A 20x20 slab is 400 square feet, the compact two-car garage. Two vehicles fit on it, though nobody ever calls it roomy. It is also a common shop and carport size, so this footprint gets quoted constantly.
Example rate math at 400 square feet
| Finish | Example rate | 20x20 (400 sq ft) |
|---|---|---|
| Standard broom | $9 / sq ft | $3,600 |
| Colored | $12 / sq ft | $4,800 |
| Stamped | $16 / sq ft | $6,400 |
Every number in that table is an example rate, not a market price. Garage floors get the broom finish almost without exception; the decorative rows matter when this same 400 square feet is a patio instead.
Concrete volume starts to matter here
At 400 square feet, yardage stops being a rounding error. The arithmetic is square feet times thickness in feet, divided by 27: about 5 yards at 4 inches, about 7.4 yards at 6 inches. Going to a garage-grade thickness adds roughly half again the concrete on the identical footprint, which is exactly why thickness belongs in the quote conversation and not in the fine print. The full method is in how to figure concrete yardage.
Add-ons on a 400 square foot job
At the example add-on rates, the usual suspects look like this:
- Tear-out of an existing slab, $4 per square foot: $1,600.
- Site prep and grading, $2 per square foot: $800.
- Rebar or mesh, $1.50 per square foot: $600.
Notice tear-out alone is almost half the base broom price. When a customer says “just replace what’s there,” the demo is the part of that sentence that costs money, and the quote should show it as its own line.
Quote it at your rates, not these
The concrete slab cost calculator opens with this size loaded, and takes any other dimensions in seconds. When you are ready to run it with your own broom rate, your own add-ons, and your own minimum, that is PriceDesk, free for 14 days.