Concrete is unforgiving about quantity. Run short mid-pour and you get a cold joint — a weak seam where fresh concrete meets partly-set concrete. Over-order and you're paying for material you'll shovel into a wheelbarrow and dump. Getting the number right the first time is the whole game.
The good news: concrete volume is just geometry. Every pour is some combination of slabs, footings, and cylinders, and each has a simple formula.
The one unit that matters: volume
Concrete is sold and mixed by volume — cubic yards in the US, cubic meters most everywhere else. Everything reduces to length × width × depth, so the only trick is keeping your units consistent before you multiply.
Depth is where people slip up
Convert depth to the same unit as length and width before multiplying. A 4-inch slab is 0.33 ft deep, not 4. Multiplying feet by inches is the most common concrete-ordering mistake there is.
Slabs and patios
- Volume = length × width × thickness (all in feet).
- Divide cubic feet by 27 to get cubic yards.
- Example: a 10 ft × 12 ft patio at 4 inches (0.33 ft) = 39.6 cu ft ÷ 27 ≈ 1.47 cubic yards.
Footings and post holes
Footings are long, narrow slabs — same formula, just tall and thin. Round post holes are cylinders: volume = π × radius² × depth. A 12-inch diameter hole (0.5 ft radius) at 3 ft deep holds about 2.4 cubic feet, so a handful of posts adds up faster than people expect.
Concrete Calculator
Enter your dimensions and get cubic yards, cubic meters, and an estimated bag count in one go.
Bags vs ready-mix
For small jobs you mix bags yourself; for large pours you order a truck. Knowing the crossover point saves both money and your back.
- An 80 lb bag of concrete yields about 0.6 cubic feet. It takes roughly 45 of them to make one cubic yard.
- Below ~0.5 cubic yards, bags usually win on cost and convenience.
- Above ~1 cubic yard, a ready-mix truck is faster, stronger (consistent mix), and often cheaper per yard.
Always add a waste factor
Order 5–10% extra
Uneven subgrade, spillage, and over-excavated holes all eat into your calculated volume. Add 10% for bags, 5% for a ready-mix order. Running out is far more expensive than a little leftover.
Do the geometry, keep your units straight, pad for waste, and you'll place a clean, continuous pour — no cold joints, no second trip to the yard.