Concrete Calculator
Estimate how much concrete you need for a slab, footing or round column. Get the volume in cubic yards and cubic metres, the number of pre-mix bags, and an optional cost — with a built-in waste allowance.
Concrete Calculator Tool
How to calculate concrete
Concrete is ordered by volume — cubic yards for ready-mix delivery, or bags for small pours. Find the volume, then convert:
Slab volume = Length × Width × Thickness
Column volume = π × radius² × Height
Cubic yards = cubic feet ÷ 27
Bags of concrete per cubic yard
| Bag size | Yield | Bags per yd³ | Bags per m³ |
|---|---|---|---|
| 80 lb | 0.60 ft³ | ≈ 45 | ≈ 59 |
| 60 lb | 0.45 ft³ | ≈ 60 | ≈ 79 |
| 40 lb | 0.30 ft³ | ≈ 90 | ≈ 118 |
| 30 kg | 0.46 ft³ | ≈ 59 | ≈ 77 |
Slab coverage per cubic yard
| Thickness | Coverage |
|---|---|
| 3 in | ≈ 108 ft² |
| 4 in | ≈ 81 ft² |
| 5 in | ≈ 65 ft² |
| 6 in | ≈ 54 ft² |
Example
A 10 ft × 10 ft slab at 4 inches thick = 100 × (4 ÷ 12) = 33.3 ft³ ≈ 1.23 cubic yards (about 1.36 yd³ with 10% waste). That's roughly 56 bags of 80 lb mix — at this size, ready-mix is cheaper.
A gravel sub-base is recommended under most slabs — see the crushed gravel calculator for the base layer.
Frequently asked questions
How do I calculate concrete?
Multiply length × width × thickness for a slab (or π × radius² × height for a column) to get the volume, then convert to cubic yards or cubic metres. The calculator also estimates the number of pre-mix bags. Add 5–10% for waste and spillage.
How many bags of concrete in a cubic yard?
A cubic yard is 27 ft³, so it takes about 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb pre-mix concrete. For more than about 1 cubic yard, ready-mix delivery is usually cheaper.
How much does a yard of concrete cover?
One cubic yard covers about 65 ft² at 5 inches thick, 81 ft² at 4 inches, or 108 ft² at 3 inches thick.
How much does concrete weigh?
Standard concrete weighs about 2.4 tonnes per cubic metre (≈4,050 lb per cubic yard).
Should I add extra concrete?
Yes — add 5–10% for spillage, over-excavation and uneven subgrade. Running short mid-pour creates a weak cold joint, so it is better to slightly over-order.
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Disclaimer: Concrete estimates are approximate and depend on subgrade, formwork and over-excavation. Order 5–10% extra to avoid running short during a pour. Costs exclude reinforcement, labour, delivery and taxes.