Use this free concrete slab calculator to find how many cubic yards or bags of concrete you need for a patio, shed base, driveway or round pad, with waste and cost included.
Home projects · feet or metersConcrete Slab Calculator
Bag counts use typical yields: 0.30, 0.45 and 0.60 cubic feet. So, check the yield printed on your bag.
Our free concrete slab calculator works out exactly how much concrete to order, so you don’t run short halfway through a pour. To begin with, enter the length, width and thickness of your slab. Then add a little extra for waste. After that, the calculator shows the volume in cubic yards, cubic feet and cubic meters, plus the number of bags you need.
How the Concrete Slab Calculator Works
Concrete is sold by volume, so the calculator multiplies the slab’s area by its thickness. Since thickness is usually given in inches, it first converts inches into feet.
Firstly, it multiplies length by width, or uses πr² for a round slab.
Then it multiplies the area by the thickness in feet.
Finally, it divides by 27 for cubic yards and adds your waste allowance.
The formula
Cubic yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27
Concrete Slab Calculator Examples
For example, here are three common projects with 10% extra, so you can see the maths in action.
To illustrate, this is a typical base for a garden shed.
- Firstly: 10 × 10 × 4 ÷ 12 33.33 ft³
- Then: plus 10% 36.67 ft³
- Also: 80 lb bags 62
Similarly, this patio uses ready-mix at $175 per cubic yard.
- Firstly: 12 × 16 × 4 ÷ 12 64 ft³
- Then: plus 10% 70.4 ft³
- Also: cost $456.30
In contrast, this round pad is for a hot tub or fire pit.
- Firstly: π × 4² × 4 ÷ 12 16.76 ft³
- Then: plus 10% 18.43 ft³
- Also: 80 lb bags 31
Concrete Slab Sizes and Volumes
Overall, this table shows cubic yards for common slab sizes without waste. In addition, the last column shows 80 lb bags for a 4-inch slab, including 10% extra.
| Slab size | 4 in (yd³) | 6 in (yd³) | 80 lb bags (4 in) |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.85 | 62 |
| 10 × 12 ft | 1.48 | 2.22 | 74 |
| 12 × 12 ft | 1.78 | 2.67 | 88 |
| 12 × 16 ft | 2.37 | 3.56 | 118 |
| 16 × 16 ft | 3.16 | 4.74 | 157 |
| 20 × 20 ft | 4.94 | 7.41 | 245 |
| 20 × 24 ft | 5.93 | 8.89 | 294 |
| 24 × 24 ft | 7.11 | 10.67 | 352 |
| 30 × 30 ft | 11.11 | 16.67 | 550 |
How Thick Should a Slab Be?
- Walkways and patios: for instance, 4 inches is the usual standard.
- Driveways: similarly, 4 to 5 inches suits cars, while 6 inches suits heavy vehicles.
- Garage and shed floors: also, 4 to 6 inches, depending on the load.
Bags or Ready-Mix Concrete?
Generally, bagged concrete works well for small jobs under about one cubic yard. However, mixing 60 or more bags by hand is slow, and the concrete may set unevenly. For larger slabs, ready-mix delivery is usually cheaper per yard, although most suppliers charge a short-load fee for small orders.
Tips for Pouring a Slab
Before pouring, compact a 4 to 6-inch gravel base, so the slab doesn’t crack as the ground settles. Also, cut control joints every 8 to 12 feet to guide cracking. Finally, keep the slab damp for about a week while it cures, as the Portland Cement Association recommends for stronger concrete.
How to Use This Calculator
- To begin with, choose feet or meters and the slab shape.
- Then enter the length and width, or the diameter for a round slab.
- After that, enter the thickness, the number of slabs and a waste allowance.
- Finally, press Calculate concrete to see the yards, bags and cost.
More Home Project Tools
These tools also help with home and garden jobs. Besides, you can browse all our construction calculators.
Concrete Slab Calculator FAQs
How many bags of concrete do I need for a 10×10 slab?
For a 4-inch slab, you need about 1.23 cubic yards, or 1.36 with 10% extra. Therefore, that works out to about 62 bags of 80 lb concrete.
How much does a concrete slab cost?
Ready-mix concrete often costs about $150 to $200 per cubic yard before delivery fees. However, a finished slab with labor usually costs much more per square foot.
How thick should a driveway slab be?
In general, most driveways are 4 to 5 inches thick for cars. However, 6 inches is better for heavy trucks or RVs.
How much extra concrete should I order?
Generally, add 5% to 10% for spills, uneven ground and form flex. So, running short in the middle of a pour is far worse than having a little left over.
How many cubic feet are in a cubic yard?
A cubic yard holds 27 cubic feet, because it is 3 feet long, wide and deep. In other words, that is about 0.76 cubic meters.
Do I need rebar or mesh in a slab?
For example, wire mesh or fiber is common for patios and walkways. Meanwhile, driveways and structural slabs usually need rebar, so check your local building code.
Disclaimer: Overall, this calculator gives an estimate for planning only. Also, ground conditions, forms and local codes affect the amount you need, so confirm large orders with your supplier or contractor.