Concrete Calculator
Free concrete calculator. Estimate cubic yards for slabs, footings, columns, and walls, plus 60/80 lb bags and delivered cost. No signup.
What is a concrete calculator?
A concrete calculator estimates how many cubic yards of concrete you need for a slab, footing, column, or wall, then converts that into an approximate number of bags or a delivered-order cost. It turns measurements into the number you actually order.
How this calculator works
- Pick the shape — slab, footing/trench, column, or wall — and enter its dimensions.
- It computes the volume in cubic feet, then converts to cubic yards (27 ft³ per yard).
- Add a waste percentage so small spillage and uneven grade don’t leave you short.
- Enter a delivered price per cubic yard to see the approximate order cost, plus the number of 60 lb or 80 lb bags if you’re mixing yourself.
How to use the result
- Double-check your dimensions before ordering — one wrong foot of slab length changes the yardage noticeably.
- Compare delivered truck concrete vs. bagged. For under a yard, bags usually win; for a patio or driveway, a truck is cheaper per yard.
- Keep the waste allowance at 5–10%. Concrete suppliers won’t take back extra — it’s cheaper to over-order a little than to pour short.
- Use the yardage as the number to get quoted by two suppliers; local rates vary by area and delivery distance.
How much concrete do I need for a slab?
A common patio slab of 20 × 10 ft at 4 inches thick is about 2.5 cubic yards. The rule of thumb: length × width × thickness (in feet) ÷ 27. For example, 20 × 10 × 0.33 = 66.7 ft³ ÷ 27 = 2.47 yd³. That matches a standard ready-mix short-load delivery.
Common mistakes
- Mixing units. Convert inches to feet before multiplying — a 4-inch slab is 0.33 ft, not 4.
- Ordering net yardage with no waste. Grade irregularities and spillage easily eat 5–10% — and you can’t order a partial truck.
- Assuming one bag covers the same as another. An 80 lb bag yields about 0.6 ft³; a 60 lb bag about 0.45 ft³. Use the right yield for the bag you buy.
Concrete volume math is pure geometry — the same length×width×depth logic used by the roofing cost calculator for surface area. For a different kind of project cost, the Pythagorean calculator helps square up a layout before you pour.