Concrete Calculator: Cubic Yards and Bags for a Slab or Pad
Calculate the concrete volume for a rectangular slab, pad or footing in cubic feet and cubic yards, and the number of bags based on the yield printed on your bag.
How the calculation works
Concrete is ordered by volume. The calculator multiplies length × width × thickness, converts inches to feet, adds an allowance, and divides by the yield of one bag.
- Volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12.
- Add extra for spillage, an uneven base, and forms that bow slightly. The field starts at 10 percent as an example.
- Cubic yards = cubic feet ÷ 27. Ready-mix concrete is sold by the cubic yard.
- Bags = cubic feet ÷ yield per bag. Bagged concrete mixes print their yield on the bag in cubic feet. The field starts at 0.6 only as an example; replace it with the figure on the bag you buy, since yields differ by bag size and product.
Worked example
A 10 × 10 ft pad, 4 inches thick, with 10 percent extra, using bags that yield 0.6 cu ft each:
| Step | Value |
|---|---|
| 10 × 10 × (4 ÷ 12) | 33.3 cu ft |
| Plus 10% | 36.7 cu ft |
| In cubic yards | 36.7 ÷ 27 ≈ 1.36 cu yd |
| Bags at 0.6 cu ft | 36.7 ÷ 0.6 ≈ 61.1, so 62 bags |
At that size, ordering ready-mix may be easier than mixing dozens of bags by hand. Many suppliers have minimum orders or short-load fees, so call for pricing.
Bags or ready-mix?
- Bags suit small jobs: setting posts, small pads, steps and repairs. Mixing is physically demanding, and each batch must be placed before the previous one sets.
- Ready-mix suits larger slabs and pours where the concrete must be placed in one continuous pour. Plan access for the truck or a pump.
Planning the base and drainage
Concrete lasts longer on a compacted, well-drained base. Slope outdoor slabs slightly so water runs off rather than pooling, and away from the house. The EPA’s moisture guidance recommends making sure the ground slopes away from the building foundation so water does not collect there. A pad for an outdoor AC unit should be level and leave the clearances in the unit’s manual; ENERGY STAR’s checklist emphasizes keeping outdoor equipment clean and unobstructed.
Thickness and reinforcement
Thickness and reinforcement depend on what the slab supports. A walkway, a pad for a trash enclosure and a driveway have very different requirements, and footings or structural slabs are governed by local building codes. If the slab carries a structure, vehicles or heavy equipment, have the design checked by a contractor or your local building department.
Limits of this estimate
The calculator assumes a rectangular area of uniform thickness. For circles, irregular shapes or thickened edges, split the area into rectangles and add the results, or calculate the volume separately. Always buy a little more than the estimate; running short in the middle of a pour creates a weak joint.