Concrete cubic yards calculator

Concrete cubic yards calculator — slabs, footings, and pours

Ordering the wrong amount of concrete is one of the most expensive mistakes on a construction job. Too little means a cold joint — a structural seam where the pour stopped and restarted — which is often unacceptable for structural slabs and always visible on finished flatwork. Too much means you're paying for material that spins back to the plant. The math is simple; the key is knowing which numbers to put in.

The formula: why 27?

One cubic yard equals 27 cubic feet — because 3 feet × 3 feet × 3 feet = 27. The concrete volume formula is:

Volume (CY) = (Length × Width × Thickness_in_feet) ÷ 27

Since thickness is usually given in inches, convert it first: thickness_ft = thickness_inches / 12. A 4-inch slab is 0.333 feet thick. A 6-inch slab is 0.5 feet thick. A 12-inch footing is 1.0 foot thick.

Standard slab thicknesses

• 3.5–4 inches — sidewalks, patios, pool decks, light-use residential slabs. Minimum thickness for residential flatwork. Anything thinner will crack under foot traffic and thermal cycling.
• 4–5 inches — residential driveways for standard passenger vehicles. Code minimum in most jurisdictions is 4 inches; 5 inches adds margin for loaded pickup trucks.
• 5–6 inches — garage floors, workshop floors, RV pads, light commercial driveways. Six inches with rebar or wire mesh handles most vehicle loads.
• 8–12 inches — perimeter and interior footings, stem walls, grade beams. Structural elements carrying wall and floor loads need this depth, often reinforced with #4 or #5 rebar at 12–18 inch spacing.
• 12–18+ inches — isolated column footings, deep frost footings in cold climates, machine pads. These are typically engineered and specified by a structural engineer.

The waste factor — why you always add extra

No pour is perfectly dimensioned. Forms bow slightly under the weight of wet concrete, adding volume beyond the design dimensions. Ground surfaces are never perfectly flat — high spots eat less concrete, low spots eat more. Some concrete sticks to the chute, to the wheelbarrow, and to tools. And mix trucks can rarely deliver a perfectly precise volume — the driver will round to the nearest tenth of a yard on the ticket.

Industry standard waste factors:

• 5% — extremely clean conditions: laser-leveled subgrade, tight formwork, experienced crew. Rarely achievable in field conditions.
• 10% — standard residential slab or footing. Use this as your default.
• 15–20% — rough ground, complex shapes, multiple footings with lots of corners, or work in tight spots where spillage is more likely.

Ready-mix trucks vs. bag concrete

Ready-mix concrete from a batch plant is the right choice for anything over 1.5–2 cubic yards. A single 80 lb bag of concrete mix yields approximately 0.60 cubic feet — meaning you need roughly 45 bags per cubic yard. At $8–10 per bag retail, one cubic yard of bag mix costs $360–$450 in materials alone, before any labor. Ready-mix typically runs $130–$180 per cubic yard delivered, including the driver. The crossover point is usually around 2 yards when you factor in mix truck short-load fees.

Bag concrete makes sense for: fence post holes, small repair patches, steps, or situations where a truck can't access the pour site. For anything structural or over 2 yards, call a batch plant.

Calculating footings differently from slabs

Slabs are simple rectangles: length × width × depth. Continuous footings are long strips with a rectangular cross-section — compute them as: total linear feet × footing width × footing depth. If a footing runs around a 40×24 building perimeter at 12 inches wide and 8 inches deep, that's 128 linear feet × 1 ft × 0.667 ft ÷ 27 = 3.16 CY for the perimeter alone.

Don't forget interior load-bearing footings. A center girder footing running the 40-foot length adds another 40 × 1 × 0.667 / 27 = 0.99 CY. Total up each section, then apply the waste factor to the grand total.

Ordering from the batch plant

When you call the ready-mix plant, give them: the total yards (already including waste), the mix design (PSI rating), whether you need air entrainment, the slump (workability — 4–5 inch slump is standard for flatwork; 6 inch is workable in tight forms), and whether you need fiber reinforcement. Most plants have a minimum load of 1 CY and charge a short-load surcharge for orders under 5–7 yards. Confirm lead time — next-day delivery is common, same-day can be tight.