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

Width:

*Enter a value

feet

Length:

*Enter a value

feet

Thickness:

*Enter a value

inches

You will need:

2.96

Cubic Yards

269

40 lb bags

174

60 lb bags

134

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

It depends on your project size! Bagged concrete is great for small DIY jobs since you can mix as you go. But if you’re pouring a large slab, ready-mix concrete is easier and faster—plus, it saves you a lot of labor.

If you’re unsure, check out our how to pour a concrete slab guide for tips!

A cubic yard of concrete is about 45 80-pound bags of concrete mix. If you’re using smaller or larger bags, the total will vary. A good trick is to calculate how much you need before you start so you don’t have to make multiple trips to the store!

Concrete prices vary, but ready-mix concrete typically runs $125 to $185 per yard if you’re doing it yourself. If you hire a contractor, they may charge $4 to $7 per square foot to pour the slab. Prices will vary depending on location and project size, so always get a few quotes!