How Many Bags of Concrete Do I Need?
For a 10 ft x 10 ft slab at 4 inches thick, you need about 33.3 cubic feet of concrete before waste. With a 5% waste factor, that is about 35.0 cubic feet, or roughly 59 bags of 80 lb mix or 78 bags of 60 lb mix. Use the calculator below for your own dimensions.
Concrete Calculator
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Concrete bags by common slab size
These examples include a 5% waste factor and use the same bag yields as the calculator above.
| Slab size | Thickness | Concrete with waste | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 4 ft x 4 ft | 4 in | 5.6 cu ft | 10 | 13 |
| 8 ft x 8 ft | 4 in | 22.4 cu ft | 38 | 50 |
| 10 ft x 10 ft | 4 in | 35.0 cu ft | 59 | 78 |
| 12 ft x 12 ft | 4 in | 50.4 cu ft | 84 | 112 |
Bag yields vary by product. Check the yield printed on the bag before buying, especially when using fast-setting, high-strength, or specialty mixes.
Understanding concrete calculations
Calculating concrete is about finding the volume of the space you need to fill. The basic unit for ordering ready-mix concrete is the cubic yard (27 cubic feet). For smaller projects, you might use bagged concrete mix, sold by weight such as 60 lb or 80 lb bags.
Our calculator above handles the most common shapes. Below are the manual formulas for reference:
- Rectangular Slab/Footing: Length (ft) × Width (ft) × Depth (in) ÷ 12 = Volume (cu ft).
- Wall: Length (ft) × Height (ft) × Thickness (in) ÷ 12 = Volume (cu ft).
- Round Column: π × (Radius in ft)² × Height (ft) = Volume (cu ft). (Radius = Diameter/2, convert inches to feet by dividing by 12).
- Tube/Footing: π × (Radius in ft)² × Depth (ft) = Volume (cu ft).
Remember to add a waste factor of 5-10% to account for spillage, over-excavation, and uneven subgrades.
Ready-mix concrete or bags?
| Project size | Usually better option | Why |
|---|---|---|
| Small repair or post hole | Bags | Easy to transport and mix in small batches. |
| Patio, shed pad, or many footings | Compare both | Bag count can become high enough that delivery may be worth pricing. |
| Over about 1 cubic yard | Ready-mix quote | Mixing dozens of bags is slow and physically demanding; truck delivery may be more practical. |
Related Tools & Guides
Explore our other construction calculators and guides to help plan your project:
- Concrete Calculator - Calculate cubic yards for slabs, footings, walls, and columns.
- Concrete Bag Calculator - Estimate bags directly when you know the bag size.
- Gravel Calculator - Calculate base material for your slab.
- How to Calculate Concrete for a Slab - Work through slab thickness and volume.
Frequently Asked Questions
What is the formula to calculate concrete volume?
The basic formula is Length (ft) × Width (ft) × Depth (in) ÷ 12 = Volume (cubic feet). To convert to cubic yards, divide the cubic feet by 27. For example, a 20ft by 10ft slab at 6 inches deep is (20 × 10 × 6) / 12 = 100 cubic feet. 100 / 27 ≈ 3.7 cubic yards.
How do I calculate concrete for a round column?
For a cylindrical column, use the formula: π × (radius in feet)² × height in feet. π is approximately 3.1416. First, convert the diameter to radius (divide by 2). For a column with a 2-foot diameter (1-foot radius) and 10 feet tall: 3.1416 × (1)² × 10 = 31.416 cubic feet. Divide by 27 to get about 1.16 cubic yards.
Should I order extra concrete, and how much?
Yes, it's standard practice to order 5-10% extra concrete to account for spillage, over-excavation, and slight variations in subgrade. For a project calculated at 4 cubic yards, ordering 4.2 to 4.4 cubic yards is recommended. It's better to have a little extra than to run short.