How this calculator works
Concrete volume is just the slab's length × width × thickness. We convert the thickness to feet, multiply, add your waste allowance, and divide by 27, because one cubic yard is 27 cubic feet.
Bag counts divide the total cubic feet by what one bag yields once mixed: about 0.30 ft³ for a 40 lb bag, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb. The weight estimate uses 150 lb per cubic foot, typical for normal-weight concrete (about 4,050 lb per cubic yard).
Worked example: a 12 × 10 ft patio, 4 in thick, with no waste: 12 × 10 × 0.333 = 40 ft³, which is 1.48 yd³ or 67 bags of 80 lb.
Concrete needed for common slab sizes
Figures are for a square-cornered slab with no waste added. Add 5 to 10% for real-world conditions, or use the calculator above.
| Slab size | 4 in thick | 5 in thick | 6 in thick | 80 lb bags (4 in) |
|---|---|---|---|---|
| 4 × 4 ft | 0.20 yd³ | 0.25 yd³ | 0.30 yd³ | 9 |
| 6 × 6 ft | 0.44 yd³ | 0.56 yd³ | 0.67 yd³ | 20 |
| 8 × 8 ft | 0.79 yd³ | 0.99 yd³ | 1.19 yd³ | 36 |
| 10 × 10 ft | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ | 56 |
| 10 × 12 ft | 1.48 yd³ | 1.85 yd³ | 2.22 yd³ | 67 |
| 12 × 12 ft | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ | 80 |
| 12 × 16 ft | 2.37 yd³ | 2.96 yd³ | 3.56 yd³ | 107 |
| 16 × 16 ft | 3.16 yd³ | 3.95 yd³ | 4.74 yd³ | 143 |
| 20 × 20 ft | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ | 223 |
| 24 × 24 ft | 7.11 yd³ | 8.89 yd³ | 10.67 yd³ | 320 |
Tips before you buy
- Order 5 to 10% extra. A short pour leaves a cold joint, and a few extra bags cost far less than a failed slab.
- Past roughly one cubic yard (about 45 bags of 80 lb), price out ready-mix. Many suppliers add a short-load fee for small orders, so ask what their minimum is.
- Measure the thickness at the thickest point if your subgrade is uneven, or average several measurements.
- Have help, tools and a plan ready before the truck arrives. You typically have limited unloading time before extra charges apply.
Quick answers by size
Tools & materials for this project
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Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45. One cubic yard is 27 ft³ and an 80 lb bag yields roughly 0.60 ft³ (27 ÷ 0.60 = 45). It takes about 60 bags of 60 lb or 90 bags of 40 lb.
How thick should a concrete slab be?
Most residential patios, walkways and shed floors are 4 inches thick. Driveways and areas that carry vehicles are typically 5 to 6 inches. Your local code and soil conditions can change that, so check before you pour.
How much does a cubic yard of concrete weigh?
Around 4,000 lb, or roughly 2 tons. Normal-weight concrete is about 145 to 150 lb per cubic foot, and a cubic yard has 27 cubic feet.
Do I need rebar or wire mesh?
Reinforcement does not stop cracks, but it holds them tight when they happen. Many residential slabs use wire mesh or rebar on a grid; see the rebar calculator to count bars. Heavy loads need an engineered design.
Should I add extra concrete for waste?
Yes, 5 to 10% is standard. Forms bow, subgrades settle, and you will spill some. Running short mid-pour is far more expensive than ordering a bit extra.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.