Step 1: Measure the Area
Start with a tape measure and the actual formed dimensions of the pour, in feet. For a rectangle, length times width gives you the square footage — a 10 ft by 10 ft slab is 100 square feet. For an L-shape or an irregular slab, break it into simple rectangles, calculate each one, and add them together; for a curved or rounded end, box it into the smallest rectangle that contains the shape and let the small over-estimate fold into your waste factor later. Measure what you are actually going to form, not a nominal number off the plan — a form that ends up an inch wide on each side quietly adds volume across a big slab.
Step 2: Choose the Thickness
Thickness is a strength decision first and a volume decision second. The standard depths are 4 inches for patios, walkways, and interior floors that carry foot traffic, and 6 inches for driveways, garage floors, and any slab that carries vehicles or heavy loads. Do not shave thickness to save concrete on something that will hold a truck — that is where slabs crack. For the math ahead, convert the thickness to feet by dividing inches by 12: 4 inches is 0.333 ft, 6 inches is 0.5 ft. Because volume scales directly with depth, the jump from 4 to 6 inches is a full 50 percent more concrete, so choose it on purpose. The load-and-thickness reasoning is covered in more depth in the concrete slab installation guide.
Step 3: Compute Cubic Yards
Concrete is ordered by the cubic yard, and the conversion hinges on one fact: one cubic yard equals 27 cubic feet. Take your area in square feet, multiply by the thickness in feet to get cubic feet, then divide by 27 to get cubic yards. Worked through on the 10 by 10 slab at 4 inches: 100 sq ft × 0.333 ft = 33.3 cubic feet, and 33.3 ÷ 27 = about 1.23 cubic yards. That is the raw volume before any margin. The Concrete Calculator performs this exact conversion from your length, width, and thickness, and for small pours it also estimates a bag count so you can compare buying methods.
Step 4: Add a Waste Factor
You never place precisely the calculated volume, so add a 5 to 10 percent waste factor on top. Concrete sticks to the chute and the wheelbarrow, forms bow outward under the load, and the sub-grade is never perfectly flat, so some depth variation is guaranteed. Lean toward 5 percent on a large, clean, well-braced rectangle and toward 10 percent on small, irregular, or hand-formed pours where the losses bite harder. On the 1.23-yard example, a 10 percent margin brings the order to about 1.35 cubic yards. This margin is not padding — it is what keeps you from coming up a wheelbarrow short with the truck pulling away.
Step 5: Place the Order
Now round to how your supplier actually sells. For a tiny pour that is bags — recall an 80 lb bag yields about 0.60 cubic feet and a 60 lb bag about 0.45 cubic feet, so divide your cubic feet by the bag yield to get a count (the ready-mix vs. bagged vs. short-load guide covers when bags beat a truck). For ready-mix, give the plant your total cubic yards, the psi mix you want (commonly 3000 to 4000 psi for residential work, 4000 and up for driveways and vehicle loads), and any additives such as fiber or air-entrainment for freeze-thaw. Crucially, ask about the short-load or minimum-load feeif your order is a partial truck — on a small pour it can rival the concrete cost, which is the whole reason to nail the volume before you dial. Check the delivered per-yard rate on the Concrete Prices by State page so the total is no surprise, and if a driveway or structural slab needs steel, set it out first using the rebar slab layout guide.
A Note on Ordering Confidence
The reason this sequence matters is that concrete does not wait. Once the truck arrives you have a working window to place and finish the whole load, and there is no pausing to recalculate. Doing the five steps ahead — measure, thickness, cubic yards, waste, order — means the number you give the plant is one you trust, and the only thing left to do on pour day is place it. If you want to sanity check the total against a rough cost, or you are weighing whether the pour is even large enough to justify a truck, the concrete cost factors guide shows where every dollar of a pour actually goes.
Frequently Asked Questions
How do I calculate how many cubic yards of concrete I need?
Multiply the slab’s length by its width to get square feet, multiply that by the thickness in feet (inches divided by 12), and divide the result by 27 to convert cubic feet to cubic yards. For example, a 10 by 10 foot slab at 4 inches is 100 sq ft times 0.333 ft, which is 33.3 cubic feet, divided by 27 is about 1.23 cubic yards. Then add 5 to 10 percent for waste. The Concrete Calculator runs the whole conversion from your measurements.
How thick should a concrete slab be?
Four inches is the standard thickness for patios, walkways, and interior floors that only carry foot traffic. Step up to six inches for driveways, garage floors, and any slab that carries vehicles or heavy loads. Thickness scales volume directly, so going from 4 to 6 inches adds 50 percent more concrete — pick it deliberately, because it is both a strength decision and a cost decision.
How much extra concrete should I order for waste?
Add 5 to 10 percent on top of your calculated volume. Use the lower end for a large, clean, well-formed rectangular slab and the higher end for small, irregular, or hand-formed pours where spillage and an uneven sub-grade eat more. The extra is cheap insurance: coming up a wheelbarrow short mid-pour can trigger a second delivery or short-load fee that costs far more than the few dollars of surplus concrete.
What happens if I order too little concrete?
Running short mid-pour is the outcome you most want to avoid. You either scramble to mix bags to finish — creating a cold joint where the fresh concrete meets the already-setting slab — or you call for a second delivery and pay another short-load or minimum-load fee. Both hurt the finished slab or the budget, which is exactly why the waste factor exists. It is always cheaper to have a little left over than to come up short.
How do I measure concrete for an irregular slab?
Break the shape into simple rectangles, calculate the square footage of each one separately, and add them together before applying the thickness and the conversion to cubic yards. For a rough curve or a rounded end, box it into the smallest rectangle that contains it — over-estimating slightly on an odd shape just folds into your waste factor rather than leaving you short.
Related Calculators
- Concrete Calculator — Convert length, width, and thickness into cubic yards and bags
- Concrete Prices by State — Current per-yard ready-mix rates to price your order
- Ready-Mix vs. Bagged vs. Short-Load — Pick the cheapest way to buy for your yardage
- Rebar Slab Layout — Set the steel before the truck arrives
Written by Daniel McCarney — AceCalc