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Concrete Calculator
Estimate how much concrete you need for a patio slab, walkway, footing or a set of fence post holes. You get cubic yards for ready-mix delivery and the number of bags if you are mixing it yourself.
Results are estimates. Soil, stone and mulch settle, weigh differently and vary by supplier. Confirm quantities and local requirements before ordering.
How to calculate it
For a slab or footing, multiply length × width × thickness, all in feet, to get cubic feet. Divide by 27 for cubic yards. For a round post hole or column, use π × radius² × depth.
Formula: cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27.
Bag counts come from the yield of each bag: roughly 0.30 cubic feet for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. Always check the yield printed on your brand of bag, because it varies slightly.
Worked example
A 12 ft × 10 ft slab poured 4 inches thick, with 10% extra:
| Volume (no extra) | 40 cu ft 1.48 cubic yards |
|---|---|
| Concrete to order | 1.63 cubic yards includes 10% extra |
| 40 lb bags | 147 bags each yields about 0.3 cu ft |
| 50 lb bags | 118 bags each yields about 0.375 cu ft |
| 60 lb bags | 98 bags each yields about 0.45 cu ft |
| 80 lb bags | 74 bags each yields about 0.6 cu ft |
| Cubic meters | 1.25 m³ |
Bags needed per cubic yard
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Tips
- Add 5–10% extra. Uneven sub-base, spillage and form movement all use more concrete than the pure volume.
- Ready-mix is usually the better choice above about 1–2 cubic yards. Trucks often charge a fee for small loads.
- Typical thicknesses: 4 inches for patios and walkways, 5–6 inches for driveways. Follow local code for footings and structural work.
- Order a small amount extra of ready-mix, since running short in the middle of a pour causes cold joints.
Frequently asked questions
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb mix. Always check the yield on your brand.
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 ft × 10 ft slab is 33.3 cubic feet, or about 1.23 cubic yards. With 10% extra, plan on about 1.4 cubic yards.
How much concrete do I need for a post hole?
A hole 10 inches across and 3 feet deep holds about 1.6 cubic feet, so about three 80 lb bags. The calculator handles the exact numbers.
Should I order extra concrete?
Yes. A 5–10% allowance is normal for ready-mix, and slightly more if your forms or sub-base are rough.