Construction
Concrete Calculator — Yards
Enter your slab dimensions to get cubic yards, cubic feet and how many bags of pre-mixed concrete you need.
How the concrete calculator works
This concrete calculator turns a slab's footprint and depth into the volume of concrete you need, expressed in cubic yards — the unit ready-mix trucks are sold by in the US. The only catch is that thickness is measured in inches while length and width are in feet, so the inches must be converted to feet (divide by 12) before multiplying.
The formula is:
- Cubic feet = length (ft) × width (ft) × (thickness in ÷ 12)
- Cubic yards = cubic feet ÷ 27 (there are 27 cubic feet in a cubic yard)
- Bags = cubic feet ÷ bag yield — an 80-lb bag yields about 0.60 ft³, a 60-lb bag about 0.45 ft³, and a 40-lb bag about 0.30 ft³ (always rounded up)
Worked example: a 10 × 10 ft, 4-inch slab
A 10 ft by 10 ft patio poured 4 inches deep needs 10 × 10 × (4 ÷ 12) = 33.3 cubic feet of concrete. Dividing by 27 gives 1.23 cubic yards. If you are bagging it instead of ordering a truck, that is 33.3 ÷ 0.60 = 56 bags of 80-lb mix, or 74 bags of 60-lb mix. For anything larger than a few cubic yards, ready-mix delivery is almost always cheaper and faster than bags.
Concrete bag yields reference
| Bag size | Approx. yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Yields vary slightly by brand, so treat bag counts as a close estimate and round up. Adding a 5–10% allowance on top protects you from running short mid-pour, which can leave a visible cold joint where fresh concrete meets concrete that has already begun to cure.
Frequently asked questions
How many cubic yards of concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches thick needs about 1.23 cubic yards (33.3 cubic feet). Change the thickness above to see how it scales — a 6-inch pour of the same footprint needs about 1.85 cubic yards.
How many 80-lb bags are in a cubic yard of concrete?
About 45 bags, since each 80-lb bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet. A 60-lb bag yields about 0.45 ft³ (≈60 bags per yard).
Why should I order extra concrete?
Subgrades are rarely perfectly level, forms bow slightly, and some concrete is always lost to spillage. Ordering 5–10% extra avoids running short mid-pour, which is far worse than having a little left over.
Does this calculator store my measurements?
No. The calculation runs entirely in your browser — your input isn't uploaded or saved.
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