# How much concrete to buy?

Computes the volume of concrete for a slab, a round column or a tube, with extra for waste, in cubic yards, cubic feet and cubic metres, and the number of premix bags.

- Page: https://www.acalculator.org/construction/concrete-calculator
- JSON spec: https://www.acalculator.org/construction/concrete-calculator.json
- Version: 0c7fb2099933

## Default answer

Example with the default inputs (Shape Slab, footing or wall, Length 10 ft, Width 10 ft, Thickness 4 in, How many 1, Extra for waste 5%, Bag size 80 lb bag): You need 1.3 yd³ of concrete, or 59 bags.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| shape | Shape | The shape of the pour. |
| len | Length | The length of the slab. |
| wid | Width | The width of the slab. |
| thick | Thickness | How thick the slab is. |
| dia | Diameter | The diameter of the column or hole, or the outer diameter of the tube. |
| inner | Inner diameter | The diameter of the hollow inside the tube. |
| h | Height or depth | The height of the column, or the depth of the hole. |
| qty | How many | How many pours of this size. |
| waste | Extra for waste | Extra concrete for spills and an uneven base, as a percent of the volume. |
| bag | Bag size | The weight of the premix bag, which sets how much concrete one bag makes. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| cubicYards | Concrete | The volume to order, waste included, in cubic yards. |
| cubicFeet | Cubic feet | The volume to order, waste included, in cubic feet. |
| cubicMeters | Cubic metres | The volume to order, waste included, in cubic metres. |
| bags | Premix bags | The number of bags of the chosen size, rounded up to a whole bag. |
| weight | Weight of the concrete | The volume to order × about 140 pounds per cubic foot. |

## Method

volume = shape volume × how many × (1 + waste ÷ 100); bags = ⌈volume ÷ bag yield⌉; cubic yards = cubic feet ÷ 27

## Assumptions

- Slab: length × width × thickness. Round column: π × diameter² ÷ 4 × height. Tube: π × (outer² − inner²) ÷ 4 × height.
- Bag yields are from the QUIKRETE Concrete Mix data sheet: 40 lb 0.30 ft³, 50 lb 0.375 ft³, 60 lb 0.45 ft³, 80 lb 0.60 ft³, 90 lb 0.675 ft³.
- The weight uses the data sheet’s unit weight of mixed concrete, about 140 lb/ft³.

## Worked examples

1. shape = slab, len = 3.048, wid = 3.048, thick = 0.1016, qty = 1, waste = 0%, bag = 80 gives cubicFeet = 0.943895, bags = 56, weight = 2,116.764393. Source: hand calculation in content.mdx: 10 × 10 × 4/12 = 33.33 ft³ = 1.23 yd³; 33.33 ÷ 0.60 = 55.6, so 56 bags of 80 lb (QUIKRETE yield).
2. shape = slab, len = 3.6576, wid = 6.096, thick = 0.1524, qty = 1, waste = 10%, bag = 60 gives cubicYards = 3.737824, bags = 294. Source: hand calculation in content.mdx: 12 × 20 × 0.5 = 120 ft³; × 1.10 = 132 ft³ = 4.89 yd³; 132 ÷ 0.45 = 293.3, so 294 bags.
3. shape = column, dia = 0.254, h = 0.9144, qty = 4, waste = 0%, bag = 50 gives cubicFeet = 0.185333, bags = 18. Source: hand calculation in content.mdx: π × (10/12)² ÷ 4 × 3 = 1.636 ft³ a hole; × 4 = 6.545 ft³; ÷ 0.375 = 17.5, so 18 bags.
4. shape = tube, dia = 0.6096, inner = 0.3048, h = 1.2192, qty = 1, waste = 0%, bag = 80 gives cubicFeet = 0.26688, bags = 16. Source: hand calculation in content.mdx: π × (2² − 1²) ÷ 4 × 4 = 3π = 9.425 ft³; ÷ 0.60 = 15.7, so 16 bags.

## FAQ

### How do I calculate concrete for a slab?

Multiply length × width × thickness, with every size in feet, to get cubic feet. Divide by 27 for cubic yards. A 10 ft × 10 ft slab 4 in thick is 10 × 10 × 4/12 = 33.33 ft³, or 1.23 yd³.

### How many bags of concrete make a cubic yard?

A cubic yard is 27 cubic feet. An 80 lb bag of QUIKRETE Concrete Mix makes about 0.60 ft³, so a cubic yard takes 27 ÷ 0.60 = 45 bags. With 60 lb bags (0.45 ft³) it takes 60 bags.

### How much extra concrete should I order?

The calculator adds the percent you type in Extra for waste. Order a little more than the exact volume: spills, a base that is lower in places and forms that bow all use concrete, and running short part way through a pour is worse than a little left over.

### How much concrete does a post hole need?

Use the round column shape with the hole’s diameter and depth. A hole 10 in wide and 3 ft deep holds π × (10/12)² ÷ 4 × 3 = 1.64 ft³. The post itself takes up some of that space, so the answer is a little more than you need.

### How much does a cubic yard of concrete weigh?

QUIKRETE gives the unit weight of its mixed Concrete Mix as about 140 lb/ft³. A cubic yard is 27 ft³, so it weighs about 27 × 140 = 3,780 lb.

### Should I buy bags or ready-mix?

Bags suit small jobs such as posts, steps and pads you can mix by hand or in a small mixer. For a large slab the bag count grows fast: 2 cubic yards is 90 bags of 80 lb, so many people order ready-mixed concrete by the cubic yard instead.

## Sources

- The QUIKRETE Companies, Concrete Mix No. 1101 data sheet (revised October 2022): bag yields and a unit weight of about 140 lb/ft³ (ASTM C138). https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf
- NIST SP 811 (2008), Appendix B: 1 ft = 0.3048 m and 1 in = 0.0254 m exactly; 1 yd³ = 27 ft³.