Concrete Calculator
Calculate how much concrete you need in cubic yards, cubic feet, and cubic meters — plus how many 60 lb or 80 lb bags for your slab.
How it works
Concrete volume formula
Volume = Length × Width × Thickness Cubic yards = ft³ ÷ 27 Bags needed = ft³ ÷ bag yield (0.45 ft³ per 60 lb, 0.60 ft³ per 80 lb)
Concrete is ordered by volume. Multiply the slab's length and width by its thickness (converted to the same unit) to get volume, then convert to cubic yards — the unit ready-mix trucks use. For small jobs, divide the cubic feet by the yield of each premixed bag.
Worked example
A 10 ft × 10 ft patio slab at 4 inches thick: 10 × 10 × (4/12) = 33.33 ft³ = 1.23 yd³. That's about 75 bags of 60 lb concrete mix or 56 bags of 80 lb mix — at that quantity, ordering ready-mix is usually easier.
Frequently asked questions
How thick should a concrete slab be?
A common guideline is 4 inches for patios and walkways, 4–6 inches for driveways with regular vehicles, and 6+ inches for heavy vehicle areas. Local codes and soil conditions may require more.
When should I order ready-mix instead of bags?
A rule of thumb: above roughly 1 cubic yard (about 45–75 bags), ready-mix delivery becomes far more practical than mixing bags by hand.
How much extra concrete should I order?
Add 5–10% to the calculated volume to cover spillage, over-excavation, and uneven ground. Running short mid-pour creates a weak seam called a cold joint.