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.
Assumptions and sources
- Assumptions used: 10% waste added; bag yields of 0.3, 0.45 and 0.6 ft³ for 40, 60 and 80 lb bags; 150 lb per ft³ for the weight.
- Bag yields used: 0.60 ft³ per 80 lb, 0.45 ft³ per 60 lb, 0.30 ft³ per 40 lb. Check your bag, since brands differ slightly.
- Ready-mix is usually ordered in ¼ yard steps. Round up rather than down.
- In metric: an 80 lb bag is about 36 kg, a 60 lb bag about 27 kg and a 40 lb bag about 18 kg.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
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 |
Concrete for other jobs: footings, posts, piers and steps
Each row uses the defaults of the calculator named, with 10% extra. A slab is only one job: footings, fence posts, piers and steps each have their own shape, so use the matching calculator for those. The bags of concrete guide compares 40, 60 and 80 lb bags with ready-mix.
| Job | Concrete with 10% extra | Calculator |
|---|---|---|
| Slab 10 × 10 ft, 4 in thick | 1.36 yd³, or 62 bags of 80 lb | Concrete calculator |
| Footing 40 ft long, 16 in wide, 8 in deep | 1.45 yd³, or 66 bags of 80 lb | Footing calculator |
| 12 fence posts in 10 in holes, 30 in deep | 21 bags of 80 lb | Fence post concrete calculator |
| One 12 in sonotube pier, 4 ft tall | 0.13 yd³, or 6 bags of 80 lb | Sonotube calculator |
| 3 steps, 4 ft wide, 7 in rise, 11 in run | 0.52 yd³, or 24 bags of 80 lb | Concrete steps calculator |
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.
How much concrete do I need for a 10 × 12 slab?
A 10 × 12 ft slab 4 in thick needs 1.63 cubic yards with 10% waste, which is 74 bags of 80 lb or 98 bags of 60 lb. See the 10 × 12 slab page for 5 and 6 inch slabs.
How many cubic yards is a 20 × 20 slab?
At 4 in thick, a 20 × 20 ft slab is 5.43 cubic yards including 10% waste. At that size ready-mix is usually easier than 245 bags of 80 lb; compare both in the bags of concrete guide.
How much concrete do I need for fence posts?
For 12 posts in 10 in holes 30 in deep, with 6 in of gravel at the bottom of each hole, about 21 bags of 80 lb. Hole size and depth change it a lot: see the fence post concrete calculator and the post spacing and depth guide.
How much concrete for a 24 × 24 garage slab?
7.8 cubic yards at 4 in thick or 11.7 yd³ at 6 in, both with 10% extra. Your local code and the loads decide the thickness; the garage slab materials list covers the base, rebar and forms.
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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.