Concrete calculator (slabs, footings, post holes)
Work out how much concrete you need for a slab, footing or round post hole, with a waste allowance, in cubic yards or cubic meters, plus the number of 80 lb or 60 lb bags.
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- Pre-mixed concrete bags
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- 80 lb bags (0.60 ft³)
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- 60 lb bags (0.45 ft³)
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- Site-mixed concrete – Cement
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Results are indicative estimates based on the values you enter. Always check your appliance labels, your tariff and the manufacturer’s data.
- Slab volume = length × width × thickness, all in the same unit (4 in = 0.333 ft).
- One cubic yard = 27 cubic feet: about 45 bags of 80 lb or 60 bags of 60 lb of QUIKRETE Concrete Mix.
- Add 5–10% for waste, uneven ground and spillage.
How to calculate concrete volume
Rectangular slab, patio or footing: volume = length × width × thickness. Convert everything to feet first: a 10 ft × 10 ft slab, 4 inches thick, is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or 33.3 ÷ 27 = 1.23 cubic yards.
Round post hole or column: volume = π × radius² × depth. A 12-inch diameter hole (radius 0.5 ft) that is 3 ft deep holds 3.1416 × 0.5² × 3 ≈ 2.36 cubic feet (before subtracting the post itself).
How many bags?
Bagged concrete mixes state their yield on the bag. For QUIKRETE Concrete Mix, the manufacturer’s data sheet gives:
| Bag size | Yield per bag | Bags per cubic yard (27 cu ft) |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
Other brands and products (fast-setting, high-strength) may differ: always check the yield printed on the bag. For the 10 × 10 ft slab above, 33.3 ÷ 0.60 ≈ 56 bags of 80 lb before waste, or 62 bags with a 10% allowance. That is about 2.5 tons of bags to lift, which is why many people order ready-mix for jobs of this size.
Metric units
Switch the calculator to metric to enter meters and centimeters. It then shows the volume in cubic meters, the number of pre-mixed bags based on the yield you enter (in liters per bag), and the cement needed for site-mixed concrete at a chosen cement content (kg per m³), which is how concrete is usually specified in Europe.
How much extra to order
- 5% for a well-prepared, level sub-base and accurate formwork.
- 10% as a sensible default for DIY projects.
- 15% for uneven ground or rough measurements.
Running short in the middle of a pour creates a weak joint, so it is better to have a little left over.
Safety and structure
This calculator estimates quantities only. It does not design anything: slab thickness, reinforcement and mix strength for structural work (house foundations, driveways for heavy vehicles, retaining walls) should be specified by a professional and follow local building codes. Wet concrete is caustic: wear gloves, long sleeves and eye protection.