Concrete Calculator
Figure out how much concrete to order — in cubic yards for the ready-mix truck or bags of premix for smaller pours. Works for slabs, footings, and round columns or sonotubes.
Volume
33.3
cu ft (0.94 m³)
Cubic yards
1.23
Order (+10% waste)
1.36
cu yd
Or in premix bags (incl. 10% waste):
80 lb bags
62
60 lb bags
82
40 lb bags
123
Ready-mix trucks usually have a 1-yard minimum and price short loads at a premium — bags make sense below about 1 cubic yard (roughly 45 × 80 lb bags). Standard slab thickness: 4" for patios and walkways, 5–6" for driveways.
How Concrete Volume Is Calculated
Concrete is sold by the cubic yard (27 cubic feet). Multiply your slab's length and width in feet by its thickness converted to feet, then divide by 27. The 10% waste allowance covers uneven subgrade, spillage, and over-excavation — running short mid-pour means a cold joint, so ordering slightly over is the standard practice.
Formula
Slab: L(ft) × W(ft) × thickness(in) ÷ 12 = cu ft Column: π × (diameter ÷ 2)² × height = cu ft Cubic yards = cu ft ÷ 27 Example: 10×10 patio, 4" thick: 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cu yd → order 1.36 Bag yields: 80 lb = 0.60 cu ft 60 lb = 0.45 40 lb = 0.30
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick, a 10×10 slab is 1.23 cubic yards — about 62 80-lb bags including 10% waste. That's a lot of mixing; above roughly one cubic yard, a ready-mix truck is usually cheaper and produces a stronger, more consistent pour.
How thick should a concrete slab be?
4 inches for patios, sidewalks, and shed bases; 5–6 inches for driveways and anything that carries vehicles. Going from 4 to 6 inches raises the volume by half, so thickness is the first thing to nail down before ordering.
What's the difference between cement and concrete?
Cement is the gray powder that acts as the glue; concrete is the finished mix of cement, sand, gravel, and water. The bags of 'concrete mix' at the hardware store are pre-blended concrete ingredients — you only add water.
Why order 10% more concrete than the math says?
Subgrades are never perfectly level, forms flex, and some mix is lost in the truck and wheelbarrows. Running short mid-pour creates a cold joint — a weak seam where fresh concrete meets partially-set concrete — so the standard practice is ordering 5–10% over the calculated volume.