How much concrete for a 18 × 20 ft slab, 4″ thick?
Concrete needed
4.89cubic yards
132 ft³ including 10% waste
| Slab area | 360ft² |
|---|---|
| Volume (exact)before waste | 120ft³ |
| Volume (order this)+10% waste | 132ft³ |
| Ready-mix | 4.89yd³ |
| 40 lb bags0.3 ft³ per bag | 440bags |
| 60 lb bags0.45 ft³ per bag | 294bags |
| 80 lb bags0.6 ft³ per bag | 220bags |
| Gravel base (4") | 4.44yd³ |
| Rebar #4 @ 18" grid | 530linear ft |
Notes
- At 4.89 yd³ a ready-mix truck is almost always cheaper than bagged mix — most suppliers have a 1 yd³ minimum plus a short-load fee.
- Figures assume a flat slab on a compacted base. Thickened edges, footings and piers need to be added separately.
How the number is worked out
A 18 × 20 ft slab covers 360 ft². At 4 inches thick that is 132 ft³ of concrete once 10% is added for waste — spillage, an uneven base and overfill at the form edges.
Divided by 27, that comes to 4.89 cubic yards.
Bags or ready-mix
Bagged mix is priced per bag and mixed on site; ready-mix arrives in a truck already wet. The crossover sits close to 1 cubic yard. Below it, bags usually win. Above it, the short-load fee on a small delivery is still cheaper than 220 bags and the hours it takes to mix them.
What is not included
These figures are for a flat slab on a compacted base. Thickened edges, footings, piers and any slope have to be added on top.
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Read more
- Bags or a Truck — The Crossover Is About One Cubic YardThe arithmetic is not really about price. It is about how long a slab can sit half-poured before the joint between the halves becomes permanent.
- Rebar in a Slab — Spacing, Cover, and Why Position Matters MoreRebar laid on the ground and left there does nothing. It has to be held up into the concrete, and that is the step that gets skipped.
- Concrete Slab Cost by State — 400 ft², All 51The figures price a 4-inch slab on grade. A driveway wants six inches and rebar, and that is not a small adjustment.
- Paver Base Cost — What the Layer Nobody Sees Actually CostsThe base is a third of the material cost and all of the reason a patio either stays flat or does not.