Concrete volume in cubic yards = (length ft x width ft x thickness in inches / 12) / 27. A 4-inch slab covers roughly 81 square feet per cubic yard. Always add a waste allowance, commonly 5% to 10%, for subgrade variation, spillage and over-excavation, since running short mid-pour is far more expensive than ordering slightly over.
Yardage math is easy to get wrong under pressure, and a short load on pour day costs a lot more than the extra concrete would have. Enter your slab or footing dimensions for cubic yards, bag counts and delivered cost.
| Input | Value |
|---|---|
| Length | 30 |
| Width | 20 |
| Thickness | 4 |
| Number of pours | 1 |
| Waste allowance | 10 |
| Ready-mix price | 165 |
| Result | Value |
|---|---|
| Concrete needed | 7.41 cu yd |
| Order with waste | 8.15 cu yd |
| Cubic feet | 200 |
| Ready-mix cost | $1,344.44 |
| 80 lb bags (small pours) | 334 |
| Slab area | 600 sq ft |
An 80 lb bag yields roughly 0.6 cubic feet, so a single cubic yard takes about 45 bags. Anything beyond a few yards is cheaper, faster and more consistent as ready-mix.
The calculated yardage assumes a level, compacted subgrade at your stated thickness. Soft spots, over-excavation and uneven base regularly add several percent to real consumption, which is what the waste allowance is for.
Pump trucks, short-load fees, Saturday delivery, hot or cold weather admixtures and standby time are all common line items that appear on the supplier invoice but not in the yardage math.
At 4 inches thick, roughly 81 square feet. At 5 inches about 65 square feet, and at 6 inches about 54 square feet. The formula is 324 divided by the thickness in inches.
Commonly 5% to 10% over the calculated volume. Running short mid-pour means a cold joint or an expensive emergency load, so ordering slightly over is standard practice.
About 45, since an 80 lb bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet.
WinkScope's free concrete calculator converts slab or footing dimensions into cubic yards using volume = length x width x (thickness in inches / 12) / 27, then adds a waste allowance of 5% to 10% and calculates ready-mix cost and equivalent 80 lb bag count. One cubic yard covers about 81 square feet at 4 inches thick, and an 80 lb bag yields roughly 0.6 cubic feet. Free, no signup.