EachCalculatorConcrete Calculator
Calculate how much concrete you need for a slab, and roughly how many pre-mixed bags that works out to.
Volume
33.33 ft³
1.23 cubic yards
60lb Bags Needed
75
or 112 × 40lb / 56 × 80lb
How It Works
Volume
Volume is length × width × thickness. Thickness is entered in inches while length and width are in feet, so thickness is converted to feet before multiplying.
Bag counts
Pre-mixed bags have a known yield in cubic feet per bag (roughly 0.3, 0.45, and 0.6 cubic feet for 40lb, 60lb, and 80lb bags respectively) — the total volume is divided by that yield and rounded up so you always have enough.
Example Calculation
A 10ft × 10ft slab poured 4 inches thick needs about 1.23 cubic yards of concrete — roughly 75 sixty-pound bags if using pre-mixed bags instead of a truck delivery.
Common Mistakes
- Forgetting to convert thickness from inches to feet before multiplying — mixing units gives a wildly wrong volume.
- Not adding a safety margin for waste, uneven ground, or spillage — contractors commonly order 5-10% extra.
Frequently Asked Questions
- Should I use bags or a truck delivery?
- For small jobs (a cubic yard or less), pre-mixed bags are usually more practical. For larger pours, ready-mix delivery is typically more cost-effective and easier to place consistently.
- Does this account for rebar or forms?
- No — this calculates concrete volume only. Rebar, forms, and other materials need to be estimated separately.