Concrete Calculator
Enter the slab dimensions to find how much concrete you need in cubic yards and cubic feet, plus how many 60 lb or 80 lb bags it would take.
| If buying bags | Bags needed |
|---|---|
| 80 lb bags (≈0.60 ft³ each) | 56 |
| 60 lb bags (≈0.45 ft³ each) | 75 |
Order a little extra — ready‑mix is usually bought by the cubic yard, and most pros add ~10% for spillage and uneven subgrade. Bag counts already round up.
How to use this calculator
Enter the length and width in feet and the thickness in inches (4 inches is typical for a patio or walkway). Set the number of slabs if you're pouring more than one. You'll get the volume in cubic yards and cubic feet, a 10%‑waste order figure, and bag counts.
How it works
Concrete is sold by volume. Multiply length × width × thickness (converting inches to feet by dividing by 12) to get cubic feet, then divide by 27 to convert to cubic yards — the unit ready‑mix trucks use.
Worked example
A 10 ft × 10 ft slab at 4 inches thick is 10 × 10 × 0.333 ≈ 33.3 ft³, or about 1.23 cubic yards. With 10% waste you'd order roughly 1.36 yd³.
The formula
Cubic feet = Length × Width × (Thickness ÷ 12) · Cubic yards = Cubic feet ÷ 27
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 to get cubic yards. This calculator does it for you and lets you enter thickness in inches.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. An 80 lb bag of pre‑mix yields about 0.60 ft³, so roughly 45 bags make a cubic yard; 60 lb bags yield about 0.45 ft³, or roughly 60 bags per yard.
Should I order extra concrete?
Yes. Most contractors add about 10% to account for spillage, uneven subgrade, and slab edges. The calculator shows a 10%‑waste figure alongside the exact amount.
Bags or ready‑mix?
For small jobs (a few cubic feet) bags are convenient. Once you need around a cubic yard or more, ready‑mix delivery is usually cheaper and far less labor than mixing dozens of bags.