ReckonBoxHow much concrete a slab really takes
Concrete volume is length times width times thickness, all in feet, divided by 27 to get cubic yards. The divide-by-27 is the step people forget, since a cubic yard is 3 feet on every side, which is 27 cubic feet.
Run the default: a 10 foot by 10 foot slab, 4 inches thick. Four inches is a third of a foot, so the volume is 10 times 10 times 0.333, which is 33.3 cubic feet, or 1.23 cubic yards. Bagged as 80-pound sacks, that is 56 bags. Bump the thickness to 6 inches and it jumps to 1.85 yards and 84 bags. Thickness moves the total as hard as area does, which is why a driveway costs so much more than a patio of the same size.
Bags or a truck
One cubic yard of concrete takes 45 eighty-pound bags, 60 sixty-pound bags, or 75 fifty-pound bags. An 80-pound bag yields about 0.6 cubic feet, and 27 divided by 0.6 is 45.
The choice a small project raises is whether to bag it or order a truck. Below about 2 cubic yards, bags win: a couple of dozen sacks, a rented mixer or a wheelbarrow, and an afternoon. Past 2 yards the bag count climbs fast, roughly 90 eighty-pound bags, and mixing that much by hand risks a cold joint where an early batch sets before the last one goes down. Ready-mix arrives uniform and pours continuously, which is why the tool flags the 2-yard line.
| 10 x 10 slab | 4 inches | 6 inches |
|---|---|---|
| Cubic yards | 1.23 | 1.85 |
| 80-pound bags | 56 | 84 |
The waste factor is not optional here
Concrete is unforgiving of a short order in a way tile is not. Once a pour starts setting you cannot top it up cleanly, so running out means a visible cold joint or a whole second pour. Add 5% to 10% for spillage, for the subgrade that is deeper in one corner than you measured, and for the bit left in the mixer. On the 1.23-yard slab, a 10% factor means ordering 1.36 yards, and ready-mix is usually sold in quarter-yard steps, so you round up to the next quarter anyway.
Where the numbers come from
Volume is pure geometry: area times depth, converted to yards. The bag counts come from each sack's yield, which the manufacturer prints on the bag, and the standard figures are 0.6 cubic feet for an 80-pound bag, 0.45 for a 60, and 0.36 for a 50. The tool takes your slab in feet and inches, does the conversion, and counts bags for the size you choose, then repeats the count with your waste factor added.
What this does not cover
This sizes the concrete for a flat slab of even thickness. It does not add the extra volume for a thickened edge, footings, or piers, which are common under a real slab and each hold their own concrete. It also does not spec the mix strength, the rebar or wire mesh, the base gravel, or the control joints that keep a slab from cracking. Those are structural choices for the project, not the volume.
Measure the depth in the deepest corner, not the shallowest, and the last bag will not be the one you are missing.
Frequently asked questions
How much concrete do I need for a 10x10 slab? A 10 foot by 10 foot slab at 4 inches thick needs 1.23 cubic yards of concrete, which is 56 eighty-pound bags. The formula is length times width times thickness in feet, divided by 27. At 6 inches thick the same slab needs 1.85 cubic yards, or 84 bags, so thickness drives the total as much as area does.
How many bags of concrete make a cubic yard? One cubic yard takes 45 eighty-pound bags, 60 sixty-pound bags, or 75 fifty-pound bags. An 80-pound bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 27 divided by 0.6 is 45 bags. Bigger bags mean fewer to haul and mix, which matters once you are past a few dozen.
When should I use ready-mix instead of bags? Ready-mix from a truck usually wins above about 2 cubic yards, where mixing bags by hand becomes slow and the concrete can start setting before you finish. Below that, bags are the practical choice for a small patio or footing. A 2 cubic yard pour is already around 90 eighty-pound bags, which is a lot of mixing.
What waste factor should I add for concrete? Add 5% to 10% extra to cover spillage, settling into an uneven subgrade, and slight over-excavation. Concrete cannot be topped up once it sets, so running short means a cold joint or a second pour. On a 1.23 cubic yard slab, a 10% factor means ordering 1.36 yards, and ready-mix is often sold in quarter-yard increments.
How thick should a concrete slab be? Thickness depends on the load: 4 inches is typical for patios, walkways, and shed floors, while 5 to 6 inches suits driveways and anything carrying vehicles. Every extra inch adds real volume, since a 6-inch slab uses 50% more concrete than a 4-inch one over the same area. This tool takes thickness in inches and converts it for the volume.