Concrete Calculator

Work out how much concrete a slab, patio, footing or set of post holes needs, in cubic yards and in 40, 60 or 80 lb bags, with an allowance for waste. Add prices to compare mixing bags with ordering ready-mix. Your inputs stay in your browser.

Your pour

What you are pouring

Shape

Patios and walkways are often 4 inches thick.

5% to 10% is common.

Bag yields

Check the bag: yields vary by product.

Compare costs (optional)

1.36 cu yd · 62 bags of 80 lb. See full results

Example: a 10 × 10 ft slab, 4 inches thick, with 10% extra. Bag yields are typical figures; check your bag. Enter your numbers to update.

Your concrete estimate

Concrete needed
1.36 cu yd
36.7 cu ft, including 10% extra
80 lb bags
62
About 4,960 lb to move
Before waste
33.3 cu ft
One piece

Bags needed by size

Bag sizeYield per bagBagsTotal weight
40 lb 0.3 cu ft1234,920 lb
60 lb 0.45 cu ft824,920 lb
80 lb 0.6 cu ft624,960 lb

Bags or ready-mix?

Add a bag price and a ready-mix price per cubic yard to compare the cost of mixing bags with ordering a truck.

How to calculate concrete volume

  1. Rectangular pour: length × width × thickness, with the thickness in feet (inches ÷ 12). The example is 10 × 10 × 4 ÷ 12 = 33.3 cubic feet.
  2. Round hole or column: π × radius² × depth. Six 10-inch holes 4 feet deep hold 13.1 cubic feet.
  3. Cubic yards = cubic feet ÷ 27, since a cubic yard is 3 × 3 × 3 feet (NIST: tables of units of measurement).
  4. Add extra for spillage, over-digging and uneven ground: the example's 10% takes it to 1.36 cubic yards.

How many bags

Divide the volume by what one bag makes and round up. Yields depend on the product; common figures are about 0.30, 0.45 and 0.60 cubic feet for 40, 60 and 80 lb bags, which is what the form starts with. The example needs 62 bags of 80 lb, about 4,960 lb to carry and mix. At 6 inches thick instead of 4, it needs 2.04 cubic yards, or 92 bags.

Bags or a ready-mix truck?

Bags suit small jobs you can mix by hand or with a rented mixer. For larger pours, ready-mix delivered by truck is easier and gives a consistent pour, but suppliers often charge a delivery or short-load fee for small orders. Enter your local prices to compare the two for your job.

What this calculator does not include

  • Gravel base, forms, rebar or mesh, and labor.
  • Steps, curbs and irregular shapes. Split them into rectangles and add the results.
  • Engineering: thickness and reinforcement for driveways or structural footings depend on your soil, loads and local building code.

Methodology and testing

Automated tests compare the volumes, bag counts and costs with an independent calculation, and check that a volume that is an exact multiple of a bag's yield is not rounded up an extra bag. How we build and test calculators.

Sources

  • Gravel Calculator

    Cubic yards and tons of gravel for rectangular or round areas, with density, compaction allowance and cost.

  • Asphalt Calculator

    Tons and cubic yards of asphalt from length, width and thickness, with waste and cost.

  • Mulch Calculator

    Cubic yards and bags of mulch for rectangular and round beds, coverage by depth, and bags vs. bulk cost.

  • Deck Calculator

    Deck boards, joists and screws from deck size, board size, gap and joist spacing, with waste and cost.

  • Cubic Yard Calculator

    Cubic yards for rectangles, circles and triangles, or the area a delivery covers, in cubic feet, cubic meters, bags and truckloads.