Concrete Calculator
A concrete calculator works out how much concrete you need by multiplying the area of your pour by its thickness, then converting to cubic yards (the unit concrete is sold in). Enter your dimensions below to get the cubic yards, the number of pre-mix bags, and an estimated cost.
Depth is usually entered in inches; the engine converts internally.
- 80 lb bags
- 0
- 60 lb bags
- 0
Enter your dimensions to see the result.
Estimate only. Material yields and prices vary by supplier and region; confirm before ordering.
How Do You Calculate How Much Concrete You Need?
To calculate concrete, you find the volume of the space you are filling. For a rectangular slab, multiply length by width to get the area, then multiply by the thickness (depth) to get the volume in cubic feet, and divide by 27 to convert to cubic yards. Concrete is ordered in cubic yards, so this is the number your supplier needs. For example, a slab 10 feet long, 10 feet wide and 4 inches thick works out to about 1.23 cubic yards. The calculator above does this math for you and adds a waste allowance.
What Unit Is Concrete Measured In?
Concrete is measured and sold in cubic yards in the United States (cubic meters in metric countries). One cubic yard of concrete fills 27 cubic feet and weighs roughly 4,050 pounds, about two tons. For small jobs you can buy pre-mixed concrete in bags instead: a standard 80 pound bag yields about 0.6 cubic feet, so it takes 45 of them to make one cubic yard, which is why bags only make sense for small pours.
How Much Concrete Do You Need by Project Type?
The right calculator depends on the shape you are pouring. A flat slab, patio or driveway uses length times width times thickness. A footing is a long narrow trench, calculated by length times width times depth. A round column or tube uses the circle formula based on diameter. A wall is height times length times thickness. Pick the matching calculator below so the formula fits your project exactly, rather than forcing every pour through one generic tool.
Should You Add Extra for Waste?
Yes. Always order a little more concrete than the exact calculation, because spillage, uneven subgrade and slight over-digging mean real jobs use more than the math suggests. A waste allowance of 5 to 10 percent is standard practice, and the calculator includes an optional 10 percent buffer. Running short mid-pour is far more costly than a small surplus, since a second delivery and a cold joint can compromise the whole slab.
Calculators in this family
How Do You Calculate Concrete?
Formula
Cubic Yards = Length (ft) x Width (ft) x Thickness (ft) / 27
- Measure the slab length, width and thickness in feet.
- Convert the thickness from inches to feet by dividing by 12 (a 4 inch slab is 0.33 ft).
- Multiply length by width by thickness for cubic feet.
- Divide by 27 for cubic yards, the unit ready-mix concrete is ordered in.
- For bagged concrete, divide the cubic feet by the bag yield (an 80 lb bag yields about 0.6 cubic feet).
Worked example
A 10 ft x 10 ft slab at 4 inches thick: 10 x 10 x 0.33 = 33 cubic feet; 33 / 27 = about 1.22 cubic yards. In 80 lb bags: 33 / 0.6 = about 55 bags.
- Figuring concrete yardage just means converting your slab volume to cubic yards by dividing cubic feet by 27.
- Order about 5 to 10 percent extra concrete for spillage, uneven subgrade and over-excavation.
- For thickened edges, footings or piers, calculate each part separately and add the volumes.
See how much concrete you need (reference table by slab size)
Frequently asked questions
- How much concrete do I need for a 10x10 slab?
- A 10 by 10 foot slab at 4 inches thick needs about 1.23 cubic yards of concrete, or roughly 1.36 cubic yards with a 10 percent waste allowance. At 6 inches thick it needs about 1.85 cubic yards.
- How many 80lb bags of concrete are in a cubic yard?
- It takes 45 standard 80 pound bags of pre-mix concrete to make one cubic yard, since each bag yields about 0.6 cubic feet and a cubic yard is 27 cubic feet. This is why bagged concrete is only practical for small jobs.
- How do I calculate concrete in cubic yards?
- Multiply length by width by thickness in feet to get cubic feet, then divide by 27. For thickness given in inches, divide the inches by 12 first. The calculator does this automatically.
- How much does a yard of concrete cost?
- Ready-mix concrete typically runs in a range that varies by region and over time, so use the cost calculator for an estimate and confirm with a local supplier. Delivery, small-load fees and the mix design all affect the final price.
- How do you figure concrete yardage?
- Multiply the slab length by width by thickness in feet to get cubic feet, then divide by 27 for cubic yards. A 10 by 10 foot slab at 4 inches thick is about 1.22 cubic yards.