Concrete Calculator
Enter your slab size to get cubic yards and exactly how many bags of concrete to buy.
Bag counts are rounded up. Consider adding 5 to 10 percent extra for waste.
For a rectangular slab. Irregular shapes need to be broken into rectangles.
How much concrete do I need? The math
Concrete is sold by volume, and the industry unit is the cubic yard. If you are asking how much concrete you need for a slab, patio, or footing, this concrete slab calculator turns your dimensions into an order quantity. The calculation starts with your slab dimensions: length times width times thickness gives cubic feet, and dividing by 27 converts to cubic yards. Thickness is entered in inches because that is how slabs are specified, and the calculator converts it to feet internally.
Bag counts come from each bag's yield. An 80lb bag of premix makes about 0.6 cubic feet of concrete, and a 60lb bag makes about 0.45. Dividing your total cubic feet by the yield and rounding up tells you how many bags to buy. Rounding up matters because you cannot buy half a bag, and running short mid-pour is a real problem.
Ordering smart
For small jobs like fence posts or a small patio, bags from the hardware store are the practical choice. Once you pass about a cubic yard, ready-mix delivered by truck usually becomes cheaper and far less labor. A good rule of thumb: if the calculator shows more than 40 or 50 bags, price out a ready-mix delivery before committing to bags.
Always add a waste factor. Pros order 5 to 10 percent extra because forms flex, subgrades are uneven, and spillage happens. It is much better to have one bag left over than to be half a wheelbarrow short with wet concrete already in the forms.
Common slab thicknesses
Most residential slabs are 4 inches thick, which suits patios, walkways and shed floors. Driveways usually go 5 to 6 inches to handle vehicle weight. If your project needs reinforcement like rebar or wire mesh, keep the thickness consistent across the whole form, because thin spots are where cracks start.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length by width by thickness (in feet) to get cubic feet, then divide by 27 for cubic yards. This calculator does it for you and also converts to bag counts.
How many 80lb bags of concrete make a yard?
One 80lb bag yields about 0.6 cubic feet, so it takes 45 bags to make one cubic yard (27 cubic feet).
How many 60lb bags of concrete make a yard?
One 60lb bag yields about 0.45 cubic feet, so it takes 60 bags to make one cubic yard.
Should I order extra concrete?
Yes, ordering 5 to 10 percent extra is standard practice. Forms are never perfectly level and some concrete is always lost in handling, so a small buffer avoids coming up short.
Is my data stored anywhere?
No. The calculation runs entirely in your browser and your measurements are never uploaded, saved or shared.
How much concrete do I need for a 10x10 slab?
A 10 by 10 foot slab at 4 inches thick needs about 1.24 cubic yards, so you would order 1.5 yards to be safe. In bags, that is roughly 56 eighty-pound bags. Enter your exact dimensions above for your project.
How many 80lb bags of concrete make a cubic yard?
One cubic yard takes about 45 eighty-pound bags, or 60 sixty-pound bags. Buying in bulk from a ready-mix truck is usually cheaper than bags for anything over half a yard.