How to use this calculator
Start by selecting your project type: slab/pad, footing/wall, column/post hole, or stairs. Each type has its own set of dimension inputs tailored to how that shape is measured on a real job site.
Enter your dimensions using the labeled fields. For slabs, you can enter length and width in both feet and inches for extra precision. Thickness, width, and depth fields use inches as the unit. The calculator updates results instantly as you type.
Adjust the waste factor percentage to account for spillage, uneven ground, and form imperfections. The default 10% is a good starting point for most projects. You can also change the ready-mix price per cubic yard to match local rates in your area.
Concrete coverage by bag size
Premix concrete bags come in several standard sizes. An 80 lb bag covers approximately 0.6 cubic feet of finished concrete, making it the most efficient option per bag. A 60 lb bag covers about 0.45 cubic feet. Smaller 40 lb and 50 lb bags are also available but require more bags for the same volume.
One cubic yard equals 27 cubic feet. That means you would need about 45 bags of 80 lb mix or 60 bags of 60 lb mix to equal one cubic yard of concrete. For anything beyond a few cubic feet, the bag count adds up quickly, which is why ready-mix delivery becomes the practical choice for larger projects.
Ready-mix vs. bags: which should you choose?
Premix bags (under 1 cubic yard): Best for small projects like fence posts, mailbox bases, small repair patches, and stepping stones. You control the pace and do not need to coordinate delivery. The downside is that mixing bags by hand is physically demanding and slow.
Ready-mix delivery (1+ cubic yards): A concrete truck delivers pre-mixed concrete directly to your site. It is faster, ensures consistent mix quality, and is more cost effective for larger pours. Most suppliers have a minimum order of 1 cubic yard and may charge short load fees for small orders.
Rule of thumb: If your project needs more than 45 bags (about 1 cubic yard), ready-mix is almost always the better choice in terms of cost, time, and quality.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10x10 foot slab at 4 inches thick requires about 1.23 cubic yards of concrete, which equals approximately 56 bags of 80 lb mix or 74 bags of 60 lb mix. Adding 10% for waste brings the total to about 62 bags of 80 lb mix.
How thick should a concrete slab be?
Standard concrete slab thickness is 4 inches for patios, walkways, and shed floors. Driveways should be 5 to 6 inches thick. Garage floors are typically 6 inches. For heavy equipment or commercial use, 8 inches or more may be required.
Should I use bags or ready-mix concrete?
For projects under 1 cubic yard, premix bags are practical and let you work at your own pace. For anything over 1 cubic yard, ready-mix delivery is more cost effective and ensures consistent quality. Most suppliers have a minimum order of 1 cubic yard.
How much does concrete cost?
Ready-mix concrete typically costs around $150 per cubic yard delivered, though prices vary by region. An 80 lb bag of premix costs $5 to $7 at most home improvement stores and yields about 0.6 cubic feet of finished concrete.