How to calculate a concrete slab
Slabs for patios, sheds, garage floors, and pads are the most common ready mix order, and the thickness matters as much as the footprint.
- Measure the length and width of the finished slab.
- Choose a thickness: 4 inches for patios and walkways, 5 to 6 inches for driveways and garage floors.
- Enter the numbers and keep 5 to 10 percent extra for the uneven subgrade.
- Order from a ready mix plant, or buy bags if the total is under about 1 cubic yard.
Worked example
For a typical concrete slab with length 12 ft, width 12 ft, depth 4 in, the volume is 48.00 cubic feet. Divide by 27 for 1.78 cubic yards. With a 10 percent allowance, plan for 1.96 cubic yards.
In concrete, 1.78 cubic yards weighs about 3.60 US tons at a planning value of 4,050 pounds per cubic yard.
Concrete Slab sizes and quantities
These common sizes are calculated with the same formula as the tool above, so you can find your number quickly.
| Size | Cubic feet | Cubic yards | With 10 percent extra | US tons |
|---|---|---|---|---|
| 10 by 10 ft, 4 in | 33.33 | 1.23 | 1.36 | 2.50 |
| 12 by 12 ft, 4 in | 48.00 | 1.78 | 1.96 | 3.60 |
| 12 by 16 ft, 4 in | 64.00 | 2.37 | 2.61 | 4.80 |
| 20 by 20 ft, 6 in | 200.00 | 7.41 | 8.15 | 15.00 |
Tips
- Compact and level the base before you measure the depth.
- A thicker edge, called a thickened edge, adds volume that the flat formula misses.
- Bring a helper or two for the pour, because concrete starts setting quickly.
Common mistakes to avoid
- Measuring the slab size but forgetting the thickness is in inches, not feet.
- Ordering the exact number with no extra and running short at the end of the pour.
- Skipping the gravel base, which often adds 4 inches of material to your total project order.
These figures are planning estimates. Real material varies in moisture, size, and compaction, so confirm weights and order quantities with your supplier.
Frequently asked questions
How many cubic yards do I need for a concrete slab?
Multiply the dimensions in feet and divide by 27. The example above, length 12 ft, width 12 ft, depth 4 in, needs 1.78 cubic yards before waste.
How much extra should I order?
Add 5 to 10 percent for waste. Add more if the ground is uneven or the material is loose.
Can I enter inches and feet together?
Yes. Each box has its own unit selector, so you can mix feet and inches.
How much does concrete slab material weigh?
1.78 cubic yards of concrete weighs about 3.60 US tons as a planning value.