Concrete Calculator
Cubic yards if you are calling a truck, bags if you are mixing by hand. Both answers, plus the gravel base and vapor barrier that go underneath.
Reviewed · Method: measured area + industry waste allowance, rounded up to real package sizes · How the numbers are calculated
Enter your dimensions and hit Calculate my concrete volume. You will get the quantity, the buyable quantity, a shopping list, and a budget range.
Figure. Volume in cubic feet is the starting point. Bags and truck orders are both derived from it, and the base is a separate volume.
Figure. Slab thickness is the number that drives cost. Going from 4 in to 6 in is a 50% increase in concrete, not a small adjustment.
Volume math, then the two ordering paths
Length times width times thickness in feet gives cubic feet; divide by 27 for cubic yards. Bag counts assume an 80 lb bag yields 0.60 cu ft mixed and a 60 lb bag yields 0.45 cu ft. A 10x10 slab at 4 inches is 33.3 cu ft, or 1.23 cubic yards — about 56 bags of 80 lb mix.
Where the bag math breaks down
Above roughly one cubic yard, bags stop making sense. Fifty-six bags is 4,480 lb of material to haul, mix and place before it sets. Ready-mix delivery has a minimum charge, often 1 to 3 yards, but a short-load fee is still cheaper than a weekend of mixing. Use bags for post holes, small pads and repairs; call the truck for anything a wheelbarrow cannot finish in 20 minutes.
Always add 5 to 10 percent
Concrete does not stretch. Uneven subgrade, spillage, and an over-dug footing all consume extra. Order 5% over for flatwork on a prepared base, 10% for footings and anything formed by hand. Never try to stretch a short pour with water — it drops strength dramatically and the surface will dust within a year.
The base is part of the job
A 4-inch compacted gravel base is standard under a slab on grade, and 6 inches under a driveway. Gravel volume uses the same length-times-width-times-depth math. Add a 6 mil vapor barrier under any interior slab or conditioned space, and control joints at 2 to 3 times the slab thickness in feet — so a 4-inch slab needs joints every 8 to 12 feet.
Slab thickness by use
A garden path or a shed base is fine at 4 in. A driveway carrying a car needs 5 in, and anything a truck or RV parks on needs 6 in with rebar. Patios sit at 4 in but should slope 1/8 in per foot away from the house.
Going from 4 in to 6 in adds 50% to the concrete volume. That is the single largest cost decision on a small pour, and it is decided before anyone opens a bag.
Bags versus a ready-mix truck
The break-even sits around one cubic yard, and it is about weight as much as money. One cubic yard is 4,000 lb of material. That is 59 bags of 80 lb to buy, load, haul, and mix inside a 45 minute window.
A short-load fee on a truck is typically $150 to $200 plus the concrete. Small jobs are genuinely cheaper with bags, but anything a truck can reach and that needs more than a yard is faster, flatter and stronger poured from a chute.
Worked example: 10 x 10 ft slab at 4 in
| Area | 10 x 10 = 100 sq ft |
| Volume | 100 x 0.33 = 33.3 cu ft |
| Cubic yards | 33.3 / 27 = 1.23 cu yd |
| 80 lb bags | 59 bags, allowing 5% loss |
| Gravel base at 4 in | 1.7 tons, about 33 cu ft |
| Vapor barrier | 100 sq ft of 6 mil |
| Budget band | $439 low, $571 mid, $740 high |
Mistakes that cost real money
Confusing bag yield with bag weight
An 80 lb bag yields about 0.6 cu ft, a 60 lb bag about 0.45. People divide the slab volume by 80 and conclude they need 4 bags. On a 1.2 cu yd slab that is off by a factor of fifteen, and the pour fails at the worst moment.
Pouring on bare soil
A 4 in gravel base is not optional. It drains water away from the slab and gives you something to compact. Without it, frost movement and clay expansion crack a slab that was mixed and finished correctly.
Ordering exactly the calculated volume
Concrete is lost to the truck chute, to an uneven subgrade, and to forms that bow under weight. Order 5 to 10% over. A short-load fee is cheaper than a cold joint in the middle of a slab.
Skipping control joints
Concrete shrinks as it cures and it will crack somewhere. A control joint decides where. Space them at roughly 2.5 times the slab thickness in feet, so a 4 in slab needs a joint every 10 ft, cut within 24 hours while the concrete is still green.
Mixing bags past the working window
Once water hits the mix you have about 30 to 45 minutes before it stops being placeable. Hauling and mixing 59 bags by hand takes longer than that. Above one cubic yard, a ready-mix truck with a short-load fee is both faster and cheaper than a failed pour.
Frequently asked questions
How many bags of concrete for a 10x10 slab?
About 56 bags of 80 lb mix at 4 inches thick, or roughly 1.23 cubic yards. That is close to the point where ready-mix delivery becomes cheaper and far less work.
How many cubic feet does an 80 lb bag of concrete make?
About 0.60 cubic feet when mixed. A 60 lb bag yields roughly 0.45 cubic feet.
How thick should a concrete slab be?
4 inches for patios, walkways and shed floors. 5 to 6 inches for driveways and anything carrying a vehicle. Footings go 8 to 12 inches deep depending on frost line and load.
How much extra concrete should I order?
5 percent over for flatwork on a well-prepared base, 10 percent for footings, post holes and hand-formed work.
Do I need rebar or wire mesh?
For slabs on grade, welded wire mesh or fiber mesh reduces cracking. Structural slabs and driveways should use rebar on chairs. Mesh laid on the ground does almost nothing.