How Much Concrete for a 15x20 Slab?
A 15 by 20 foot slab is 300 square feet. How much concrete for a 15x20 slab at the common 4-inch thickness: about 3.70 cubic yards. Most people order 4.25 yards to cover waste and an uneven base.
Concrete calculator →The math for a 15x20 slab
- Area: 15 ft × 20 ft = 300 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 300 × 0.333 = 100.0 cubic feet.
- Cubic yards: 100.0 ÷ 27 = 3.70 yd³.
- With 10% extra: 4.07 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Concrete for a 15x20 Patio?
Concrete for a 15x20 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 100.0 | 3.70 | 4.07 | 167 | 223 |
| 5 in | 125.0 | 4.63 | 5.09 | 209 | 278 |
| 6 in | 150.0 | 5.56 | 6.11 | 250 | 334 |
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 Concrete for a 12x20 Slab?
Truck or bags?
A cubic yard of concrete is 27 cubic feet and weighs roughly 4,000 pounds. Most ready-mix trucks carry up to about 9 to 10 yards, though local trucks and rules vary. If your total is under a yard or two, bagged mix or a short-load supplier may be cheaper.
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
Reinforcement does not stop cracks, but it holds them tight. Wire mesh or rebar set mid-depth on chairs is common for a slab that carries vehicles; fiber-reinforced mix is an option for light-duty slabs. Joints about 8 feet apart control where cracks form.
Questions people ask
How many yards of concrete for a 15x20 slab at 4 inches?
About 3.70 cubic yards, or 4.07 with 10 percent extra.
How many yards for a 15x20 slab at 6 inches?
About 5.56 cubic yards.
How many 80 lb bags for a 15x20 slab?
About 167 bags at 4 inches thick, before any waste allowance.
How many square feet is a 15x20 slab?
300 square feet.
How much does a cubic yard of concrete weigh?
Roughly 4,000 pounds, or about 2 tons, for normal-weight concrete.
When is bagged concrete practical?
Usually for jobs under about a cubic yard. Beyond that, mixing dozens of bags by hand gets slow and batches may not blend well.
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 15x16 Slab?
- How Much Concrete for a 15x18 Slab?
As an Amazon Associate, AsphaltTons.com earns from qualifying purchases. Product links below are sponsored.