# Free online concrete calculator

> Concrete volume tells you how much ready-mix to order or how many bags to buy. This calculator covers rectangular slabs, footings, and cylindrical columns in imperial and metric.

Calculate how much concrete you need for slabs, footings, and cylindrical columns in cubic yards, cubic feet, and cubic meters. Also shows how many 60-lb and 80-lb bags to buy.

**Interactive version:** https://onlinecalculator.me/construction/concrete/
**Category:** Construction

## How to use

1. Choose your shape: Slab, Footing, or Column.
2. Pick Imperial (feet and inches) or Metric (meters).
3. Enter dimensions — length, width, and depth for slabs/footings; radius and height for columns.
4. Set the count if you're pouring multiple identical elements.
5. Read cubic yards, cubic feet, cubic meters, and bag counts in the results.

## Formulas

**Rectangular slab or footing (imperial):**
```
cubic feet = length (ft) × width (ft) × depth (in) ÷ 12
cubic yards = cubic feet ÷ 27
```

**Cylindrical column (imperial):**
```
cubic feet = π × (radius in ÷ 12)² × height (ft)
cubic yards = cubic feet ÷ 27
```

**Metric (all dimensions in meters):**
```
cubic meters = length × width × depth   (slab/footing)
cubic meters = π × radius² × height     (column)
```

**Bag counts:**
```
60-lb bags = ceil(cubic feet ÷ 0.45)
80-lb bags = ceil(cubic feet ÷ 0.60)
```

## Unit conversions

| From | To cubic yards |
|---|---|
| Cubic feet | divide by 27 |
| Cubic meters | multiply by 1.30795 |

## Worked example

Driveway slab: 20 ft × 10 ft × 4 in deep

```
cubic feet  = 20 × 10 × (4 ÷ 12) = 66.67 ft³
cubic yards = 66.67 ÷ 27         = 2.47 yd³
60-lb bags  = ceil(66.67 ÷ 0.45) = 149 bags
80-lb bags  = ceil(66.67 ÷ 0.60) = 112 bags
```

Order 2.5 cubic yards of ready-mix (always round up slightly when ordering) or 112 bags of 80-lb concrete mix.

## Notes

- Always add 5–10% extra when ordering ready-mix to account for spillage and uneven sub-grade.
- Bag yield varies slightly by brand; the calculator uses the standard 0.45 ft³ for 60-lb and 0.60 ft³ for 80-lb.
- For large pours (>1 yd³), ready-mix is usually more cost-effective and consistent than bags.

## Frequently asked questions

### How do I calculate cubic yards of concrete?

For a rectangular slab: (length ft × width ft × depth in) ÷ 324 = cubic yards. The ÷ 324 converts depth from inches to feet (÷ 12) and then cubic feet to cubic yards (÷ 27). The calculator does this automatically.

### How many 60-lb bags of concrete do I need per cubic yard?

A 60-lb bag yields about 0.45 cubic feet, so you need roughly 60 bags per cubic yard (27 cu ft ÷ 0.45). An 80-lb bag yields about 0.60 cubic feet, so about 45 bags per cubic yard.

### How thick should a concrete slab be?

Typical residential slabs are 4 inches thick. Driveways are usually 4–6 inches. Structural slabs and heavy equipment pads start at 6 inches. The calculator accepts any depth you enter.

### What does the count field do?

Count multiplies the volume by the number of identical slabs, footings, or columns. If you're pouring 8 concrete piers all the same size, enter 8 in the count field.

### How do I calculate concrete for a round column?

Volume = π × radius² × height. The calculator uses this formula when you select the Column shape. Enter the radius (not the diameter) and the height.

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

Approximately 4,050 lb (about 1.84 metric tons). A standard ready-mix truck carries about 10 cubic yards, which weighs roughly 40,000 lb.

## Sources

- [Concrete (composition and materials)](https://en.wikipedia.org/wiki/Concrete) — Wikipedia
- [Cement & Concrete overview](https://www.cement.org/cement-concrete/) — American Cement Association
- [Volume (measuring three-dimensional space)](https://en.wikipedia.org/wiki/Volume) — Wikipedia

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