Slab calculator
Concrete Slab Calculator
Estimate concrete volume for patios, driveways, shed pads, sidewalks, and rectangular slabs. Get instant cubic yards, bag count, waste factor, and cost estimate.
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How to Calculate Concrete for a Slab
Calculating concrete for a slab is straightforward. Measure the length, width, and depth of your slab, then multiply these dimensions to get the volume in cubic feet. Divide by 27 to convert to cubic yards, which is how ready-mix concrete is typically sold.
For example, a 20-foot by 10-foot patio with a 4-inch depth requires:
Volume = 20 × 10 × (4/12) = 66.7 cubic feet = 2.47 cubic yards
Always add a waste factor of 5-10% to account for spillage, over-excavation, and variations in subgrade. This ensures you order enough concrete to complete your project without delays.
Common Slab Thickness Guidelines
- Patios and walkways: 4 inches (100mm) is standard for residential use
- Driveways: 4-6 inches depending on vehicle weight; 6 inches recommended for trucks
- Shed pads: 4 inches is typical for light structures
- Garage floors: 4-6 inches with reinforcement
Ready-Mix vs Bagged Concrete
For slabs larger than 1 cubic yard (27 cubic feet), ready-mix delivery is usually more cost-effective and ensures consistent quality. For smaller projects, bagged concrete from a home center works well. An 80 lb bag yields approximately 0.6 cubic feet of mixed concrete.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
A 10x10 foot slab at 4 inches deep requires approximately 1.23 cubic yards (33.3 cubic feet) of concrete. With a 10% waste factor, order 1.35 cubic yards or round up to 1.5 yards for ready-mix delivery.
How thick should a concrete patio be?
Standard residential patios are 4 inches thick. If you plan to place heavy items like a hot tub or large planters, consider 5-6 inches with reinforcement. Always check local building codes for specific requirements in your area.
How many bags of concrete for a 10x10 slab?
A 10x10 slab at 4 inches deep requires about 1.23 cubic yards. Using 80 lb bags (0.6 cubic feet each), you would need approximately 56 bags. With 10% waste, plan for 62 bags.
What is the waste factor for concrete slabs?
A 5-10% waste factor is standard for slab work. Use 5% for simple rectangular slabs with good access, 10% for complex shapes, poor access, or uneven subgrade. Professional contractors often use 10% as a standard practice.