Use this tool to estimate cubic yardage, bag counts, and recommended order amounts for slabs, footers, and post holes. Our calculator includes a built-in 10% waste buffer to help you avoid costly shortages on the job site.
Get instant results including cubic yards, number of bags, estimated cost, and recommended order volume.
Concrete volume is calculated by multiplying three dimensions: Length × Width × Thickness. The result is in cubic feet, which is divided by 27 to get cubic yards — the unit ready-mix suppliers use. Every dimension must be in the same unit before you multiply, which is why this calculator automatically converts inches to feet.
Ordering precisely what the math says is a common and costly mistake. Running short mid-pour creates cold joints where old concrete partially sets before new material is added — a structural weak point that can crack under load.
Pro Tip
Most driveways require a thickness of at least 4 inches for passenger vehicles. For heavier loads such as trucks, RVs, or equipment, increase to 6 inches and consider adding wire mesh or rebar for reinforcement.
A properly prepared sub-base is the foundation of any long-lasting concrete slab. Compacted gravel, typically 4 inches deep for driveways and patios, provides drainage and uniform support that prevents soil movement from causing cracks. Without it, even a perfectly mixed pour will heave and crack within a few seasons.
Rebar transforms concrete from a material that resists compression into one that also handles tension, the force that pulls apart a slab when the ground shifts or a heavy load bears down. A standard #4 rebar grid at 12-inch spacing is the go-to choice for residential driveways and slabs.
Everything you need to know to order the right amount of concrete, the first time.
For a concrete slab, multiply the length by the width by the thickness. A 10 ft × 10 ft slab that is 4 inches (0.33 ft) thick requires about 1.23 cubic yards of concrete, or roughly 56 bags of 80 lb mix. Always add 10% to this figure when calling your supplier.
For round pads or columns, use: Volume = π × r² × Depth, where r is the radius (half the diameter). A 12-inch diameter column that is 3 feet deep requires approximately 0.087 cubic yards (2.36 cubic feet) of concrete, roughly 6 bags of 80 lb mix.
Foundation footings and piers use the same formula as a rectangular slab applied to a square base. Multiply the side length by itself, then by the depth of the footer. Use the Square Footer mode in the calculator for an instant result.
Both bag sizes contain the same concrete mix; the only difference is weight and yield per bag.
Ready-mix concrete is ordered by the cubic yard. Always add at least 10% to your calculated volume to account for waste, spillage, and slight variations in sub-grade depth. One cubic yard of concrete weighs approximately 4,050 lbs and will fill an area of 81 square feet at a 4-inch thickness.
Common questions about concrete estimation and ordering.
How many 80 lb bags of concrete are in a cubic yard?
It takes 45 bags of 80 lb concrete to make one cubic yard. Each 80 lb bag yields approximately 0.60 cubic feet when mixed, and there are 27 cubic feet in a cubic yard (27 ÷ 0.60 = 45). For projects requiring more than half a cubic yard, ordering ready-mix concrete by truck is usually more economical.
What is the standard thickness for a concrete patio?
The standard thickness for a residential concrete patio is 4 inches. This is sufficient for foot traffic and light outdoor furniture. If you plan to drive a vehicle onto the patio, increase the thickness to at least 6 inches and consider adding wire mesh or rebar reinforcement.
How do I calculate concrete for a post hole?
To calculate concrete for a round post hole, use the circular column formula: Volume = π × r² × Depth, where r is the radius of the hole (half the diameter). For example, a 12-inch diameter hole that is 3 feet deep requires about 0.087 cubic yards (2.36 cubic feet) of concrete, roughly 6 bags of 80 lb mix.
When should I order ready-mix concrete instead of bags?
Order ready-mix concrete when your project requires more than about half a cubic yard (roughly 20–25 bags of 80 lb mix). At that scale, ready-mix is more economical, less labor-intensive, and delivers a more consistent mix than hand-mixing bags. Most driveways and large patios qualify.
What does a 10% waste factor mean?
A 10% waste factor means you order 10% more concrete than the bare calculated volume. This accounts for uneven sub-grade depth, spillage at form edges, and minor over-pours. The calculator defaults to +10%, which is the industry-standard recommendation for exterior slabs and footings.