Concrete Slab Calculator
Standard patio is 4 inches
0% (Exact)
5% (Recommended)
10% (Safe)
Calculation Results
0
Cubic Yards
0
Cubic Feet
0
80lb Bags
0
60lb Bags
*Note: Bag counts are estimates based on standard premix yield (approx 0.60 cu ft per 80lb bag).
How to Estimate Concrete for a Slab
Calculating the correct amount of concrete is crucial for any construction project, whether you are pouring a simple patio, a driveway, or a foundation for a shed. Ordering too little results in expensive "short load" fees or cold joints in your slab, while ordering too much is a waste of money.
The standard formula for calculating concrete volume is:
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Since concrete is sold by the Cubic Yard, you must divide your total cubic feet by 27 (since there are 27 cubic feet in one cubic yard).
Standard Slab Thickness Guide
Choosing the right thickness is just as important as the measurements:
- 4 Inches: Standard for residential sidewalks, patios, and garage floors for passenger cars.
- 5-6 Inches: Recommended for driveways that handle heavier vehicles, trucks, or RVs.
- 6+ Inches: Heavy-duty foundations or areas supporting significant structural weight.
Bags vs. Ready-Mix Truck
Once you have your calculation, you need to decide how to buy the concrete:
- Premix Bags (60lb or 80lb): Best for small projects under 1 cubic yard (approx. 45-60 bags). It requires manual mixing and is labor-intensive.
- Ready-Mix Truck: Best for projects over 1 cubic yard. The concrete arrives pre-mixed, ensuring consistency and saving immense physical labor.
The Importance of the "Waste Factor"
Construction sites are rarely perfect. Uneven subgrades (the ground beneath the slab), spillage during pouring, and form spreading can all increase the volume needed. We highly recommend adding a 5% to 10% safety margin to your order. It is far cheaper to dump a small amount of extra concrete than to halt a pour to buy a few more bags.
Steps for a Successful Pour
- Measure Twice: Accurate measurements of your formwork are the foundation of your calculation.
- Prepare the Subgrade: Ensure the ground is compacted and level. If the ground is uneven, your slab thickness will vary, throwing off your math.
- Calculate Volume: Use the calculator above to determine total cubic yards and bag counts.
- Account for Reinforcement: While rebar or wire mesh takes up some volume, it is negligible for the volume calculation. Do not subtract it.