Square footage tells you how much ground a concrete slab covers, but it will not tell you how much concrete to buy. Concrete is sold by volume, not area, because a slab has depth — a thin patio and a thick driveway can share the same footprint yet need very different amounts of material. To order correctly you have to turn your square footage into cubic yards.
This guide walks through that conversion in plain steps. You will learn why concrete is measured by volume, how to fold thickness into the math, how to divide down to cubic yards, and how to decide between bagged concrete and a ready-mix delivery. We will cover typical slab thicknesses, how much a standard bag covers, and how much extra to order, then finish with a full worked example for a common slab size.
Why concrete is measured by volume
When you pour a slab you are filling a three-dimensional space, so what matters is volume, not just the flat area on top. A slab that is 100 square feet and 4 inches thick holds a completely different amount of concrete than the same 100 square feet poured 8 inches thick. Area alone cannot capture that difference, which is why suppliers quote and deliver concrete by the cubic yard.
The key is to treat the slab as a box: length, width, and depth. Square footage already gives you length times width. Adding the third dimension — the thickness — turns that flat area into a volume you can actually order. Everything in this guide flows from that one idea: area times thickness equals volume.
The core formula: area x thickness
Start with the slab's area in square feet, which is simply length × width. Then multiply by the thickness — but the thickness has to be expressed in feet, not inches, so that all three measurements share the same unit. Convert inches to feet by dividing by 12: a 4-inch slab is 4 ÷ 12 = 0.333 feet, and a 6-inch slab is 6 ÷ 12 = 0.5 feet.
Multiplying area (in square feet) by thickness (in feet) gives you the volume in cubic feet. For example, a 200-square-foot slab poured 4 inches thick is 200 × 0.333 = about 66.6 cubic feet of concrete. Getting the units right at this stage is the single most important step — mixing inches and feet is the most common reason a concrete estimate comes out wildly wrong.
Converting cubic feet to cubic yards
Ready-mix concrete is ordered by the cubic yard, so the last conversion turns cubic feet into cubic yards. One cubic yard is a cube three feet on every side, which equals 3 × 3 × 3 = 27 cubic feet. To convert, divide your cubic feet by 27.
Continuing the example above, 66.6 cubic feet ÷ 27 = about 2.47 cubic yards. That is the amount of concrete the slab actually contains. Keep this figure handy, because you will round it up before placing an order — no supplier delivers a perfectly exact volume, and you always want a little extra rather than a little short.
Choosing the right slab thickness
Thickness is driven by what the slab will carry. Light-duty surfaces can be thinner, while anything that supports vehicles or heavy loads needs more depth to resist cracking. Picking the correct thickness up front matters because it directly changes your volume — and therefore your cost — more than almost any other choice.
Use these common thicknesses as a starting point, and check local building codes for anything structural. When in doubt, err on the thicker side for surfaces that will bear weight, since adding depth after the pour is not an option.
- Patios and walkways — 4 inches is standard for foot traffic.
- Driveways and garage floors — 5 to 6 inches to handle vehicle weight.
- Sheds and light outbuildings — 4 inches for most, more if heavily loaded.
- Slabs for heavy equipment or RVs — 6 inches or more, often with reinforcement.
Bagged concrete vs. ready-mix
For small jobs you can mix bagged concrete yourself. An 80-pound bag of standard concrete mix yields about 0.6 cubic feet once mixed, a 60-pound bag yields about 0.45 cubic feet, and a 40-pound bag yields about 0.3 cubic feet. To find how many bags you need, divide the slab's volume in cubic feet by the yield per bag. Our example slab of 66.6 cubic feet would take roughly 66.6 ÷ 0.6 = 111 of the 80-pound bags — a clear sign this job is better suited to ready-mix.
As a rule of thumb, once you get past about 1 cubic yard (27 cubic feet, or around 45 of the 80-pound bags) it becomes cheaper and far less exhausting to order ready-mix delivered by the truck. Ready-mix is sold by the cubic yard and often carries a minimum order and short-load fees, so it pays to know your cubic-yard figure before you call. Bagged mix wins for small repairs, post footings, and slabs under about half a cubic yard.
Ordering ready-mix and adding overage
When you order ready-mix, round your cubic-yard figure up. Trucks typically dispense in quarter-yard increments, and you cannot pour half a slab and send the truck back for more without the first pour starting to set. Rounding up protects you from the expensive problem of running out mid-pour.
On top of rounding, add a waste and overage allowance of 5 to 10 percent. Uneven subgrade, spillage, over-excavated edges, and slight variations in slab depth all consume a little more concrete than the math predicts. For our 2.47-cubic-yard slab, a 10 percent allowance brings it to about 2.72 cubic yards, which you would round up to 2.75 or 3 cubic yards when ordering. It is always cheaper to have a wheelbarrow's worth left over than to be short.
Worked example: a 20 x 10 slab at 4 inches
Walk through a full patio calculation for a slab going into the back yard. The slab is 20 feet by 10 feet, so the area is 20 × 10 = 200 square feet. You choose a standard 4-inch thickness for foot traffic, and convert that to feet: 4 ÷ 12 = 0.333 feet.
Multiply area by thickness to get cubic feet: 200 × 0.333 = about 66.6 cubic feet. Divide by 27 to reach cubic yards: 66.6 ÷ 27 = about 2.47 cubic yards. That is the base volume. Add a 10 percent overage — 2.47 × 1.10 = about 2.72 cubic yards — and round up to 2.75 or 3 cubic yards for the order. If you decided to pour the same slab at 6 inches instead, the volume would jump to 200 × 0.5 ÷ 27 = about 3.7 cubic yards, a reminder that thickness drives the number as much as area does.