Concrete Calculator
How much concrete do you need — and is it cheaper by the bag or by the truck? Enter your sizes and get cubic yards, bag counts and cost.
How this concrete calculator works
Slabs and footings: length × width × thickness (inches ÷ 12) = cubic feet. Divide by 27 to get cubic yards.
Round posts: π × radius² × depth. A 12-inch hole 3 feet deep holds about 2.4 cubic feet — four 80 lb bags.
Bags: an 80 lb bag yields about 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³. Bag counts are rounded up and include your waste percentage.
Tips before you pour
- Compact the base (usually 4 inches of gravel) before pouring a slab.
- Ask your supplier about short-load fees — small truck orders often carry a surcharge.
- Footings and foundations must follow your approved plans and local frost depth.
Estimates only — not engineering, legal or code advice. Check requirements with your local building department and a licensed professional before you build.
Common questions
How many bags of concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab 4 inches thick is 33.3 cubic feet (1.23 cubic yards). That's about 56 bags of 80 lb concrete, or 62 bags with 10% extra for waste and uneven ground.
How much does one 80 lb bag of concrete make?
About 0.60 cubic feet. A 60 lb bag makes about 0.45 cubic feet and a 40 lb bag about 0.30 cubic feet. It takes 45 bags of 80 lb concrete to make one cubic yard.
When should I order a concrete truck instead of bags?
For anything over about 1 cubic yard, ready-mix is usually easier and often cheaper once you count your time. Many suppliers charge a short-load fee under 3–5 yards, so ask when you call.
How thick should a concrete slab be?
4 inches is standard for patios, walkways and shed floors. Driveways are usually 4–6 inches, and garage floors 4–6 inches with reinforcement. House foundations are engineered — follow your plans.
How much extra concrete should I order?
Order 5–10% extra. Ground is never perfectly flat, forms bow slightly, and running short in the middle of a pour is far worse than having a little left over.