How Much Concrete for an 18x45 Slab?
How much concrete for an 18x45 slab depends on thickness. The slab covers 18 × 45 = 810 square feet, and at 4 inches thick that is about 10.00 cubic yards, or 11.00 yards with a 10 percent allowance. Mixed from bags, it is roughly 450 bags of 80 lb mix.
Concrete calculator →The math for an 18x45 slab
- Area: 18 ft × 45 ft = 810 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 810 × 0.333 = 270.0 cubic feet.
- Cubic yards: 270.0 ÷ 27 = 10.00 yd³.
- With 10% extra: 11.00 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Gravel for a 18x45 Driveway?
Concrete for an 18x45 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 270.0 | 10.00 | 11.00 | 450 | 600 |
| 5 in | 337.5 | 12.50 | 13.75 | 563 | 750 |
| 6 in | 405.0 | 15.00 | 16.50 | 675 | 900 |
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 Many Tons of Asphalt for a 18x45 Driveway?
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.
Keeping cracks under control
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 12 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 18x45 slab at 4 inches?
About 10.00 cubic yards, or 11.00 with 10 percent extra.
How many yards for an 18x45 slab at 6 inches?
About 15.00 cubic yards.
How many 80 lb bags for an 18x45 slab?
About 450 bags at 4 inches thick, before any waste allowance.
How many square feet is an 18x45 slab?
810 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 18x36 Slab?
- How Much Concrete for an 18x40 Slab?
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