How Much Concrete for an 18x24 Slab?
For an 18x24 slab poured 4 inches thick, plan on about 5.33 cubic yards of concrete. That comes from 432 square feet times 0.333 feet of depth, divided by 27. How much concrete for an 18x24 slab at other thicknesses is in the table below.
Concrete calculator →The math for an 18x24 slab
- Area: 18 ft × 24 ft = 432 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 432 × 0.333 = 144.0 cubic feet.
- Cubic yards: 144.0 ÷ 27 = 5.33 yd³.
- With 10% extra: 5.87 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Gravel for a 18x24 Parking Pad?
Concrete for an 18x24 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 144.0 | 5.33 | 5.87 | 240 | 320 |
| 5 in | 180.0 | 6.67 | 7.33 | 300 | 400 |
| 6 in | 216.0 | 8.00 | 8.80 | 360 | 480 |
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: Recycled Concrete Aggregate for a 18x24 Parking Pad
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
Under the slab, most residential pours sit on about 4 inches of compacted gravel. The base spreads the load, drains water away and keeps the slab from cracking as the soil freezes and thaws. Compact the base in thin lifts, and dampen it just before the pour so it does not pull water out of the mix.
Joints and reinforcement
Plan the joints before the pour. Panels close to square crack less than long, narrow ones, and joint spacing around 10 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 18x24 slab at 4 inches?
About 5.33 cubic yards, or 5.87 with 10 percent extra.
How many yards for an 18x24 slab at 6 inches?
About 8.00 cubic yards.
How many 80 lb bags for an 18x24 slab?
About 240 bags at 4 inches thick, before any waste allowance.
How many square feet is an 18x24 slab?
432 square feet.
How many 80 lb bags make a cubic yard?
About 45. Each 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet.
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 an 18x20 Slab?
- How Much Concrete for an 18x22 Slab?
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