Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 12 ft; Width 12 ft; Thickness 4 in = 0.3333 ft
- Volume = 12 × 12 × 0.3333 × 1 = 48 ft³
- Net volume = 48 ft³ ÷ 27 = 1.7778 yd³ (× 0.0283168 = 1.3592 m³)
- Order quantity = 1.7778 yd³ × (1 + 10%) = 1.9556 yd³ = 52.8 ft³
- 80 lb bags = 52.8 ft³ ÷ 0.6 ft³ per bag = 88 → 88 bags (always round up)
- Weight = 52.8 ft³ × 150 lb/ft³ = 7,920 lb
What this slab is used for
Twelve by twelve feet (144 ft²) is a generous patio, a base for a 10×12 or 12×12 shed or gazebo, and about the footprint of a small workshop addition. It is large enough that two people can furnish it as an outdoor room with a seating group and a table, and small enough that a patio roof or pergola can span it with one set of posts.
Use 4 in for outdoor living space. For a shed or gazebo, a 4 in slab with a thickened perimeter of 8 in or more gives a stiffer edge that carries wall loads without the cost of a 6 in slab throughout. The thickened edge is poured in the same pour and adds a little volume that the calculator below does not include.
Concrete for a 12×12 ft slab at each thickness
The order comes to 1.96 yd³, about two yards. That is the volume at which most people decide on a truck, because the bag count of 88 is more than most people want to lift. Ask the plant about its minimum order and compare it with the cost calculator before booking.
| Thickness | Net yd³ | Order yd³ | Order m³ | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 3 in | 1.33 | 1.47 | 1.12 | 66 | 88 | 5,940 lb |
| 4 in | 1.78 | 1.96 | 1.50 | 88 | 118 | 7,920 lb |
| 5 in | 2.22 | 2.44 | 1.87 | 110 | 147 | 9,900 lb |
| 6 in | 2.67 | 2.93 | 2.24 | 132 | 176 | 11,880 lb |
The formula is 12 × 12 × thickness ÷ 12 ÷ 27 for the net volume. The order adds 10% for waste and rounds the bag counts up. Use the calculator above for another thickness or waste allowance.
Bags or ready-mix for 12×12 ft?
Bags: 88 bags of 80 lb mix at 4 in, a total of 7,040 lb of dry product. Ready-mix: 1.96 yd³, material about $294–$781 at the published range, last verified 2026-10-04, before delivery and short-load fees. Bagged concrete for this slab costs about $484–$704. Because the order is under two cubic yards, price both routes: the delivery and short-load fees can make bags competitive. See bagged vs ready-mix.
What to watch for
- Forgetting the extra volume of a thickened edge or footing under a shed wall.
- Making the slab exactly the size of the shed. Add 6 to 12 in on each side so that water does not run back under the wall.
- Letting the middle of the slab sag on poorly compacted fill.
- Not planning how the concrete reaches the back corner. A truck chute reaches only a few feet, so a 12 ft depth often needs a wheelbarrow and a helper.
Layout and joints
Because 12 ft is at the upper end of the usual spacing for a 4 in slab, put one joint in each direction and treat the slab as four 6×6 ft panels.
For a 4 in slab, joint spacing of 8 to 12 ft is the usual range, with a 15 ft maximum for any thickness. Read the control joints guide for depth and timing.
Before you order
- Measure the forms, not the excavation, and check the thickness in several places.
- Add the volume of any thickened edge, apron or step separately.
- Plan the base: use the gravel base calculator to estimate stone for 144 ft².
- If the design calls for steel, estimate it with the rebar calculator.
- Put the quantity on the Pour List to combine it with footings and steps.
Frequently asked questions
How many yards of concrete for a 12×12 slab?
1.78 yd³ net at 4 in, 1.96 yd³ with 10% waste.
How many bags of concrete for a 12×12 slab?
88 bags of 80 lb mix or 118 bags of 60 lb mix at 4 in with 10% waste.
How much does a 12×12 slab weigh?
About 7,920 lb (3.96 US tons) at 4 in with 10% waste, using 150 lb/ft³.
How many yards for a 12×12 slab at 6 in?
2.93 yd³ with 10% waste.
How much does a 12×12 concrete slab cost in materials?
About $294–$781 for ready-mix concrete at 4 in, before delivery fees, based on the ranges on the cost page (verified 2026-10-04).
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.