Free Concrete Calculator
Use this concrete calculator to see how many cubic yards or bags of concrete you need for a slab, patio or driveway.
Concrete is ordered by the cubic yard from a ready-mix plant, or bought in bags that you mix yourself. A typical 80 lb bag makes about 0.60 cubic feet, so one cubic yard takes about 45 of them.
Enter your slab size and thickness. For footings and post holes, use the dedicated footing and post hole calculators.
Concrete slab calculator example
A 10 × 10 ft slab, 4 in thick: 100 sq ft × 4 in ÷ 12 = 33.3 cu ft, plus 10% extra = 36.7 cu ft. Divide by 27 to get 1.36 cubic yards, which weighs about 2.72 tons, or 62 × 80 lb bags (0.60 cu ft).
How deep should concrete be?
| Use | Thickness | Notes |
|---|---|---|
| Walkway or patio | 4 in | |
| Shed slab | 4 in | |
| Driveway (cars) | 4 to 6 in | Check local code and soil conditions. |
| Garage floor or heavier loads | 5 to 6 in | Often with reinforcement. |
Bags of concrete per cubic yard
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb bag | 0.3 cu ft | 90 |
| 60 lb bag | 0.45 cu ft | 60 |
| 80 lb bag | 0.6 cu ft | 45 |
Buying tips
- Bags make sense up to about 1 cubic yard. Beyond that you would be mixing 45+ heavy bags, and ready-mix delivery is usually easier and often cheaper.
- Many ready-mix plants charge a short-load fee for small orders. Ask for the minimum order and fee when you get a quote.
- Order a little extra. Running short in the middle of a pour leaves a cold joint, which is far worse than a few dollars of leftover concrete.
- The bag yields used here (0.30, 0.45 and 0.60 cu ft for 40, 60 and 80 lb) are typical. Check the yield printed on your brand's bag.
How much does concrete weigh?
A cubic yard of concrete typically weighs 3,800 to 4,200 lb (1.9 to 2.1 tons). The calculator uses 4,000 lb (2 tons) per cubic yard. Moisture and grading change the weight, so if your supplier sells by the ton, ask for their figure.
Frequently asked questions
How many bags of concrete do I need?
Work out the volume in cubic feet (length × width × thickness in feet), add about 10%, then divide by the bag yield: 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb or 0.30 for 40 lb. The concrete bag calculator does this for every bag size, including from a volume you already know.
How many bags of concrete are in a yard?
About 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb.
How many yards of concrete do I need?
Multiply length × width (ft) × thickness (in) ÷ 12 to get cubic feet, then divide by 27. Add 5 to 10% and round up to the next quarter yard when ordering ready-mix.
How many square feet does a yard of concrete cover?
About 81 sq ft at 4 in thick (each square foot needs 1/3 cu ft), 65 sq ft at 5 in and 54 sq ft at 6 in.
How do I calculate the volume of concrete for odd shapes?
Split the area into rectangles, circles and triangles and add each one in the calculator. It adds the volumes for you.
Is this a cement calculator too?
Yes. People often say cement when they mean concrete. Cement is one ingredient of concrete, mixed with sand, gravel and water. Bagged concrete mix already contains the cement, so this calculator gives the number of bags of mix to buy.
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick: 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cubic yards. With 10% extra, order about 1.4 yards, or 62 bags of 80 lb.
How thick should a concrete slab be?
About 4 inches for patios, walkways and sheds, and 5 to 6 inches for driveways and garage floors.
How much does a cubic yard of concrete weigh?
About 4,000 lb (2 tons).
How accurate is this concrete calculator?
The concrete calculator gives the exact volume for the measurements you enter, then adds 10% extra (you can change it). Weights use a typical 2 tons per cubic yard; real loads vary with moisture, so confirm with your supplier if you buy by the ton. Measure carefully: an error of one inch in depth changes a 3 inch layer by a third.
How the concrete calculator works
Volume = area × thickness. Each area is worked out from its shape (length × width, π × radius², or ½ × base × height), converted to square feet, multiplied by the thickness in feet, and added up. Cubic feet ÷ 27 = cubic yards.The extra percentage is then added, and bag counts are rounded up to whole bags. See how we calculate for every figure we use.