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Concrete Slab Calculator: How Much Concrete Do I Need?

Use this calculator to estimate the amount of concrete and number of bags you’ll need for a concrete foundation. Enter the length and width in feet. Next, enter the desired thickness (depth) in inches of your concrete slab foundation.

Concrete Slab Calculator

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You will need:


Cubic Yards



40 lb bags


60 lb bags


80 lb bags

How to Measure Concrete

Manually calculate your concrete slab volume in cubic yards (yd3) by following the formula below: 

Volume in yards = (Length in feet x Width in feet x Depth in feet) ÷ 27

Step 1: Measure the square footage

Get the surface area by multiplying the length and width of your garage footing or foundation.

For example, a 25’x25’ two-car garage will be 625 sq. ft. 

Step 2: Determine the slab thickness 

A concrete slab is usually at least four to six inches thick, unless otherwise determined by your local building codes. Next, convert the inches into feet by dividing the number by twelve. This will be the depth of your foundation.

Example: For a slab four inches thick, calculate 4 ÷ 12 = 0.333 ft.

Step 3: Multiply the square footage with the slab thickness

This results in the volume of your concrete slab in cubic feet. 

Example: For a 625-sq.-ft. slab that is 0.333 feet thick, calculate 625 x 0.333 = 208.13 ft3.

Step 4: Convert the cubic feet to cubic yards

You’ll need to do this since concrete purchases are measured in cubic yards (yd3). To get this number, divide the square footage above by twenty-seven. 

Example: 208.13 ft3 ÷ 27 = 7.71 yd3

Step 5: Determine the number of bags needed

Once you have your volume above, you can approximate the number of premixed bags you’ll need using the formulas below:

  • 90 40-lb bags make 1 yard
  • 60 60-lb bags make 1 yard
  • 45 80-lb bags make 1 yard 

For example, to approximate the number of 40-pound bags needed for a 7.71-yd3 concrete slab, you will multiply 90 x 7.71, equaling about 694 bags needed. 

Quick Tip: It’s better to overestimate the amount you need so you don’t come up short.

Frequently Asked Questions

Concrete ready mix can cost anywhere from $125–$185 per cubic yard if you do the job yourself. If hiring a contractor, they may charge between $4 and $7 per square foot to pour the slab. Keep in mind that slabs will vary depending on the size of your project, so it’s best to determine the type and size of the project in advance.

The slab’s volume will vary by the size and weight of your concrete bags. For example, one 80-pound bag of concrete can cover about 0.022 cubic yards, which equates to about forty-five bags for one cubic yard.

Choosing a type of concrete mix depends on the size and timeline of your project. While ready-mix concrete is more expensive, it’s better for large buildings since it’s delivered in large batches and is ready to use. Bagged (site-mixed) concrete is preferable for smaller projects like a backyard shed, where you’ll most likely pour a concrete slab yourself.

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