Ordering concrete for a slab is one of those jobs where a small measurement mistake turns into a very expensive one. This concrete slab calculator works out the volume in cubic feet and cubic yards, converts that into the number of bags or truck loads you need, adds your chosen waste allowance and prices the whole job.
What Is the Concrete Slab Calculator?
This calculator answers the two questions that matter when you are ordering a slab: how much concrete do I need, and what will it cost. It works in feet and inches for the input and gives the volume back in cubic feet, cubic yards and cubic metres.
The bag count includes your waste allowance, which is not optional. Even with careful forms and a level base, the practical figure is 5 to 10 percent above the theoretical volume, and running short mid-pour is far worse than over-ordering.
How the Concrete Volume Works
Volume in cubic feet = Length × Width × (Thickness in inches ÷ 12)
That figure converts to cubic yards by dividing by 27, which is how concrete is sold by the yard from a mixer truck. Each bag size covers a fixed volume of finished concrete: 0.45 cubic feet for a 60 pound bag, 0.60 for an 80 pound bag and 0.71 for a 94 pound bag.
The cost per square foot row is worth checking against a local quote, because it makes the thickness decision concrete. Moving from 4 inches to 6 inches adds 50 percent more material, which is usually the biggest lever on the total.
Choosing the Right Thickness
A 4 inch slab is the standard for patios, sheds and interior floors in most climates. Driveways, ramps and slabs in areas with frost heave generally need 6 inches, and a slab carrying a vehicle should be reinforced and thickened at the edges.
Thickness also depends on the base. A well-compacted gravel base with a membrane underneath allows a thinner slab, while a slab poured straight onto soil needs more concrete to spread the load. Local building codes set minimums that always override a rule of thumb.
Bags or Ready Mix
Below roughly one cubic yard, bags are usually the practical choice, but watch the cost difference. Bagged concrete carries a premium per cubic foot that grows with the volume ordered, because you pay for the packaging and the carrying.
A ready mix truck delivers by the cubic yard and is normally cheaper above about one and a half cubic yards, but you need a clear route for the truck and a formwork area ready to receive it. Ordering a half yard of ready mix as a top-up later is usually more expensive than ordering the whole load at once.
Frequently Asked Questions
How many cubic feet of concrete is in an 80 pound bag?
About 0.60 cubic feet, which is one third of a cubic yard. A 60 pound bag covers 0.45 cubic feet and a 94 pound bag covers about 0.71.
How much extra concrete should I order for waste?
Add 5 to 10 percent for a small slab with good forms and a level base, and up to 15 percent for a large pour with hand finishing. The figure includes spillage, slight over-excavation and the volume taken up by the reinforcement mesh.
What thickness do I need for a driveway?
Most driveways use 4 to 6 inches of concrete over a compacted base. In areas with freezing winters, 6 inches is the safer choice because it reduces the risk of cracking along the length of the slab.
How much concrete do I need for a 10 by 10 foot slab?
At 4 inches thick that is 33.3 cubic feet, which is 1.23 cubic yards. With 80 pound bags that works out at 56 bags before waste, or 62 bags with the 10 percent allowance this calculator applies.
Does the slab area change the amount of reinforcement I need?
It does. The mesh or rebar requirement depends on both the area and the thickness, and local codes often specify it directly. Check the code for your area rather than relying on a rule of thumb, because this is the part of a slab most likely to be inspected.