Concrete Slab Calculator
- Updated
- Imperial and metric
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Results
1.36 yd³ of concrete including waste; order 1.5 yd³
| Bag size | Yield per bag | Bags | Weight |
|---|---|---|---|
| 40 lb | 0.300 ft³ | 123 | 4,920 lb |
| 50 lb | 0.375 ft³ | 98 | 4,900 lb |
| 60 lb | 0.45 ft³ | 82 | 4,920 lb |
| 80 lb | 0.60 ft³ | 62 | 4,960 lb |
Over about one cubic yard (0.75 m³), a ready-mix truck is usually easier than mixing 62 bags by hand. Compare prices with the concrete cost calculator.
How to use the concrete calculator
- Choose the shape: a rectangular slab or footing, a round slab, a column or tube, or steps.
- Enter the dimensions. Lengths are in feet (or meters); thickness and diameters are in inches (or centimeters).
- For several identical footings or pads, enter how many. For slabs with a deeper perimeter, turn on thickened edges.
- Keep 5–10% waste unless your forms and base are perfectly level. Running short mid-pour is the expensive mistake.
Concrete volume formulas
Round slab: π × (Diameter ÷ 2)² × Thickness
Column or tube: π × (Diameter ÷ 2)² × Height
Steps: Width × Run × Rise × (1 + 2 + … + number of steps)
Cubic yards = cubic feet ÷ 27 · Cubic meters = cubic feet × 0.0283
Convert every measurement to feet first: a 4-inch slab is 4 ÷ 12 = 0.333 ft thick. In metric, keep everything in meters: 100 mm is 0.1 m. The steps formula treats the stairs as solid concrete poured against the ground, which is how most small exterior steps are built.
Worked example: a 12 × 16 ft patio
A 12 × 16 ft patio 4 inches thick is 12 × 16 × 0.333 = 64 ft³, or 2.37 yd³. Adding 10% waste gives 70.4 ft³ (2.61 yd³), so you would order 2.75 cubic yards of ready-mix. Mixing it from bags would take 118 bags of 80 lb concrete, which is why a truck is the better choice at this size.
How much concrete per 100 square feet?
| Thickness | Cubic yards per 100 ft² | Area one cubic yard covers |
|---|---|---|
| 3 in | 0.93 yd³ | 108 ft² |
| 4 in | 1.23 yd³ | 81 ft² |
| 5 in | 1.54 yd³ | 65 ft² |
| 6 in | 1.85 yd³ | 54 ft² |
| 8 in | 2.47 yd³ | 41 ft² |
| Thickness | m³ per 10 m² | Area one m³ covers |
|---|---|---|
| 75 mm | 0.75 m³ | 13.33 m² |
| 100 mm | 1 m³ | 10 m² |
| 125 mm | 1.25 m³ | 8 m² |
| 150 mm | 1.5 m³ | 6.67 m² |
| 200 mm | 2 m³ | 5 m² |
Figures are before waste.
How thick should a concrete slab be?
| Use | Typical thickness |
|---|---|
| Walkways, patios, shed floors, hot tub pads | 4 in (100 mm) |
| Driveways for cars | 5–6 in (125–150 mm) |
| Garage floors and heavy vehicle areas | As specified by plans and local code, often 4–6 in with thickened edges |
| Footings | As specified; commonly 8–12 in deep and twice the wall width |
These are common starting points, not engineering advice. Anything that supports a structure, or that a building inspector will check, should follow your plans and local building code.
Tips for ordering concrete
- Round up, don't down. Ready-mix is often sold in quarter- or half-yard steps; the calculator rounds your order up to the next quarter yard (or 0.1 m³).
- Ask about short loads. Many plants charge a fee for orders below a minimum (often a few yards). For small pours, bags may be cheaper.
- Measure the base, not the plan. An uneven base can add a surprising amount of concrete. Check depth in several places before ordering.
- Plan the finish. Have tools, help and a curing plan ready before the truck arrives; most suppliers allow only limited unloading time.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
At 4 inches thick, a 10 × 10 ft slab is 33.3 cubic feet, or about 1.23 cubic yards, before waste. With 10% extra you would order 1.5 cubic yards of ready-mix (rounded up to the nearest quarter yard) or buy 62 bags of 80 lb concrete.
How do I calculate cubic yards of concrete?
Multiply length (ft) × width (ft) × thickness (in) ÷ 12 to get cubic feet, then divide by 27 to get cubic yards. In metric, multiply length (m) × width (m) × thickness (cm) ÷ 100 to get cubic meters.
How thick should a concrete slab be?
Four inches (100 mm) is standard for patios, walkways and shed floors. Driveways and slabs that carry vehicles are usually 5 to 6 inches (125–150 mm). Garages and structural slabs should follow your plans and local building code.
How much extra concrete should I order?
Order 5% to 10% more than the calculated volume to cover an uneven base, spillage and forms that bow slightly. Running short in the middle of a pour leaves a cold joint, which is far worse than a little leftover concrete. This calculator defaults to 10%.
Should I use bagged concrete or ready-mix?
For pours under about one cubic yard, bags are usually practical. Above that, mixing 45+ bags by hand is slow and a ready-mix truck is usually easier and often cheaper. Compare both with your local prices in the concrete cost calculator.