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.
How Many Bags of Concrete Do You Need? (60 lb & 80 lb Yields)
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.
Bags for a 4-inch slab: the 1.8 sq ft rule
At the 4-inch residential standard, one 80 lb bag covers 1.8 square feet, its 0.60 cubic feet spread a third of a foot deep. That single figure turns any 4-inch slab into a one-step estimate: divide the area by 1.8 for the bag count, then add waste. A 12×12 patio (144 sq ft) works out to 80 bags before the 10% buffer.
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.
Working in metric: cubic metres and kg bags
The calculator is fully bilingual in units. Tap Metric (Global) above the inputs and every field switches to metres and centimetres, while the results ticket converts to cubic metres, litres, and metric bag sizes. The anchor number to know: one cubic metre of concrete takes about 86 × 25 kg bags (or 54 × 40 kg), using the same yield density printed on the bags. Mixed units work too, measure a slab in feet and its thickness in centimetres, and the maths sorts itself out.
Real-world example: a 12×24 backyard patio
The project
A homeowner pouring a 12 by 24 ft patio slab at the residential 4-inch standard, with the default 10% waste buffer applied.
Inputs
- Length: 12 ft
- Width: 24 ft
- Thickness: 4 in
- Waste factor: 10%
| Result | Value |
|---|---|
| Slab area | 288 sq ft |
| Volume (exact) | 3.56 yd³ (96 ft³) |
| Volume (+10% waste) | 3.91 yd³ |
| 80 lb bags needed | 177 |
| Recommended supply | Ready-mix truck |
At just under 4 cubic yards, this pour is well past the ~1-yard crossover, ready-mix delivered by truck beats hand-mixing 177 bags. See our bags vs. ready-mix guide.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
A cubic yard of concrete takes 45 eighty-pound 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?
You 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?
Coverage depends on thickness: at 4 inches thick, one cubic yard of concrete 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.
How many 25 kg bags of concrete make a cubic metre?
A cubic metre of concrete takes about 86 × 25 kg bags, each bag yields roughly 11.7 litres of mixed concrete, and a cubic metre is 1,000 litres. In 40 kg bags it's about 54. Switch the calculator to Metric (Global) and it counts 20, 25, and 40 kg bags for your exact dimensions.
How many bags of concrete do I need for a 4-inch slab?
For a 4-inch slab, divide the slab's square footage by 1.8 to get the 80 lb bag count, each 80 lb bag yields 0.60 cubic feet, which covers 1.8 square feet at 4 inches thick. A 10×10 slab (100 sq ft) needs 56 bags before waste, about 62 with the standard 10% buffer.