What this page is for
This calculator plans a whole slab job: a patio, a walk, a shed pad or a garage floor. It gives the ready mix order in cubic yards, the gravel under the slab and the steel inside it. For post holes, tube forms and deck footings, use the concrete footing calculator, which works in holes and bags.
How it works
- Concrete = length × width × thickness, in feet
- Cubic yards = cubic feet ÷ 27
- Ready mix to order = cubic yards, plus extra
- Bags = cubic feet, plus extra, ÷ what one bag makes, rounded up
- Gravel = length × width × base depth
- Bar length = slab side − 2 × edge distance
- Bars one way = bar length the other way ÷ spacing, rounded up, plus 1
- Bars to buy = total bar length, plus extra, ÷ the length sold, rounded up
- Mesh sheets along one side = (slab side − overlap) ÷ (sheet side − overlap), rounded up
Example: a 12 by 12 foot patio, 4 inches thick. Concrete = 12 × 12 × 0.333 = 48 cubic feet, which is 48 ÷ 27 = 1.78 cubic yards. With 10 percent extra, order 1.96 cubic yards. In bags, 48 × 1.10 = 52.8 cubic feet, and an 80 lb bag makes 0.60, so that is 88 bags. A 4 inch gravel base is also 48 cubic feet, or 1.78 cubic yards.
Rebar at 24 inches on center, held 3 inches back from each edge: each bar is 144 − 6 = 138 inches, or 11 feet 6 inches. 138 ÷ 24 = 5.75, so 6 spaces and 7 bars each way. That is 14 bars × 11.5 feet = 161 feet. With 10 percent extra it is 177 feet, so buy 9 bars 20 feet long. With mesh instead, sheets 8 feet by 12.5 feet lapped 8 inches cover it with 2 sheets.
For mesh, the calculator lays the sheets in rows and columns, tries them turned both ways, and keeps the smaller count.
How thick, and what goes under it
Quikrete's slab guides say a sidewalk should be at least 4 inches thick, and small slabs for foundations and bases normally run 4 to 6 inches depending on the load. The International Residential Code sets 3.5 inches as the least for a concrete floor in a house. Dubuque, Iowa, asks for at least 4 inches in a small garage slab. Heavier loads call for a thicker slab, so ask your building department what it wants for a driveway or a garage.
For the base, Quikrete says to spread and compact 3 to 4 inches of gravel. The code asks for a 4 inch base of sand, gravel or crushed stone under a house floor slab that sits below grade, unless the soil already drains well. The gravel number here is the volume of the finished layer. Gravel settles when it is compacted, so order a little more.
Ordering ready mix
Ready mix is sold by the cubic yard. The National Ready Mixed Concrete Association says to order 4 to 10 percent more than the plan dimensions give you. The extra covers spills and a base that is not perfectly flat. A truck carries 8 to 12 cubic yards, so a patio or a shed pad is well under one load. Suppliers often have a minimum order, so call and compare. Mixing by hand suits small pads. One cubic yard takes 45 bags of 80 lb mix.
Concrete by slab size
Each of these pages works one slab size out at 4, 5 and 6 inches thick: cubic yards, bags, gravel, rebar and what the concrete costs.
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Rebar or wire mesh
The code section on floor slabs does not set a bar size or spacing. It says that where steel is used, it has to sit between the middle and the upper third of the slab while the concrete is placed, so hold it up on supports. Local rules do set numbers. Dubuque, Iowa, for example, asks for 6 by 6 welded wire or #4 bars no more than 2 feet apart both ways in a small garage slab. Fridley, Minnesota, recommends steel in every slab and says to overlap joined bars by at least 40 bar diameters, which is 20 inches for a 1/2 inch #4 bar.
The edge distance keeps the steel covered. The default of 3 inches is the cover the International Residential Code requires for steel in foundation concrete cast against the earth. Rebar is stocked in 20 foot lengths and longer. The mesh both cities name is 6 by 6 inch squares of 10 gauge wire, which the trade writes as 6x6 W1.4xW1.4. The Wire Reinforcement Institute lists 8 foot by 12.5 foot as a common sheet size in most of the United States and 8 by 16 or 8 by 20 on the west coast. Stock varies from place to place, so enter the size you can buy. For the overlap, the structural rule is one wire spacing plus 2 inches. The institute notes that a slab on the ground can use less. Follow your plans if you have them.
Joints
Concrete shrinks as it cures, and control joints tell it where to crack. Quikrete says to cut them at least a quarter of the slab depth. Its guides give every 6 to 8 feet for a 4 inch slab, and 10 feet as the most.
Common questions
How much concrete do I need for a 10x10 slab?
A 10 by 10 foot slab 4 inches thick holds 33.3 cubic feet, which is 1.23 cubic yards. With 10 percent extra, order 1.36 cubic yards of ready mix, or buy 62 bags of 80 lb mix.
How many yards of concrete do I need for a 20x20 slab?
At 4 inches thick, a 20 by 20 foot slab holds 4.94 cubic yards, so order about 5.4 with 10 percent extra. At 6 inches thick it holds 7.41 cubic yards, so order about 8.1.
How thick should a concrete slab be?
Quikrete calls for at least 4 inches for a sidewalk and 4 to 6 inches for small slabs, depending on the load. The International Residential Code minimum for a house floor slab is 3.5 inches. Heavier loads call for a thicker slab, so check with your building department before you pour a driveway or a garage floor.
Do I need rebar or wire mesh in a concrete slab?
The International Residential Code section on floor slabs does not require steel in a plain slab on the ground, but some building departments recommend or require it. Steel does not stop cracks. It keeps them narrow. Ask your building department what it wants for your slab.
How much gravel do I need under a concrete slab?
Quikrete says 3 to 4 inches, compacted. For a 10 by 10 foot slab, a 4 inch base is 33.3 cubic feet, or 1.23 cubic yards. Order a little more, because gravel settles when it is compacted.
How far apart should rebar be in a slab?
There is no single rule. It depends on the load, the soil and your local code. As one example, Dubuque, Iowa, allows #4 bars up to 2 feet apart both ways in a small garage slab. Use the spacing on your plans or from your building department.
Tools for this job
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- Form stakes. Hold the form boards in place while you pour.
- Concrete float. Smooths the surface after the concrete is leveled.
- Edger. Rounds the slab edges so they do not chip.
- Groover. Cuts control joints so cracks follow a straight line.
- Finishing trowel. Gives the slab its final smooth surface.
- Rubber boots. Keep wet concrete off your skin while you stand in the pour.
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Sources
- Quikrete, Pouring and Finishing Concrete Slabs
- Quikrete, Concrete Sidewalks and Small Slabs project guide
- Quikrete Concrete Mix data sheet (yield per bag)
- National Ready Mixed Concrete Association, CIP 31: Ordering Ready Mixed Concrete
- International Residential Code 2021, Chapter 5: concrete floors on the ground (R506)
- International Residential Code 2021, Chapter 4: foundations (R403, concrete cover over steel)
- Wire Reinforcement Institute, Manual of Standard Practice: Structural Welded Wire Reinforcement
- City of Dubuque, Iowa, minimum requirements for a detached garage slab
- City of Fridley, Minnesota, Garage Slabs handout
- PJ's Rebar, stock rebar sizes and lengths
Facts on this page were last checked against these sources in October 2026.