Ordering concrete is unforgiving. You cannot buy a little more halfway through a pour, and a short load leaves a cold joint in exactly the wrong place. The arithmetic is straightforward once you know the sequence, and it takes less than a minute with a tape measure and a calculator. This guide covers slab and footing volumes, the cubic feet to cubic yards conversion, the bagged alternative for small pours, and why ordering slightly over is normal practice.
Quick answer: Multiply length by width by thickness in feet to get cubic feet, then divide by 27 to get cubic yards. A 10 ft by 10 ft slab, 4 inches thick, is 100 times 0.333, or 33.3 cubic feet, which is about 1.23 cubic yards. Add 5% to 10% for spillage and uneven subgrade, so order roughly 1.3 to 1.4 cubic yards.
Step 1: get everything into feet
Volume formulas need consistent units, and the most common mistake is mixing inches with feet. Convert thickness from inches to feet by dividing by 12, so a 4 inch slab is 0.333 feet thick and a 6 inch slab is 0.5 feet. Do the same for footing width and depth, which are almost always quoted in inches. If you prefer to work in meters, keep every dimension in meters and the result is cubic meters, which convert to cubic yards by multiplying by about 1.308.
Step 2: volume for slabs, footings and columns
Slab: length in feet times width in feet times thickness in feet.
Footing or strip foundation: length times width times depth, all in feet.
Round column: 3.1416 times radius in feet times radius in feet times height.
Steps and curbs: break them into rectangles, work each one, then add the volumes.
Irregular areas: split the pour into simple rectangles and add the parts together.
Step 3: convert cubic feet to cubic yards
Concrete is sold by the cubic yard, and there are 27 cubic feet in a cubic yard, because a yard is 3 feet in each direction. Divide your total cubic feet by 27. For the 10 by 10 foot slab at 4 inches, that is 33.3 cubic feet divided by 27, or about 1.23 cubic yards. Suppliers often apply a minimum charge or a short load fee below a certain volume, so a very small pour can cost nearly as much as a larger one delivered on the same truck.
Step 4: bagged concrete for small pours
For a small project, bags of ready-mix are simpler than a delivery truck. An 80 pound bag of concrete mix yields about 0.6 cubic feet, and a 60 pound bag about 0.45 cubic feet. The 10 by 10 foot slab at 4 inches needs roughly 56 bags of the 80 pound mix, which is a great deal of mixing by hand. Bagged concrete makes sense for fence posts, small pads and isolated footings, but anything approaching a full slab is usually cheaper and far faster delivered.
Step 5: add waste, then round up
Add 5% to 10% to your calculated volume before ordering. Subgrade is rarely perfectly level, formwork shifts slightly under load, and some concrete always stays behind in the chute or the pump. You cannot add material to a pour that has started to set, so a slight overage is normal and expected practice. Order in the increment your supplier offers, and have somewhere ready to use any leftover, such as a small pad or a set of steps.
Frequently Asked Questions
How many cubic yards is a 10x10 slab?
At 4 inches thick, a 10 by 10 foot slab is 33.3 cubic feet, or about 1.23 cubic yards. At 6 inches thick it is 50 cubic feet, or about 1.85 cubic yards. Add 5% to 10% for waste and round up to whichever increment your supplier delivers.
How many 80 pound bags of concrete make a yard?
One 80 pound bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet, so it takes roughly 45 bags to make a cubic yard. That is practical for post holes and small pads but impractical for a driveway or a full slab, where delivered ready-mix is faster and usually cheaper per yard.
How thick should a concrete slab be?
Four inches is common for a patio, walkway or shed floor over well-compacted ground. Five or six inches with reinforcement is typical for driveways and anything that will take vehicle weight. Thickness changes volume directly, so settle it before ordering, and check local requirements for the specific use you have in mind.
Do I need rebar or mesh?
Reinforcement helps control cracking and holds a slab together if cracking does occur. Welded wire mesh is common in slabs on grade, and rebar is usual in footings and structural slabs. Position matters as much as quantity: reinforcement works when it sits in the middle of the slab rather than resting on the ground beneath it.
What is a short load fee?
Suppliers often charge extra when a delivery is below a minimum volume, because the truck and driver are committed either way. The threshold and the fee vary by supplier and region. If your pour is small, ask about the minimum before ordering and compare the delivered cost against bagged concrete for the same volume.