Concrete Slab & Bag Calculator
Comprehensive Guide to Calculating Concrete for Slabs and Footings
Ordering too little concrete can ruin a project, while ordering too much is a waste of money. Our professional Concrete Calculator helps you estimate the exact volume of concrete needed for your project, whether you are pouring a patio, a driveway, or a garden path.
How to Calculate Concrete Volume
Calculating the volume of a rectangular slab is a straightforward geometric calculation. The formula is:
Length (ft) × Width (ft) × Thickness (ft) = Total Cubic Feet
Since most people measure thickness in inches, you must divide the inches by 12 to convert them to feet before multiplying. To find the cubic yardage (which is how most ready-mix concrete is sold), divide the total cubic feet by 27.
Steps for an Accurate Estimate:
- Measure Twice: Ensure your length and width measurements are exact.
- Check Depth: Subgrades are rarely perfectly level. If your slab varies between 3.5 and 4.5 inches, use 4.5 as your calculation depth to ensure coverage.
- Include Waste: We recommend adding a 10% waste factor for spillage, uneven subgrade, and forms bulging.
Concrete Bag Requirements
If you are mixing concrete by hand using pre-mixed bags, you need to know the yield of each bag size. On average:
- 80lb Bag: Yields approximately 0.60 cubic feet.
- 60lb Bag: Yields approximately 0.45 cubic feet.
- 40lb Bag: Yields approximately 0.30 cubic feet.
Example Calculation
If you have a patio that is 10 feet long, 10 feet wide, and 4 inches thick:
- Convert 4 inches to feet: 4 / 12 = 0.333 ft.
- Multiply: 10 × 10 × 0.333 = 33.33 Cubic Feet.
- Add 10% waste: 33.33 × 1.10 = 36.66 Cubic Feet.
- Divide by 27 for yards: 36.66 / 27 = 1.36 Cubic Yards.
- For 80lb bags: 36.66 / 0.60 = 62 Bags.
When to Use Ready-Mix vs. Bags
As a general rule of thumb, if your project requires more than 1 cubic yard (roughly 45-50 bags of 80lb concrete), it is usually more cost-effective and physically easier to order a ready-mix truck. Hand-mixing 50 bags of concrete is labor-intensive and makes it difficult to achieve a uniform finish before the first batches start to set.
Tips for a Successful Pour
Always ensure your forms are sturdy and well-staked. For driveways or heavy-load areas, consider using rebar or wire mesh to increase tensile strength. If pouring in hot weather, keep the concrete moist during the curing process to prevent cracking.