How Much Concrete for an 18x36 Slab?
How much concrete for an 18x36 slab depends on thickness. The slab covers 18 × 36 = 648 square feet, and at 4 inches thick that is about 8.00 cubic yards, or 8.80 yards with a 10 percent allowance. Mixed from bags, it is roughly 360 bags of 80 lb mix.
Concrete calculator →The math for an 18x36 slab
- Area: 18 ft × 36 ft = 648 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 648 × 0.333 = 216.0 cubic feet.
- Cubic yards: 216.0 ÷ 27 = 8.00 yd³.
- With 10% extra: 8.80 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Gravel for a 18x36 Parking Pad?
Concrete for an 18x36 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 216.0 | 8.00 | 8.80 | 360 | 480 |
| 5 in | 270.0 | 10.00 | 11.00 | 450 | 600 |
| 6 in | 324.0 | 12.00 | 13.20 | 540 | 720 |
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: Recycled Concrete Aggregate for a 18x36 Parking Pad
Truck or bags?
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.
Base and forms
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, mesh and rebar
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 18x36 slab at 4 inches?
About 8.00 cubic yards, or 8.80 with 10 percent extra.
How many yards for an 18x36 slab at 6 inches?
About 12.00 cubic yards.
How many 80 lb bags for an 18x36 slab?
About 360 bags at 4 inches thick, before any waste allowance.
How many square feet is an 18x36 slab?
648 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 18x32 Slab?
- How Much Concrete for an 18x35 Slab?
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