Concrete Calculator — Cubic Yards, Bags and Cost

Concrete is sold by volume, and in the United States the unit is the cubic yard. The core formula for any rectangular pour is length × width × thickness, with every dimension in feet, which gives cubic feet. Divide by 27 to convert to cubic yards, because a cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. The only common trap is thickness: slabs are described in inches, so divide inches by 12 before multiplying. Filling the hollow cores of a block wall is ordered the same way, by volume, and the block wall calculator works out the cubic yards of core fill for you.

Here is a full worked example. A patio measures 10 ft × 12 ft and is poured 4 in thick. Four inches is 4 ÷ 12 = 0.333 ft, so the volume is 10 × 12 × 0.333 = 40 cubic feet. Dividing by 27 gives 40 ÷ 27 = 1.48 cubic yards. Footings use the same box maths with depth in place of thickness, and if you have several footings, rebar or a frost depth to meet, the dedicated footing calculator handles them together. Columns and post holes are cylinders, so use π × radius² × height, and for a post set in concrete subtract the volume the post displaces. If you are pouring into round form tubes, the form tube calculator also works out how many tubes to buy and how to cut them. Stairs are the sum of each step's block, which is width × run × rise × (steps × (steps + 1) ÷ 2) for solid steps, plus any platform.

If you are mixing from bags, divide your cubic feet by the yield printed on the bag. An 80 lb bag yields about 0.60 cubic feet, which means 45 bags per cubic yard. A 60 lb bag yields 0.45 cubic feet (60 bags per yard) and a 40 lb bag yields 0.30 cubic feet (90 bags per yard). Always round up to a whole bag, since a part bag cannot be bought. For the 40 cubic foot patio above, that is 40 ÷ 0.60 = 66.7, so 67 bags of 80 lb. Fresh concrete weighs about 150 lb per cubic foot, so the same patio is roughly 6,000 lb, or 3 tons, before you add the sub-base. If you need the load or haulage side of that number, such as what a floor can carry or how many trips a trailer needs, the concrete weight and load check covers it.

The break-even between bags and ready-mix is roughly 1 cubic yard. Below that, bagged concrete is convenient and avoids the delivery and short-load fees that suppliers commonly charge on small orders. Above it, mixing by hand stops making sense: one yard is 45 bags of 80 lb, and each bag weighs 80 lb, so you would be lifting, mixing and placing about 3,600 lb of material before it starts to set. Ready-mix arrives ready to place, is usually cheaper per yard at that scale, and gives a more consistent mix, which matters for strength and for avoiding cold joints in a large pour.

Finally, plan for waste. Real pours always use more concrete than the drawing suggests, because forms bow slightly, the subgrade is never perfectly level, some material is spilled, and the bottom of the truck chute or wheelbarrow never empties completely. A 10% allowance is the usual default: it is large enough to absorb these small losses, yet small enough not to leave a large pile of leftover concrete. Use 5% for a tidy pour into precise forms and 15% or more for rough excavation, stepped footings or hard-to-reach sites. Running short mid-pour creates a cold joint that weakens the slab, and a second delivery usually triggers another minimum charge, so it is almost always cheaper to over-order slightly.