How to calculate concrete for your project
Every concrete estimate comes down to three steps. First, measure your pour: length and width in feet, and thickness or depth in inches, since that is how plans and building departments specify it. Second, convert those dimensions to volume. The calculator does this instantly, turning your measurements into cubic feet and then dividing by 27 to get cubic yards — the unit ready-mix suppliers sell by. Third, add a waste factor. Concrete spreads into uneven subgrade, clings to the mixer and wheelbarrow, and forms rarely sit perfectly level, so ordering the exact computed volume almost guarantees coming up short mid-pour. A 10% cushion is the standard allowance for typical residential work; go higher for rough or sloped ground, lower for precise formwork you have poured before. If you are pouring several identical footings or piers, set the quantity field instead of running the numbers one at a time.
The concrete volume formula, explained
For any rectangular pour, the math is simply length × width × depth — with every dimension in the same unit. Because thickness is measured in inches while length and width are in feet, divide the inches by 12 first:
Cubic yards = Volume (ft³) ÷ 27
Worked example: a 10 × 10 ft patio slab poured 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet. Divide by 27 and you get 1.24 cubic yards; add the standard 10% waste factor and you should order about 1.36 cubic yards, or 62 eighty-pound bags. Round columns use the cylinder formula instead — π × radius² × height — and continuous footings use their width and depth in inches the same way slabs use thickness.
Concrete bag yields: 40 lb vs 60 lb vs 80 lb
Bagged concrete mix is sold by weight, but what matters for your estimate is the volume each bag produces once mixed with water. These yields are printed on the bag and are consistent across major brands:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Bags make sense for small jobs — fence posts, a mower pad, repairs — where the convenience outweighs the mixing labor. Once a pour approaches a full cubic yard (45 eighty-pound bags is a long afternoon of continuous mixing), most people are better served ordering ready-mix delivered by truck: the concrete arrives consistent, arrives all at once, and eliminates cold joints between batches mixed an hour apart.
Common slab and footing thicknesses
Patios and walkways are typically poured at 4 inches. Driveways run 4 to 6 inches depending on vehicle weight — go thicker if trucks or RVs will park on it. Garage and shed slabs are commonly 4 to 6 inches with thickened edges. Footings are a different story: their required depth is governed by the frost line and soil conditions where you live, and those requirements are set by your local building code. Always confirm footing dimensions with your local building department before pouring — the calculator will size any dimensions you give it, but it can't know your frost depth.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
45 bags. Each 80 lb bag yields 0.60 cubic feet of mixed concrete, and a cubic yard is 27 cubic feet — 27 ÷ 0.60 = 45. For 60 lb bags it takes 60, and for 40 lb bags it takes 90.
How thick should a concrete slab be?
Four inches is the standard for patios, sidewalks, and shed pads. Driveways and garage slabs are usually 4–6 inches, moving toward the thicker end when heavier vehicles will use them. Structural slabs and footings should follow your local building code.
Why do I need a waste factor?
Because real pours never match the math exactly. Subgrade is uneven, concrete clings to mixing equipment, and forms flex slightly. A 10% allowance is the accepted standard for residential work — running short mid-pour risks a cold joint, which is a far more expensive problem than a little leftover concrete.
How many square feet does a yard of concrete cover?
It depends on thickness. At 4 inches thick, one cubic yard covers 81 square feet. At 6 inches, 54 square feet. The formula is 324 ÷ thickness in inches = square feet covered per cubic yard.
Is it cheaper to mix bags or order ready-mix?
For small jobs, bags win because ready-mix trucks carry minimum-order and short-load fees. As volume grows, ready-mix takes over: the per-yard price of truck-delivered concrete falls well below the equivalent in bags once you factor in the sheer number of bags and hours of mixing labor. The practical crossover for most homeowners is right around one cubic yard.
Can I pour concrete in separate layers or batches?
A single element — one slab, one footing — should be poured continuously. If fresh concrete is placed against concrete that has already begun to set, the two never bond into one piece; that seam is called a cold joint and it's a permanent weak plane. This is exactly why the calculator pads your order: finishing a pour matters more than saving one bag.