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Concrete Calculator
Work out how much concrete you need for a slab, patio, wall, footing, round column, fence post holes or stairs: cubic yards, cubic feet or cubic meters, the number of 40, 60 or 80 lb bags, and the cost of bags or ready-mix.
Cost (optional)
Premixed concrete bags
Cost
- Ready-mix
Under about 1 cubic yard, bags are usually easier and cheaper than a ready-mix truck.
Bag yields: 80 lb = 0.60 cu ft, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.30 (as printed on US bags). Bags are rounded up.
Enter the sizes to see how much concrete you need.
Concrete for common slab sizes
About the Concrete Calculator
The concrete calculator works out how much concrete a job needs. Pick the shape (a slab or patio, a wall, a footing, a round column or Sonotube, fence post holes or stairs), enter the sizes in feet and inches or meters and centimeters, and it shows the cubic yards or cubic meters to order, with extra for waste, and how many premixed bags of each size that makes. Add your local prices to compare bags with a ready-mix truck.
How to use it
- 1 Choose the shape: slab, wall, footing, column, post holes or stairs.
- 2 Pick feet and inches or meters and centimeters, and enter the sizes.
- 3 Set how many, and the extra for waste (10% is usual).
- 4 Read the cubic yards to order and the number of 40, 60 or 80 lb bags; add prices to see the cost.
Frequently asked questions
How do I work out cubic yards of concrete?
Multiply length × width in feet by the thickness in inches, divide by 12 to get cubic feet, then divide by 27. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 4 ÷ 12 = 33.33 cubic feet, or 1.23 cubic yards.
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb. An 80 lb bag makes about 0.6 cubic feet of concrete and a cubic yard is 27 cubic feet.
How much extra concrete should I order?
Order 5 to 10% more than the exact volume. The base is rarely perfectly flat, forms bow a little and some is spilled, and running short in the middle of a pour is far worse than having a little left over.
Should I use bags or ready-mix?
Bags suit small jobs under about one cubic yard, such as post holes, steps and small pads. Above that, ready-mix is usually cheaper and much less work. A truck carries up to about 10 cubic yards, and many suppliers charge a short-load fee for small orders.
How much concrete do I need for a fence post?
A 10 inch hole 36 inches deep with a 4 × 4 post (3.5 inches actual) takes about 1.4 cubic feet: three 80 lb bags or four 60 lb bags. Choose “Post holes” and enter the number of holes for the whole fence.
How thick should a concrete slab be?
4 inches suits patios, walkways and shed floors. Driveways and garage floors are usually 5 to 6 inches. Check local building code, and follow an engineer for anything structural.
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