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
feet
feet
inches
You will need:
2.96
Cubic Yards
or
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
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!
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!
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!
Helpful Resources
Helpful Resources
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