How Much Concrete for an 18x20 Slab?
An 18 by 20 foot slab is 360 square feet. How much concrete for an 18x20 slab at the common 4-inch thickness: about 4.44 cubic yards. Most people order 5.00 yards to cover waste and an uneven base.
Concrete calculator →The math for an 18x20 slab
- Area: 18 ft × 20 ft = 360 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 360 × 0.333 = 120.0 cubic feet.
- Cubic yards: 120.0 ÷ 27 = 4.44 yd³.
- With 10% extra: 4.89 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Many Tons of Asphalt for a 18x20 Driveway?
Concrete for an 18x20 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 120.0 | 4.44 | 4.89 | 200 | 267 |
| 5 in | 150.0 | 5.56 | 6.11 | 250 | 334 |
| 6 in | 180.0 | 6.67 | 7.33 | 300 | 400 |
Bag counts assume about 0.60 cubic feet per 80 lb bag and 0.45 per 60 lb bag, rounded up.
How thick to pour it
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 Gravel for a 18x20 Parking Pad?
Placing the order
Ready-mix is priced and batched by the cubic yard (27 cubic feet). Plants commonly add a small-load fee when an order is under a set minimum, so ask about it when you call. Ordering a little extra costs far less than a second truck.
Getting the ground ready
Prep matters as much as the yardage. Strip sod and soft soil, compact the subgrade, then add a compacted stone base, commonly about 4 inches. A level, firm base also keeps your thickness even, which keeps the yardage close to the math.
Joints, mesh and rebar
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 an 18x20 slab at 4 inches?
About 4.44 cubic yards, or 4.89 with 10 percent extra.
How many yards for an 18x20 slab at 6 inches?
About 6.67 cubic yards.
How many 80 lb bags for an 18x20 slab?
About 200 bags at 4 inches thick, before any waste allowance.
How many square feet is an 18x20 slab?
360 square feet.
How much does a cubic yard of concrete weigh?
Roughly 4,000 pounds, or about 2 tons, for normal-weight concrete.
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 16x60 Slab?
- How Much Concrete for an 18x18 Slab?
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