Concrete Calculator
Enter the length, width and thickness of your slab or footing to find how much concrete you need — in cubic yards and in 60 lb or 80 lb bags.
Concrete is ordered by volume, so estimating a job means working out the space you're filling in cubic yards — the unit ready-mix suppliers use. This calculator takes your slab or footing dimensions, converts them to volume, adds a waste allowance for spillage and uneven subgrade, and also shows how many bags you'd need if you're mixing your own. It works for patios, driveways, footings, shed bases and steps.
How to calculate concrete
Multiply length by width by thickness to get the volume in cubic feet, then divide by 27 to convert to cubic yards. Remember thickness is in inches, so divide it by 12 first.
Cubic yards = Cubic feet ÷ 27
Bags = Cubic feet ÷ yield per bag (80 lb ≈ 0.60, 60 lb ≈ 0.45 cu ft)
| Slab | Volume |
|---|---|
| 10 × 10 ft at 4 in | 1.23 cu yd |
| 20 × 10 ft at 4 in | 2.47 cu yd |
| 24 × 24 ft at 4 in (garage) | 7.11 cu yd |
Worked example
A 20 ft × 10 ft patio, 4 inches thick, with 10% waste:
| Cubic feet = 20 × 10 × (4 ÷ 12) | 66.7 |
| Cubic yards = 66.7 ÷ 27 | 2.47 |
| With 10% waste | ≈ 2.7 cu yd |
Bags or ready-mix?
Mixing bags by hand makes sense for small jobs — a post footing, a small pad, or repairs — up to roughly half a cubic yard. Beyond that, the number of bags climbs fast (a cubic yard is about 45 eighty-pound bags), and ready-mix delivered by truck is usually cheaper, faster and more consistent. Most suppliers have a minimum order and charge for short loads, so get a quote and round your order up rather than down; running out partway through a pour creates a weak cold joint.