18×20 Concrete Slab Cost
A 18×20 slab covers 360 square feet. Here's the concrete, bags and cost for 2026 — a common size for a garage.
Concrete (4", waste in)
4.89 yd³
220 × 80 lb bags
Materials (ready-mix)
$671 – $1,104
delivered, 4" slab
Installed by a pro
$1,980 – $4,320
4" slab, national average
18×20 slab cost calculator
Adjust thickness, waste or region — the cost updates live.
Waste & region
Order extra for spillage & uneven subgrade. 10% is typical.
Installed cost estimate
$1,980 – $4,320
360 sq ft · 4.89 yd³ concrete
Bags of concrete needed (rounded up)
220
80 lb
294
60 lb
352
50 lb
440
40 lb
- Ready-mix (delivered)
- $671 – $1,104
- Bagged mix (80 lb, DIY)
- $1,216 – $1,989
- Installed by a pro
- $1,980 – $4,320
Estimate only — not a quote. Bag counts are rounded up and include a 10% waste allowance.
18×20 slab cost by thickness
| Thickness | Concrete | 80 lb bags | Ready-mix | Installed |
|---|---|---|---|---|
| 4" | 4.89 yd³ | 220 | $671 – $1,104 | $1,980 – $4,320 |
| 5" | 6.11 yd³ | 275 | $824 – $1,336 | $2,250 – $4,590 |
| 6" | 7.33 yd³ | 330 | $977 – $1,568 | $2,520 – $4,860 |
Includes a 10% waste allowance; bag counts rounded up. National US averages, January 2026.
You'll also need
Sized for this exact slab — tap any item to open its calculator with your measurements already filled in.
18×20 concrete slab: what to expect
A 18×20 slab covers 360 square feet. At the standard 4" thickness it takes about 4.89 cubic yards of concrete once you allow 10% for waste, which is roughly 220 × 80 lb bags if you mix by hand. Stepping up to 6" — sensible if it'll carry a vehicle — raises that to about 7.33 cubic yards.
Budget around $1,980 – $4,320 to have a 4" slab this size poured and finished by a contractor, or roughly $671 – $1,104 in ready-mix if you're doing the labor yourself. On top of the concrete you'll need about 5.11 yd³ of gravel base and 26 sticks of rebar.
Compare nearby sizes
Planning something bigger or smaller? Jump straight to the cost for a nearby size.
Volumes use standard bag yields and a 10% waste allowance; costs are national US averages that vary by region, mix and finish. See our methodology for exactly how each figure is worked out, or check the reference sources: