Concrete: how much to order and how to plan the pour
Concrete is ordered by volume: length times width times thickness, converted to cubic yards, plus a margin for uneven ground and spillage. A 10 x 10 ft slab 4 inches thick takes 1.36 cubic yards with 10% extra, which is 62 bags of 80 lb mix or 82 bags of 60 lb. Past a cubic yard or so, ready-mix from a truck is usually easier than mixing bags by hand.
This page gathers everything we have on concrete: the calculators for slabs, footings, posts, tubes and steps, the guides on thickness, mix and reinforcement, and quick answers for the sizes people pour most often. Start with the table, then jump to the part of the job you are planning.
Concrete for common slab sizes
Worked out by the concrete calculator with 10% extra. Use 4 inches for patios and walks; driveways and slabs that carry vehicles are usually poured thicker.
| Slab (feet) | Cubic yards at 4 in | 80 lb bags at 4 in | 60 lb bags at 4 in | Cubic yards at 6 in |
|---|---|---|---|---|
| 10 x 10 | 1.36 | 62 | 82 | 2.04 |
| 10 x 12 | 1.63 | 74 | 98 | 2.44 |
| 12 x 12 | 1.96 | 88 | 118 | 2.93 |
| 12 x 16 | 2.61 | 118 | 157 | 3.91 |
| 16 x 16 | 3.48 | 157 | 209 | 5.21 |
| 20 x 20 | 5.43 | 245 | 326 | 8.15 |
| 24 x 24 | 7.82 | 352 | 470 | 11.73 |
Bags or ready-mix?
One cubic yard is 27 cubic feet, and with no allowance for waste that is 45 bags of 80 lb mix or 60 bags of 60 lb. It also weighs about 4,050 lb, all of which you lift, mix and place. For small jobs (posts, a footing, a step or a few square feet of repair) bags make sense. For a patio or a driveway, price a ready-mix delivery, including any short-load fee for small orders, before you rent a mixer.
Whole-project lists for slabs, patios and driveways
A slab is more than concrete. It needs a compacted gravel base, forms, reinforcement, expansion joints and a cure. The project calculators take one set of measurements and list all of it, using the same volume formula as the single-material calculator.
Footings, post holes, tubes and steps
Round and odd shapes are where guesses go wrong. A single 12 inch tube 4 feet deep holds 3.46 cubic feet with 10% extra, which is 6 bags of 80 lb mix. Continuous footings, fence posts and poured steps each have their own calculator, because each has its own geometry.
Mixing your own and reinforcing it
If you are mixing from cement, sand and gravel rather than bagged mix, the mix calculator turns a volume and a ratio into bags of cement, sand, gravel and water. Rebar or wire mesh does not stop concrete cracking, but it holds the pieces together when it does; the rebar calculator lays out a grid and counts bars and ties.
Block walls and mortar
Concrete block is counted by wall area rather than volume, and the mortar is counted from the number of blocks. For walls that hold back soil, use the retaining wall planner, which also covers drainage stone.
Every calculator for this topic
Guides
- Common Mistakes When Pouring a Concrete Slab (and Fixes)Five slab mistakes that run you short or waste money. A 10 × 10 ft slab at 4 in needs 1.36 cu yd, but a 6 in pour needs 2.04.
- How Many Bags of Concrete Make a Yard? 80, 60 and 40 lbOne cubic yard of concrete is 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb. A chart from a quarter yard to five, plus slab sizes.
- How Much Concrete for Steps? Yards and Bags by Step CountThree concrete steps 4 ft wide need 0.52 cubic yards or 24 bags of 80 lb mix. A chart for 1 to 6 steps at 3, 4 and 5 ft wide.
- 10x12 Concrete Slab Materials List: Concrete, Base and RebarA 10x12 ft slab 4 in thick needs 1.63 cubic yards of concrete (74 bags of 80 lb), 2.5 tons of gravel and 9 rebar bars.
- Concrete vs Pavers vs Gravel Patio: Which to ChooseCompare a concrete, paver and gravel patio for a 12 x 12 ft space: what each needs to buy, the work involved and when each is the right choice.
- How Much Does a Concrete Slab Cost? Bags vs Ready-MixA 20 × 10 ft slab at 4 in needs 2.72 yd³ or 123 bags of 80 lb. Charts of yards and bags, and how to compare bags with delivered ready-mix.
- How Much Rebar Do I Need for a Slab? Spacing, Bars and Weight ChartA 20 × 12 ft slab with #3 rebar on 16 inch centers takes 20 bars of 20 ft and 144 lb of steel, with 135 ties. Charts for 12, 16 and 24 inch spacing.
- How Thick Should a Concrete Slab Be? Thickness by Use and CostA patio slab is 4 inches thick, a driveway 4 to 6. A 10 × 10 ft slab at 4 inches takes 1.36 cubic yards. Thickness by use, concrete volume and joint spacing.
- Concrete Mix Ratios: Cement, Sand, Gravel and Water Per YardThe 1:2:4 concrete mix ratio and others: bags of cement, cubic feet of sand and gravel and gallons of water per cubic yard, plus when to buy ready-mix.
- How Many Bags of Mortar for Concrete Blocks? Wall Size ChartHow many 80 lb bags of mortar mix for concrete blocks? A chart by number of blocks and by wall size, the blocks per bag, and mixing and laying tips.
- How Much Does Concrete Weigh? Per Yard, Slab and BagConcrete weighs about 150 lb per cubic foot and 4,050 lb per cubic yard. Weight charts for slabs by size and thickness, pounds per square foot, and bags.
- Concrete Driveway Materials List: Yards, Base, Rebar, JointsWhat to buy for a 20×12 ft concrete driveway: cubic yards and truck loads, gravel base, rebar, forms and control joints, with a size chart for other drives.
- Garage Slab Materials List: Concrete, Base, Vapor BarrierWhat to buy to pour a 24×24 ft garage or shed slab: concrete in yards and truck loads, gravel base, vapor barrier, rebar, forms and joints, with a size chart.
- Concrete Patio Materials List: What to Buy for a 12×12 SlabEverything you need to pour a 12×12 ft concrete patio: concrete, gravel base, rebar, forms and tools, with quantities and a size chart for other patios.
- How Big Should a Patio Be? Sizes for Tables, Grills & LoungingPatio sizes by use: bistro set, dining for 4, 6 or 8, lounge seating and a grill, with the concrete or pavers each size needs, and the rule for walking space.
- How Many Bags of Concrete Do I Need? Slab and Post ChartsHow many 40, 60 and 80 lb bags of concrete a slab, footing or fence post needs, with charts, the formula, and when ready-mix is the better buy.
Other guides that use these calculators: 12x12 Deck Material List: Boards, Joists, Screws, Footings · 16x16 Deck Material List: Boards, Joists, Screws, Footings · 8x8 Deck Material List: Boards, Joists, Screws, Footings · Common Mistakes When Building a Fence (Posts, Gates, Concrete) · How to Calculate Cubic Yards: Formula, Chart and Examples · Wood Privacy Fence: Individual Pickets vs Prefab Panels · How Many Bricks Do I Need? Per Square Foot and by Wall Size · Pergola Materials List: Posts, Beams, Rafters and Slats · Shed Foundation Materials List: Gravel Pad or Concrete Slab · Slope and Grade: How Much Fall for Drainage, Patios and Ramps
Answers for common sizes
Already know the size? These pages give the worked answer for one common size each, with the inputs filled in so you can adjust them.
Project planners
Planning the whole job rather than one material? These step-by-step planners put the calculators above in the order you will need them.
Frequently asked questions
How many bags of concrete are in a cubic yard?
With no waste, 45 bags of 80 lb mix or 60 bags of 60 lb mix make one cubic yard (27 cubic feet). Add 5 to 10% for an uneven base.
How much concrete do I need for a 10 x 10 slab?
At 4 inches thick, 1.36 cubic yards including 10% extra, or 62 bags of 80 lb mix.
How much does a cubic yard of concrete weigh?
About 4,050 lb (2.03 tons) for normal-weight concrete, which is why bagged pours of more than a yard are hard work.
How thick should a concrete slab be?
Four inches is the usual thickness for patios, walks and shed floors. Driveways and garage floors that carry vehicles are usually 5 to 6 inches. Check what your local code asks for.
Related topics
Estimates only. Quantities depend on your materials, site and supplier, so check coverage and sizes on the label. Follow local building codes for anything structural. How we calculate.