How to use the concrete slab calculator
- Enter the slab dimensions in feet for length and width, and inches for thickness.
- Set the waste factor (5-10% is typical) to cover spillage and uneven subgrade.
- Choose the bag size you will buy (80 lb, 60 lb, or 40 lb).
- Optionally add a cost per cubic yard to see a material cost estimate.
- Read the results: volume in cubic feet and yards, yards with waste, number of bags, and total cost.
How the volume is calculated
The slab volume in cubic feet is length x width x (thickness / 12). For a 10 ft x 10 ft slab at 4 in thick: 10 x 10 x (4 / 12) = 33.33 ft3. Cubic yards are cubic feet divided by 27: 33.33 / 27 = 1.23 yd3. The waste factor multiplies the yardage by 1 + waste/100. This concrete slab calculator performs the conversion automatically so you do not have to remember the factor of 27.
Bag yields and bag count
Pre-mixed concrete bags are sold by weight with a standard yield: 80 lb = 0.60 ft3, 60 lb = 0.45 ft3, 40 lb = 0.30 ft3. The tool divides the waste-adjusted cubic feet by the selected bag yield and rounds up to the next whole bag. For the 10 x 10 ft example with 10% waste: 33.33 x 1.10 = 36.66 ft3; 80 lb bags: 36.66 / 0.60 = 61.1 -> 62 bags. Yields are typical manufacturer figures and vary slightly by brand and mixing water.
Common slab sizes reference
| Area (ft2) | Thickness (in) | Yards (no waste) | Yards (10% waste) |
|---|---|---|---|
| 100 | 4 | 1.23 | 1.36 |
| 200 | 4 | 2.47 | 2.72 |
| 400 | 6 | 7.41 | 8.15 |
| 600 | 5 | 9.26 | 10.19 |
When to use this concrete slab calculator
- Planning a patio, driveway, sidewalk, or footing.
- Estimating how many bags to buy from a home center.
- Getting a quick material cost before calling a ready-mix supplier.
- Comparing bagged concrete versus delivered ready-mix for cost savings.
Common mistakes
- Forgetting to convert thickness from inches to feet.
- Omitting a waste factor: you will run short on pour day.
- Using the wrong bag yield (for example, assuming 80 lb = 0.5 ft3).
- Entering dimensions in meters or centimeters: the tool expects feet and inches.
- Not accounting for thickened edges or footings that require extra concrete.
Background on concrete ordering
Ready-mix concrete is sold by the cubic yard. One cubic yard equals 27 ft3. Most residential projects order a little extra (5-10%) to account for spillage, form deflection, and uneven subgrade. Bagged concrete is convenient for small jobs; a typical 80 lb bag yields 0.6 ft3, so 45 bags make one cubic yard. For larger pours, ready-mix delivery is cheaper and ensures consistent quality. A concrete slab calculator helps you decide which route makes sense for your project size.
Reinforcement and subgrade preparation
Volume calculations cover the concrete only. Most slabs need reinforcement: welded wire mesh or rebar placed at mid-depth. A compacted gravel subbase (4-6 in) improves drainage and reduces cracking. Control joints spaced at 24-30 times the slab thickness control where cracks appear. These items add cost but are essential for durability.
Worked examples
| Example | Input | Result |
|---|---|---|
| 10 x 10 ft patio, 4 in thick | Length (ft): 10; Width (ft): 10; Thickness (in): 4; Waste factor (%): 10; Bag size: 80; Cost per cubic yard ($): 125 | Cubic feet: 33.33; Cubic yards: 1.23; Cubic yards with waste: 1.36; Bags needed (80 lb): 62; Estimated cost: $170.00 |
| 20 x 12 ft driveway, 6 in thick | Length (ft): 20; Width (ft): 12; Thickness (in): 6; Waste factor (%): 5; Bag size: 60; Cost per cubic yard ($): 130 | Cubic feet: 120.00; Cubic yards: 4.44; Cubic yards with waste: 4.67; Bags needed (60 lb): 280; Estimated cost: $607.10 |
| Small footing, 3 x 3 ft, 8 in thick | Length (ft): 3; Width (ft): 3; Thickness (in): 8; Waste factor (%): 10; Bag size: 40; Cost per cubic yard ($): 0 | Cubic feet: 6.00; Cubic yards: 0.22; Cubic yards with waste: 0.24; Bags needed (40 lb): 22; Estimated cost: $0.00 |
| Large slab, 30 x 40 ft, 5 in thick | Length (ft): 30; Width (ft): 40; Thickness (in): 5; Waste factor (%): 10; Bag size: 80; Cost per cubic yard ($): 120 | Cubic feet: 500.00; Cubic yards: 18.52; Cubic yards with waste: 20.37; Bags needed (80 lb): 917; Estimated cost: $2,444.40 |
FAQ
How accurate is the bag count?
The calculator uses manufacturer-specified yields (0.60, 0.45, 0.30 ft3). Real-world yield can vary slightly with mixing water, so buy one extra bag for safety.
Can I use metric units?
This tool expects feet and inches. Convert meters to feet (1 m = 3.28084 ft) before entering values.
What waste factor should I use?
5% for well-prepared, level subgrade; 10% for typical residential pours; up to 15% for difficult sites or complex forms.
Does the cost estimate include delivery or labor?
No. The cost field only multiplies the waste-adjusted cubic yards by the per-yard price you enter. Add delivery, pump, and labor separately.
Why does the tool show cubic feet and cubic yards?
Bag yields are given in cubic feet, while ready-mix is sold by the cubic yard. Showing both lets you compare bagged vs. delivered concrete.
Can I calculate multiple slabs at once?
This concrete slab calculator handles one rectangular slab per run. For multiple areas, calculate each separately and sum the yardage.
Is the concrete slab calculator free to use?
Yes, the tool is completely free and runs in your browser without any server calls or data collection.