Concrete Calculator
Calculate how much concrete you need for slabs, footings, and columns.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Concrete Calculator
This calculator estimates the volume of concrete you need for a slab, footing, or round column, then converts it to cubic yards and tells you exactly how many bags to buy. Here is how to get an accurate estimate:
- Choose your shape. Select Slab for a flat poured surface like a driveway, patio, or floor. Select Footing for a continuous strip or pad footing. Select Round Column for cylindrical piers or posts.
- Enter your dimensions. For a slab, enter the length and width in feet and the thickness in inches. A typical residential slab is 4 inches thick. Driveways often use 4-6 inches. Footings use length, width, and depth all in feet. Columns use the diameter and height in feet, plus how many columns you are pouring.
- Set your waste factor. The default is 10%, which accounts for uneven subgrades, spillage, and leftover concrete in the mixer. For complex pours or rough terrain, increase this to 15-20%. For small, simple pours you can reduce it to 5%.
- Read your results. The calculator shows cubic yards (the unit concrete trucks use) and the number of pre-mixed bags you will need in both 80 lb and 60 lb sizes. Bag counts always round up so you do not run short.
Results update instantly as you type. Use the Share button to send your inputs to a contractor or supplier, or Copy to paste the result into a message or spreadsheet.
About the Concrete Calculator
Concrete is sold by the cubic yard for ready-mix truck deliveries (typically a minimum of 1 yard per order) and by the bag for small projects. One cubic yard equals 27 cubic feet. A standard 80 lb bag of pre-mixed concrete covers approximately 0.022 cubic yards, which means you need about 45 bags per cubic yard. A 60 lb bag covers about 0.017 cubic yards, requiring roughly 60 bags per yard.
This calculator uses the standard industry formulas: slabs multiply length by width by thickness (converted from inches to feet), then divide by 27. Footings multiply length by width by depth and divide by 27. Round columns use the circle area formula (pi times radius squared) multiplied by height and quantity, then divided by 27. All calculations run entirely in your browser. No data is stored or transmitted.
Frequently Asked Questions
How many cubic yards of concrete do I need for a 20x20 slab at 4 inches thick?
A 20x20 foot slab at 4 inches thick requires approximately 4.94 cubic yards of concrete before waste. With a standard 10% waste factor, order about 5.43 cubic yards. That translates to roughly 245 bags of 80 lb mix or 326 bags of 60 lb mix if you are doing it without a truck. For a project this size, a ready-mix delivery is almost always the better choice economically.
Should I order concrete by the bag or by the yard?
For projects under about 0.5 cubic yards (roughly 22 bags of 80 lb mix), bagged concrete from a hardware store is practical and cost-effective. For anything larger, a ready-mix truck delivery is faster, less labor-intensive, and usually cheaper per yard once you account for the cost of bags and your time mixing them. Most ready-mix companies have a minimum order of 1 cubic yard. If you need between 0.5 and 1 yard, you can sometimes pay a short-load fee or rent a concrete trailer.
Why add a waste factor to the concrete calculation?
No excavation is perfectly uniform, and concrete settling, spillage, and mixer residue all reduce your usable volume. Running out of concrete mid-pour is far more disruptive than having a small amount left over. A 10% waste factor is a standard buffer for most residential projects. Increase it to 15-20% for rough or uneven subgrades, forms with irregular shapes, or when pouring in cold weather when you need to work quickly.
How thick should a concrete slab be?
For most residential uses, 4 inches is the minimum recommended thickness. Patios and walkways can use 4 inches with proper base preparation. Driveways that support passenger vehicles should be 4-6 inches. Slabs supporting heavy vehicles or equipment should be 6 inches or more. Footings for structures typically need to be at least as wide as the wall above them and deep enough to reach below the frost line in cold climates. Always check local building codes, as requirements vary by region.