Concrete Calculator: How Much Concrete Do I Need?

Whether you’re buying concrete for a slab, column, or footing, our concrete calculator will estimate how many bags to buy. We’ll also show you how to manually calculate how much concrete you need based on cubic feet and cubic yards.

How Much Concrete Do I Need – Calculator

Concrete comes in cubic yards and cubic feet. You can order premixed concrete by the yard, while bags of Quikrete and similar brands are sold by cubic foot.

To determine the cubic feet of concrete for a simple square or rectangle shape, multiply the length x width x depth in feet.

Cubic yards = length in ft x width in ft x depth in ft

To determine the number of cubic yards of concrete, multiply length x width x depth in ft and divide the total by 27.

Cubic feet = length in ft x width in ft x depth in ft ÷ 27

If you need concrete for a column or circular footing, the formula to determine cubic yards and cubic feet is different.

Cubic yards = radius in ft x radius in ft x depth in ft x pi ÷ 27

Cubic feet = radius in ft x radius in ft x depth in ft x pi

What Size Bags Does Concrete Come In?

Bags of concrete come in four sizes – 40, 50, 60, and 80 lb bags.

• A 40 lb bag of concrete covers 0.011 yards or 0.3 cu ft.
• One 50 lb bag of concrete covers 0.013 cubic yards or 0.37 cu ft.
• A 60 lb bag of concrete covers 0.017 cu yards or 0.45 cu ft.
• An 80 lb bag of concrete covers 0.022 cu yards or 0.60 cu ft.

Depending on the bag size, there are between 40-80 bags of concrete on one pallet. There are 80 bags of 40 lb concrete on a pallet, 64 bags of 50 lb concrete, 56 bags of 60 lb concrete, and 42 bags of 80 lb concrete.

Example: How Many Bags of Concrete for a 10 x 10 Slab?

To determine how many bags of concrete for a 10′ x 10′ slab, you must decide on concrete thickness. In many scenarios, like for walkways and patios, a 4-inch thickness is recommended.

For our concrete formula to work, we must convert our 4-inch slab thickness to feet – we’ll do this by dividing 4 by 12 since there are twelve inches in a foot.

• Convert slab thickness of 4 inches into feet by dividing 4 by 12
• 4 ÷ 12 = 0.33 ft
• Cubic feet = 10 ft length x 10 ft width x 0.33 ft depth

There are 33 cubic feet in a 10’ x 10’ patio with a 4-inch slab thickness

• To convert cubic feet to cubic yards, divide by 27

There are 1.22 cubic yards in a 10’ x 10’ x 4” concrete slab

For a 10’ x 10’ x 4” concrete slab, you’ll need 56 80 lb bags, 72 60 lb bags, 84 50 lb bags, or 111 40lb bags of concrete.

Figuring Concrete for a Circular Slab or Column

To determine concrete needs for a circular slab or column, you’ll need to know the radius in ft and the desired thickness in ft. 

To find the radius of your circle, divide the diameter by two. You can then determine the desired thickness by dividing the total depth in inches by 12. For example, a 4-inch thickness is equivalent to 0.33 feet (4 ÷ 12), while a 40-inch thickness is equivalent to 3.33 ft (40 ÷ 12.)

For our example, we’ll calculate cubic feet, cubic yards, and bags of concrete for a circular patio that’s 8 feet in diameter and 6 inches thick.

Cubic feet = radius in ft x radius in ft x depth in ft x pi

Cubic yards = radius in ft x radius in ft x depth in ft x pi ÷ 27

• Determine the radius of the circle by dividing the diameter by two. 8 ÷ 2 = 4-foot radius
• Determine the depth in feet by dividing desired thickness in inches by 12. 6 ÷ 12 = 0.5 ft
• Use 3.14 for pi
• Cubic feet = 4 ft x 4ft x 0.5 ft x 3.14

There are 25.12 cubic feet in a concrete slab with an 8-foot diameter and 6-inch thickness.

To convert cubic feet to cubic yards, divide by 27

There are 0.93 cubic yards in an 8-foot circular concrete slab with a 6-inch thickness

For a 25.12 cu ft or 0.93 cu yard slab, you’ll need 42 80 lb bags, 59 60 lb bags, 68 50 lb bags, or 84 40lb bags of concrete.

How Do You Calculate Square Feet into Cubic Yards of Concrete?

The square footage of a concrete slab is length x width in feet. Convert square footage to cubic footage by multiplying it by your desired concrete thickness in feet. Then divide cubic feet by 27 to determine cubic yards.

Example: Your concrete slab is 120 square feet, and you’d like a concrete thickness of 5 inches.

• Step 1: Convert 5 inches into feet by dividing by 12. 5 ÷ 12 = 0.42 ft
• Step 2: Determine cubic feet by multiplying the square footage by the desired depth in ft. 120 square feet x 0.42 ft depth = 50.4 cubic feet
• Step 3: Divide cubic feet by 27 to determine cubic yards. 50.4 ÷ 27 = 1.87 cu yds