What a 12x12 slab is for
A 12x12 slab is a square patio big enough for a table for four with a grill to one side, or the floor of a 12x12 shed, workshop or gazebo. At 144 square feet it is under the 200 square foot line where the model code starts asking for a shed permit.
Concrete for a 12x12 slab at 4, 5 and 6 inches
Every inch of thickness on a 12x12 slab is 12 cubic feet, or 0.44 cubic yards. The ready mix order and the bags in this table include 10 percent extra. Change the thickness or the extra in the calculator above.
| Thickness | Cubic feet | Cubic yards | Ready mix to order | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 48 | 1.78 | 1.96 cu yd | 88 | 118 |
| 5 in | 60 | 2.22 | 2.44 cu yd | 110 | 147 |
| 6 in | 72 | 2.67 | 2.93 cu yd | 132 | 176 |
The 4 inch row by hand: 12 × 12 × 0.333 = 48 cubic feet, and 48 ÷ 27 = 1.78 cubic yards. With 10 percent extra that is 52.8 cubic feet. An 80 lb bag makes 0.60 cubic feet and a 60 lb bag makes 0.45, so it takes 88 of one or 118 of the other.
Quikrete says a sidewalk should be at least 4 inches thick and small slabs normally run 4 to 6 inches, depending on the load. The concrete slab calculator page covers what the building code asks for.
Bags or a truck for a 12x12 slab
From bags, it is 88 bags of 80 lb mix at 4 inches, 7,040 lb to lift and mix, about $501 at McCoy's price of $5.99 a bag, or $5.69 for 42 or more.
From a truck, it is 1.96 cubic yards, about $216 to $294 at HomeAdvisor's $110 to $150 per cubic yard delivered. The National Ready Mixed Concrete Association says a truck mixer carries 8 to 12 cubic yards, so this is less than one truck load. HomeAdvisor says an order under a full truckload can carry a short-load fee of $40 to $60 per cubic yard, and suppliers often have a minimum order.
By these prices a 12x12 slab costs about $207 to $285 less from a truck than from bags, and it saves lifting 7,040 lb of mix. A short-load fee or a minimum order comes out of that gap, so get a delivered price for 1.96 cubic yards and compare it with $501. It is a bag job only if that quote comes in higher or a truck cannot reach the slab. The association notes that a loaded truck weighs about 80,000 lb and may not be able to cross loose dirt or a residential curb.
| Thickness | 80 lb bags | Bags weigh | Bags cost | Ready mix | Ready mix cost |
|---|---|---|---|---|---|
| 4 in | 88 | 7,040 lb | $501 | 1.96 cu yd, less than one truck load | $216 to $294 |
| 5 in | 110 | 8,800 lb | $626 | 2.44 cu yd, less than one truck load | $268 to $366 |
| 6 in | 132 | 10,560 lb | $751 | 2.93 cu yd, less than one truck load | $322 to $440 |
Prices are from the sources below, checked in October 2026; multiply the table by your own to compare. Gravel, steel, forms, finishing and labor are extra.
Pouring a 12x12 slab
Twelve feet across needs a 14 foot screed board, which is heavy and long enough that two people should work it. The middle of the slab is 6 feet from every edge, so a bull float with extension handles does the floating from outside the forms. At just under 2 cubic yards, this is where hand mixing turns into a hard day: 88 bags of 80 lb mix is more than three and a half tons.
Gravel base and slope
A 4 inch gravel base under a 12x12 slab is 48 cubic feet, or 1.78 cubic yards. Quikrete says to spread and compact 3 to 4 inches, and a 3 inch base is 36 cubic feet, or 1.33 cubic yards. Those are volumes of the finished layer. Gravel settles when it is compacted, so order a little more.
Outside, Quikrete says to slope the slab away from buildings and gives 1/4 inch per foot as the usual fall. Either way across a 12x12 slab, that is 3 inches from one edge to the other. Set the forms to the slope, and the screed follows them.
Rebar or wire mesh
With bars 24 inches on center both ways, held 3 inches back from each edge, a 12x12 slab takes 7 bars each way, all 11 feet 6 inches long. That is 161 feet of bar. With 10 percent extra, buy 9 bars 20 feet long. Every bar is shorter than the 20 foot length sold, so none has to be joined.
With wire mesh instead, 2 sheets 8 feet by 12.5 feet, lapped 8 inches, cover it. The spacing and the sheet size are where the calculator starts. Your plans or your building department set the steel for your slab.
Control joints
Quikrete says control joints in a 4 inch slab go every 6 to 8 feet and about a quarter of the slab deep, which is 1 inch. Keeping them no more than 8 feet apart cuts a 12x12 slab into 2 panels each way. That makes 4 panels, each 6 feet square, with 24 feet of joint to cut.
Before you pour a 12x12 slab
For a gazebo or pergola, find out where the posts land before you pour. Posts need anchors set in the wet concrete or bolted in later, and a heavy roof may need a deeper footing under each post instead of a plain 4 inch slab. The kit or plan says which.
Other slab sizes
8x1010x1010x1212x1210x2012x2016x2020x2020x2424x2424x3030x40
Common questions
How much concrete do I need for a 12x12 slab?
A 12 by 12 foot slab 4 inches thick holds 48 cubic feet of concrete, which is 1.78 cubic yards. With 10 percent extra, order 1.96 cubic yards of ready mix. At 5 inches thick it holds 2.22 cubic yards and at 6 inches 2.67.
How many 80 lb bags of concrete do I need for a 12x12 slab?
88 bags at 4 inches thick, with 10 percent extra. They weigh 7,040 lb in all and cost about $501 at the store price checked in October 2026. At 5 inches thick it is 110 bags and at 6 inches 132.
How thick should a 12x12 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. On a 12x12 slab each extra inch adds 0.44 cubic yards, so going from 4 to 6 inches takes the ready mix order from 1.96 to 2.93 cubic yards. Ask your building department what it wants for a driveway or a garage floor.
Will a 12x12 slab hold a gazebo?
Most gazebo kits bolt their posts to a slab like this, but the kit sets what it needs. A heavy roof or a windy site may call for a deeper footing under each post. Read the kit's instructions before you pour, so the anchors go in the right place.
How thick should a 12x12 patio be?
Four inches is the usual thickness for a patio that people walk on. Quikrete calls for at least 4 inches for a sidewalk, and 4 to 6 inches for small slabs, depending on the load. Go thicker if a car, a hot tub or a heavy structure will sit on it.
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.
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
- International Residential Code 2021, Chapter 1: work exempt from permit (R105.2)
- McCoy's Building Supply: Quikrete Concrete Mix, 80 lb bag
- HomeAdvisor: concrete delivery cost, per cubic yard and short-load fees
Facts on this page were last checked against these sources in October 2026.