volume accurately is essential for any construction project, whether you're pouring a small patio or a large driveway. Our concrete calculator uses industry-standard formulas to ensure accuracy:
For Rectangular Slabs:
Formula: Length (ft) × Width (ft) × Thickness (in) ÷ 12 = Cubic Feet
Example: A 10×10 patio at 4-inch thickness requires: 10 × 10 × (4÷12) = 33.33 cubic feet
For Cylindrical Post Holes:
Formula: π × Radius² (ft) × Depth (ft) = Volume per hole
Example: A 12-inch diameter hole, 24 inches deep: π × (0.5)² × 2 = 1.57 cubic feet
Types of Concrete Mixes
Understanding different concrete mixes is crucial for project success:
Standard Mix (3,000-4,000 PSI)
Most common for residential projects. Ratio: 1 part cement, 2 parts sand, 3 parts gravel. Perfect for driveways, patios, and walkways.
High-Strength (5,000+ PSI)
Used for structural applications, foundations, and heavy-duty industrial floors. Contains less water and more cement for increased strength.
Lightweight Concrete
Uses lightweight aggregates like expanded shale or clay. Perfect for non-structural applications where weight reduction is important.
Bagged Concrete vs. Ready-Mix
Choosing between bagged concrete and ready-mix depends on your project size:
| Project Size | Recommended | Cost Per Yard |
|---|---|---|
| Small (≤ 0.5 yd³) | Bagged Concrete | $150-200 |
| Medium (0.5-2 yd³) | Either Option | $120-180 |
| Large (≥ 2 yd³) | Ready-Mix Delivery | $100-150 |
Frequently Asked Questions
Q: How many 80lb bags make a yard?
A: Approximately 45 bags (80lb each) equal 1 cubic yard of concrete. Our calculator automatically factors this in.
Q: How thick should a concrete driveway be?
A: Residential driveways should be 4-6 inches thick, with 6 inches recommended for heavy vehicles or frequent use.
Q: How long does concrete take to cure?
A: Concrete reaches 70% strength in 7 days, full strength in 28 days, but continues to strengthen for years.
Professional Tip from Experts
"Always order 10-15% more concrete than calculated. It's cheaper to have a little extra than to run short during a pour. Leftover concrete can be used for stepping stones or small projects."
Step-by-Step Concrete Pouring Guide
- Site Preparation: Clear area, remove vegetation, compact soil
- Formwork Setup: Build sturdy forms with 2×4 or 2×6 lumber
- Reinforcement: Install rebar or wire mesh for structural integrity
- Mixing: Follow manufacturer's water ratio (usually 4-6 quarts per 80lb bag)
- Pouring: Work in sections, avoiding segregation of materials
- Finishing: Screed, float, trowel, and edge for smooth finish
- Curing: Keep moist for 7 days for maximum strength
Seasonal Considerations
Concrete pouring is highly weather-dependent. Here's what to consider:
Summer Pouring
Early morning pours recommended to avoid rapid drying. Use evaporation retarders and cure properly with water or curing compounds.
Winter Pouring
Temperatures must remain above 40°F for 48 hours. Use insulating blankets and cold-weather concrete mixes with accelerators.
Cost-Saving Tips
Buy in Bulk
Order full pallets or truckloads when possible for better pricing.
Plan Ahead
Schedule deliveries in advance to avoid rush charges.
Safety Considerations
- Skin Protection: Wear gloves and boots - wet concrete is caustic
- Eye Protection: Wear safety glasses when mixing
- Respiratory Protection: Use masks when working with dry mix
- Lifting Safety: Use proper lifting techniques for heavy bags