How Much Concrete for a 15x15 Slab?
How much concrete for a 15x15 slab depends on thickness. The slab covers 15 × 15 = 225 square feet, and at 4 inches thick that is about 2.78 cubic yards, or 3.06 yards with a 10 percent allowance. Mixed from bags, it is roughly 125 bags of 80 lb mix.
Concrete calculator →The math for a 15x15 slab
- Area: 15 ft × 15 ft = 225 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 225 × 0.333 = 75.0 cubic feet.
- Cubic yards: 75.0 ÷ 27 = 2.78 yd³.
- With 10% extra: 3.06 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Concrete for a 15x15 Patio?
Concrete for a 15x15 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 75.0 | 2.78 | 3.06 | 125 | 167 |
| 5 in | 93.8 | 3.47 | 3.82 | 157 | 209 |
| 6 in | 112.5 | 4.17 | 4.58 | 188 | 250 |
Bag counts assume about 0.60 cubic feet per 80 lb bag and 0.45 per 60 lb bag, rounded up.
Choosing the thickness
Four inches is the usual thickness for patios, walks and shed floors. Slabs that carry cars or a heavy hot tub are often poured at 5 to 6 inches, and edges are sometimes thickened where loads concentrate.
Related: How Much Paver Base for a 15x15 Patio?
Ordering ready-mix
Concrete is ordered by the cubic yard, and a yard is 27 cubic feet. Ready-mix plants usually sell in quarter- or half-yard steps, so round up to the next step rather than down. Running short in the middle of a pour leaves a cold joint, which is a weak seam in the slab.
Prep before the pour
Uneven ground is the main reason pours run short. Every half inch of extra depth across the slab adds concrete you did not plan for. Screed the base flat, check it with a string line and set the forms to finished height before the truck arrives.
Joints and reinforcement
Cut or tool control joints so the slab cracks where you want it to. A common rule is spacing them, in feet, at no more than two to three times the slab thickness in inches, so about 8 feet apart for a 4-inch slab, and about a quarter of the slab depth deep.
Questions people ask
How many yards of concrete for a 15x15 slab at 4 inches?
About 2.78 cubic yards, or 3.06 with 10 percent extra.
How many yards for a 15x15 slab at 6 inches?
About 4.17 cubic yards.
How many 80 lb bags for a 15x15 slab?
About 125 bags at 4 inches thick, before any waste allowance.
How many square feet is a 15x15 slab?
225 square feet.
Should I add extra for waste?
Yes. Adding 5 to 10 percent covers uneven base, spillage and form bulges.
How thick should the base under the slab be?
About 4 inches of compacted gravel is common for residential slabs, more on soft or poorly drained soil.
Related guides
- How Much Area Does a Yard of Concrete Cover?
- How Many Bags of Concrete in a Yard? 40, 60, 80 lb
- Gravel Base Under a Concrete Slab: Depth and Tons
- How Many Yards in a Concrete Truck? Mixer Sizes
- Concrete Short Load Fee: What It Is and How to Cut It
- 60 lb vs 80 lb Concrete Bags: Yield, Count, and Cost
- Rebar vs Wire Mesh in a Slab: Which One to Use
- Cost to Pour a Concrete Slab: Per Square Foot
- How Much Concrete for a 14x50 Slab?
- How Much Concrete for a 14x60 Slab?
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