A 20x30 slab is 600 square feet: the deep two-car garage, or the small shop. The extra six feet of depth over a standard 20x24 is the cheapest usable space most people will ever add to a build, since the forms, crew, and mobilization are already paid for and only the footage grows.
Example math at 600 square feet
| Finish | Example rate | 20x30 (600 sq ft) |
|---|---|---|
| Standard broom | $9 / sq ft | $5,400 |
| Colored | $12 / sq ft | $7,200 |
These are example rates for doing the math out loud, not market prices. Shop and garage slabs get broom finish; the colored row is what the same footprint runs when it is a patio or pool surround at the example rate.
Yardage grows faster than the footprint feels
Concrete volume is where slabs this size surprise people. The arithmetic: square feet times thickness in feet, divided by 27. At 4 inches, 600 square feet is about 7.4 yards. At a shop-grade 6 inches, it is about 11 yards. The footprint did not change, but the material did, by half. Get comfortable with that math before quoting anything this size or bigger; how to figure concrete yardage covers it start to finish.
Thickness is not optional detail on this slab. If vehicles or equipment live on it, it gets poured heavier than patio flatwork, and the quote has to carry the extra yards.
The add-ons, priced at the example rates
- Tear-out of an existing slab at $4 per square foot: $2,400.
- Site prep and grading at $2 per square foot: $1,200.
- Rebar or mesh at $1.50 per square foot: $900.
All three on top of the broom example takes the job from $5,400 to $9,900. Same slab, honest scope. That spread is exactly why the line items belong on the quote where the customer can see them.
Run your own numbers
The concrete slab cost calculator opens preloaded at 20x30 and takes any other size, and how to price a concrete slab shows how to build the rate itself.
When the rate is yours, PriceDesk turns this whole page into a ten-second quote on your phone. The first 14 days are free.