How much concrete do I need? Measuring & cubic yards
Concrete is ordered by the cubic yard, but you measure your project in feet and inches. This guide shows the one conversion that ties them together, so you can walk into a ready-mix plant or a hardware store knowing exactly how much you need.
The one formula that does the work
Every "how much concrete" question is really a volume question. Concrete fills a shape, and a flat slab is just a very thin box: length × width × thickness. The only trick is that your thickness is in inches while length and width are in feet, so you divide the thickness by 12 to convert it to feet first:
Cubic feet = length (ft) × width (ft) × (thickness in ÷ 12)
That gives you cubic feet. Concrete is sold by the cubic yard, and a cubic yard is a 3 ft × 3 ft × 3 ft cube = 27 cubic feet. So:
Cubic yards = cubic feet ÷ 27
That is the whole thing. Area × thickness gives volume; divide by 27 to order. Our concrete calculator does exactly this and also splits the answer into bags, and the concrete slab calculator handles L-shaped pours as two rectangles added together.
A worked example, start to finish
Say you are pouring a 12 ft × 12 ft patio, 4 inches thick. Work it in order:
- Area: 12 × 12 = 144 square feet.
- Thickness in feet: 4 ÷ 12 = 0.333 ft.
- Cubic feet: 144 × 0.333 = 48 cubic feet.
- Cubic yards: 48 ÷ 27 = 1.78 cubic yards.
So a 12×12 slab at 4 inches needs about 1.78 cubic yards of concrete. Most plants sell in quarter- or half-yard increments, so you would order 2 yards and have a small cushion.
How much area does a cubic yard cover?
Because coverage is pure geometry (27 cubic feet spread over a chosen thickness), it never changes and is worth memorizing:
| Thickness | Area covered by 1 cubic yard |
|---|---|
| 3 inches | ≈ 108 sq ft |
| 4 inches | ≈ 81 sq ft |
| 6 inches | ≈ 54 sq ft |
(To get these: 27 ÷ thickness-in-feet. At 4 inches that is 27 ÷ 0.333 = 81 sq ft.) A quick sanity check on any order: divide your square footage by the coverage above.
Measuring the real world, not a textbook rectangle
Few projects are a perfect rectangle. A few habits keep your number honest:
- Break odd shapes into rectangles. An L-shaped patio is two rectangles; add their volumes. A circle is π × radius² × thickness. The slab calculator does the L-shape for you.
- Measure the deepest reality, not the plan. Subgrade is rarely dead level; a slab that is supposed to be 4 inches often averages 4½. Even half an inch across a big pad is real concrete.
- Account for thickened edges and footings. A monolithic slab with a thickened perimeter, or sonotube footings, hold extra volume — add them separately (see the sonotube / column / footing calculator).
Always order a little extra
Running short mid-pour is the one mistake you cannot fix — a cold joint between yesterday's and today's concrete is a permanent weak line. So professionals add 5–10% for spillage, over-excavation, uneven subgrade and the concrete left in the chute and wheelbarrow. On our 48-cubic-foot patio, a 10% cushion is 48 × 1.10 = 52.8 cubic feet, or about 1.96 cubic yards — which is why you would round up to 2 yards.
Turning volume into an order
Once you know the cubic yards, you have two ways to buy it. For big pours, ready-mix delivered by truck is almost always cheaper and easier — see concrete cost per yard. For small jobs (roughly under a cubic yard) bagged mix can win; the bags-of-concrete calculator converts your volume into 40, 60 and 80 lb bags, and ready-mix vs bags finds the breakeven point between the two.
Rounder and odder shapes
Not every pour is a rectangle, but every shape reduces to the same area × thickness idea. A round pad or a column uses the circle area, π × radius², in place of length × width — a 10 ft diameter patio is π × 5² = 78.5 sq ft, then × thickness as usual. A driveway that widens toward the street is a trapezoid: average the two widths, then multiply by the length. An L-shape is two rectangles; a T-shape is three. And a slab with a thickened edge or an integral footing holds extra concrete around its perimeter that a flat-slab calculation quietly misses — add those beams as their own prisms. The one habit that keeps every shape honest is to split it into pieces you can actually measure, compute each piece, and add the volumes; the concrete slab calculator does the L-shape for you, and the sonotube / column / footing calculator handles round tubes and footings.
A quick pre-order checklist
Before you place an order, run down five questions. Did I measure the real thickness, not the plan thickness — is the subgrade actually level? Did I add every piece: the main slab, thickened edges, footings, steps and curbs? Did I convert cubic feet to cubic yards by dividing by 27? Did I add 5–10% for waste, spillage and the concrete left in the chute? And did I round up to the increment my plant or store sells in — quarter- or half-yard for ready-mix, whole bags for bagged mix? Miss any one and you either run short mid-pour (the expensive, unfixable mistake) or pay for concrete you cannot use. The calculator handles the arithmetic; this checklist keeps the inputs honest, which is where most ordering errors actually come from.
The bottom line
Measure length, width and thickness; multiply for cubic feet; divide by 27 for cubic yards; add 5–10%. That single chain answers "how much concrete do I need" for a slab, patio, driveway, sidewalk or footing. Everything else — bags, cost, rebar, gravel base — builds on this number. Remember that these are material-quantity guides: confirm bag yields against your product's spec, and for load-bearing work let a licensed engineer size the concrete and reinforcement.