How Much Concrete for an 18x30 Slab?
An 18 by 30 foot slab is 540 square feet. How much concrete for an 18x30 slab at the common 4-inch thickness: about 6.67 cubic yards. Most people order 7.50 yards to cover waste and an uneven base.
Concrete calculator →The math for an 18x30 slab
- Area: 18 ft × 30 ft = 540 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 540 × 0.333 = 180.0 cubic feet.
- Cubic yards: 180.0 ÷ 27 = 6.67 yd³.
- With 10% extra: 7.33 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Gravel for a 18x30 Driveway?
Concrete for an 18x30 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 180.0 | 6.67 | 7.33 | 300 | 400 |
| 5 in | 225.0 | 8.33 | 9.17 | 375 | 500 |
| 6 in | 270.0 | 10.00 | 11.00 | 450 | 600 |
Bag counts assume about 0.60 cubic feet per 80 lb bag and 0.45 per 60 lb bag, rounded up.
Thickness for this job
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 Many Tons of Asphalt for a 18x30 Driveway?
Ordering ready-mix
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.
Base and forms
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.
Keeping cracks under control
Plan the joints before the pour. Panels close to square crack less than long, narrow ones, and joint spacing around 8 feet is typical for a 4-inch slab. Cut joints with a groover while the concrete is soft or with a saw within the first day.
Questions people ask
How many yards of concrete for an 18x30 slab at 4 inches?
About 6.67 cubic yards, or 7.33 with 10 percent extra.
How many yards for an 18x30 slab at 6 inches?
About 10.00 cubic yards.
How many 80 lb bags for an 18x30 slab?
About 300 bags at 4 inches thick, before any waste allowance.
How many square feet is an 18x30 slab?
540 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 an 18x26 Slab?
- How Much Concrete for an 18x28 Slab?
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