How much concrete do I need?
Choose your shape, enter the size and the calculator returns cubic yards for a ready-mix order and the number of 80 lb, 60 lb or 40 lb bags for a DIY pour. It adds a waste allowance so you do not run short mid-pour.
Concrete calculator formula
Every concrete estimate is volume. Convert everything to feet, multiply, then divide cubic feet by 27 to get cubic yards.
| Shape | Cubic feet |
|---|---|
| Slab, patio, driveway | length (ft) x width (ft) x thickness (in) / 12 |
| Triangle slab | 0.5 x base (ft) x height (ft) x thickness (in) / 12 |
| Footing | length (ft) x width (in) x depth (in) / 144 |
| Round column | 3.1416 x (diameter in / 24)² x height (ft) |
Concrete calculator for square feet
If you know the area in square feet, multiply by thickness in inches and divide by 12 to get cubic feet. This table shows how much concrete 100 square feet needs, and how much area one cubic yard covers.
| Thickness | Cubic yards per 100 sq ft | Sq ft covered by 1 yard |
|---|---|---|
| 4 in | 1.23 | 81 |
| 5 in | 1.54 | 65 |
| 6 in | 1.85 | 54 |
Concrete calculator for yards and bags
One cubic yard is 27 cubic feet. That equals about 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix. For roughly one yard or more, compare the price of ready-mix delivery against bags, and ask about short-load fees.
Quick reference: bags for common slabs (4 in thick, no waste)
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 4 x 4 ft | 0.20 | 9 |
| 8 x 8 ft | 0.79 | 36 |
| 10 x 10 ft | 1.23 | 56 |
| 12 x 12 ft | 1.78 | 80 |
Mixing your own? Try the 1:2:4 mix calculator
Our concrete mix ratio calculator turns a volume into cement, sand and gravel for 1:2:3, 1:2:4 and other ratios.
Tips before you order
- Measure the dug-out depth in several spots. Uneven ground is the most common reason for running short.
- Round up and add 5 to 10% waste.
- Have forms, base and helpers ready before the truck arrives.
- Read how much concrete a patio needs and how long concrete takes to cure.
Frequently asked questions
How do I calculate concrete in cubic yards?
Multiply length (ft) x width (ft) x thickness (ft) to get cubic feet, then divide by 27. If thickness is in inches, divide it by 12 first. Example: 10 x 10 ft at 4 in = 33.3 cubic feet = 1.23 cubic yards.
How many bags of concrete are in a yard?
About 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix, based on typical yields of 0.60, 0.45 and 0.30 cubic feet per bag. Always check the yield printed on your bag.
How much concrete do I need per square foot?
It depends on thickness. At 4 inches, one square foot needs 0.33 cubic feet. One cubic yard covers about 81 sq ft at 4 in, 65 sq ft at 5 in and 54 sq ft at 6 in.
How do I calculate concrete for a triangle-shaped area?
Area = half of base x height. Multiply by thickness in feet for cubic feet. Choose Triangle-shaped slab in the calculator above, or split an irregular area into rectangles and triangles and add them.
What is a 1:2:4 concrete mix?
It means 1 part cement, 2 parts sand and 4 parts gravel by volume. Use our mix ratio calculator to see how much of each material you need.
How much extra concrete should I order?
Add 5% to 10% for uneven ground, spillage and over-excavation. If the ground is very uneven, lean toward 10%.
Do I need a concrete calculator app?
No. This calculator works in your phone browser. You can save the page to your home screen.
Do bag yields differ by brand?
Yes, slightly. The calculator uses common yields (80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³). Check your bag and round up.
Does the calculator include the gravel base?
No. It estimates concrete only. Gravel, forms, rebar and wire mesh are separate quantities.
Does it support metric units?
Not yet. Inputs are in feet and inches. For reference, 1 cubic meter is about 1.31 cubic yards, and 1 cubic yard is about 0.76 cubic meters.