How Much Concrete for an 18x25 Slab?
How much concrete for an 18x25 slab depends on thickness. The slab covers 18 × 25 = 450 square feet, and at 4 inches thick that is about 5.56 cubic yards, or 6.11 yards with a 10 percent allowance. Mixed from bags, it is roughly 250 bags of 80 lb mix.
Concrete calculator →The math for an 18x25 slab
- Area: 18 ft × 25 ft = 450 sq ft.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 450 × 0.333 = 150.0 cubic feet.
- Cubic yards: 150.0 ÷ 27 = 5.56 yd³.
- With 10% extra: 6.11 yd³.
The concrete calculator runs the same steps for any size and thickness.
Related: How Much Gravel for a 18x25 Driveway?
Concrete for an 18x25 slab by thickness
| Thickness | Cubic feet | Cubic yards | Yards +10% | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 in | 150.0 | 5.56 | 6.11 | 250 | 334 |
| 5 in | 187.5 | 6.94 | 7.64 | 313 | 417 |
| 6 in | 225.0 | 8.33 | 9.17 | 375 | 500 |
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 Many Tons of Asphalt for a 18x25 Driveway?
Placing the order
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
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.
Joints and reinforcement
Reinforcement does not stop cracks, but it holds them tight. Wire mesh or rebar set mid-depth on chairs is common for a slab that carries vehicles; fiber-reinforced mix is an option for light-duty slabs. Joints about 10 feet apart control where cracks form.
Questions people ask
How many yards of concrete for an 18x25 slab at 4 inches?
About 5.56 cubic yards, or 6.11 with 10 percent extra.
How many yards for an 18x25 slab at 6 inches?
About 8.33 cubic yards.
How many 80 lb bags for an 18x25 slab?
About 250 bags at 4 inches thick, before any waste allowance.
How many square feet is an 18x25 slab?
450 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 18x22 Slab?
- How Much Concrete for an 18x24 Slab?
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