Concrete slab & footing cost: figuring cubic yards
Concrete is sold by the cubic yard, so estimating a slab or footing starts with a volume calculation. The /27 rule turns your dimensions into cubic yards; your own prices turn that into a cost.
Whether you are pouring a basement slab, a garage floor or the footings a foundation rests on, the estimate begins the same way: concrete is sold by the cubic yard, so you convert your dimensions to a volume first, then multiply by your own $/cu yd and add labor. The one convention you need is that there are 27 cubic feet in a cubic yard. The slab cost tool and footing cost tool do the arithmetic; the cubic-yard conversions table shows the math in general.
The /27 rule for slabs
A slab's volume in cubic yards is:
cu yd = area (sq ft) × (thickness in ÷ 12) ÷ 27
You divide thickness by 12 to convert inches to feet, multiply by the area to get cubic feet, then divide by 27 to get cubic yards. Worked example: a 400 sq ft slab at 4 inches thick is 400 × (4 ÷ 12) ÷ 27 = 4.94 cubic yards. At $160 per cubic yard the concrete is 4.94 × $160 = $790.12. Add labor priced per square foot — 400 × $4 = $1,600 — for a total of about $2,390. The concrete answers "how much to order"; the labor is usually the larger share of the bill.
Footings are the same idea, in three dimensions
A footing is a long, buried concrete beam under a wall or column, so its volume uses length, width and depth:
cu yd = linear ft × (width in ÷ 12) × (depth in ÷ 12) ÷ 27
Worked example: a 120 ft footing that is 16 inches wide and 8 inches deep is 120 × (16 ÷ 12) × (8 ÷ 12) ÷ 27 = 3.95 cubic yards. Multiply by your $/cu yd for the concrete. Footing dimensions are dictated by load and soil — they are engineered — so use the tool to price the volume, not to decide the size.
Order a little extra
Real pours are never perfectly to plan: the subgrade is uneven, forms flex, and some concrete is lost to spillage. It is standard to order a modest overage (commonly around 5–10%) so you do not run short mid-pour, which is far more costly than a little waste. Round up when ordering, and check the supplier's minimum delivery. The driveway tool handles the same slab math for flatwork priced per square foot.
Why we do not store a concrete price
Ready-mix prices move with cement, fuel and your local market, and delivery minimums and short-load fees vary by supplier. So this site keeps no price: you enter $/cu yd and labor from your own quote, and the volume math — which never changes — does the rest. That is what makes the estimate correct today and correct next year. To compare a couple of quotes on a common basis, use the cost-per-square-foot normalizer.
What else goes into a slab besides concrete
The cubic-yard figure tells you how much concrete to order, but a slab or footing bill has several other lines, and on many jobs the concrete itself is the smaller share. There is site prep and excavation — clearing, digging and compacting the subgrade; a gravel base under the slab for drainage and support; forms to shape the pour; reinforcement such as rebar or wire mesh; a vapor barrier under interior slabs; and the labor to place, finish and cure the concrete. Delivery adds its own costs: ready-mix suppliers charge a short-load fee below a minimum quantity, which is why very small pours can cost surprisingly much per yard.
This is why the tools ask for a labor figure alongside the concrete price: for flatwork, finishing labor often rivals or exceeds the material. When you compare quotes, check what each includes — base, forms, rebar, finishing and cleanup — because a low concrete price can hide a thin scope. Use the volume math to get the order quantity right, then build the rest of the estimate from your contractor's line items, and normalize competing quotes with the cost-per-square-foot tool so you are comparing complete jobs.
Footings and structural slabs are engineered
Figuring the volume is arithmetic anyone can do. Deciding a footing's size, its rebar, or whether a slab is structural is engineering — it depends on load and soil bearing and belongs to a licensed professional. Footings and structural slabs are also permit-and-inspection work. The cubic-yard and cost figures here are planning estimates from your own prices, not a design and not a bid.