How Much Concrete for a 12x12 Slab? Yards and Bags

How much concrete for a 12x12 slab depends almost entirely on thickness. At the common 4-inch depth, a 12 by 12 foot pad takes about 1.78 cubic yards. Most people round that up to 2 yards so a slightly low spot in the base or a bowed form board does not leave them short. At 6 inches, the same footprint needs about 2.67 cubic yards. Below are the numbers for each common depth, what that means in bags, and how much gravel goes underneath.

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The math in three steps

A 12x12 slab covers 144 square feet. To turn that into concrete:

Convert thickness to feet. Divide the inches by 12. Four inches is 0.333 feet, 5 inches is 0.417 feet, and 6 inches is 0.5 feet.
Find cubic feet. Multiply 144 square feet by the thickness in feet. At 4 inches that is 48 cubic feet.
Convert to cubic yards. Divide by 27, since a cubic yard holds 27 cubic feet. 48 ÷ 27 = 1.78 cubic yards.

The concrete calculator does the same steps for any size, which helps when the pad is not quite square.

Concrete for a 12x12 slab by thickness

Thickness	Cubic feet	Cubic yards	With 8% extra
4 inches	48	1.78	about 1.9
5 inches	60	2.22	about 2.4
6 inches	72	2.67	about 2.9

The extra column covers the small losses that happen on every pour: a base that settles a little under the screed, concrete left in the chute or wheelbarrow, and forms that flex outward. Five to ten percent is a normal cushion for a small slab. If the base has not been graded carefully, lean toward the high end.

Which thickness fits the job

4 inches is the usual choice for a patio, a walkway pad, a grill or hot tub area that sees foot traffic, or a small shed with light contents.
5 inches gives more margin for a shed holding a riding mower, a workshop with heavy tools, or soil that has a reputation for movement.
6 inches makes sense for a pad that will see vehicle wheels, such as a small parking spot or the landing in front of a garage door.

Thickness is only part of strength. A well-compacted base, the right mix, and control joints matter just as much. A 12x12 slab usually gets one joint each way, splitting it into four 6x6 panels, which keeps random cracking in check.

Bags versus a truck

Bagged concrete is sold by weight, but what matters is the yield printed on the bag. A typical 80-pound bag makes about 0.6 cubic feet, and a 60-pound bag makes about 0.45 cubic feet.

Thickness	80 lb bags	60 lb bags
4 inches	about 80	about 107
5 inches	about 100	about 134
6 inches	about 120	about 160

Eighty bags is roughly 3 tons of material to lift, mix, and place, and it has to go in quickly enough that each batch bonds with the last. That is a lot for a weekend crew. For anything over about a yard, most people find a ready-mix delivery easier, even after a short-load fee. A bag count per cubic yard guide walks through the tradeoff in more detail.

Gravel base under the slab

Most 12x12 slabs sit on 4 inches of compacted crushed stone. That base is the same 48 cubic feet as the concrete at 4 inches, or about 1.8 cubic yards. Compacted crushed stone weighs roughly 1.4 to 1.5 tons per cubic yard, so plan on about 2.5 to 2.75 tons. Spread it in two lifts and compact each one; compacting gravel base in lifts explains why a single thick layer tends to stay soft in the middle.

A quick ordering checklist

Measure the formed area, not the area you meant to form. A board set an inch wide on two sides adds up.
Check depth at several points with a tape and a string line. A base that dips an inch in the middle can eat a quarter yard by itself.
Tell the ready-mix dispatcher the slab size and thickness, and ask what minimum they deliver and what they charge below it.
Have the forms, base, reinforcement, and tools ready before the truck arrives. Waiting time is usually billed.

Questions people ask

How many yards of concrete do I need for a 12x12 slab?

About 1.78 cubic yards at 4 inches thick, 2.22 at 5 inches, and 2.67 at 6 inches. Add 5 to 10 percent for waste and uneven base.

How many 80 lb bags of concrete for a 12x12 slab?

Roughly 80 bags at 4 inches thick, based on about 0.6 cubic feet per bag. Check the yield printed on the bag you buy, since it varies slightly by brand and mix.

Is 4 inches thick enough for a 12x12 slab?

For patios, walkways, and light sheds, 4 inches on a compacted base is common. Go to 5 or 6 inches for vehicle traffic or heavy equipment.

How much gravel goes under a 12x12 slab?

At 4 inches of compacted crushed stone, about 1.8 cubic yards, which is roughly 2.5 to 2.75 tons.

Should I use bags or ready-mix for a 12x12 slab?

Bags work, but 80 or more bags is heavy, slow work. Many people order ready-mix for anything over about one yard, even with a short-load fee.

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