Free concrete calculator
Concrete Slab Cost Calculator
Estimate slab cubic yards, ready-mix cost, finish cost, labor, and total installed price for patios, sheds, garage pads, and flatwork.
- Slab area
- 400 sq ft
- Concrete with 10% waste
- 5.4 cu yd
- Ready-mix material
- $750
- Installed cost per sq ft
- $7.37 - $12.25
Concrete Slab Estimating Notes
- Use 4 inches for patios and sidewalks; use 5 to 6 inches for heavier garage or driveway slabs.
- Add 10% waste before ordering concrete, especially if the subgrade is uneven.
- Stamped finishes can cost more than the concrete itself, so compare plain and decorative results before quoting.
- Base prep is where slabs quietly get expensive. 4 to 6 inches of compacted gravel is standard, and poor compaction is the leading cause of early cracking.
- Control joints should be cut at roughly 24 to 30 times the slab thickness — about every 10 ft on a 4-inch slab — within the first 24 hours.
- Concrete is often under half the total bill. Labour, forming, base, reinforcement, and finishing usually outweigh the mix itself.
Concrete Slab Calculator FAQs
How much does a 20x20 concrete slab cost?
A 20x20 slab is 400 square feet. In 2026, a plain 4 inch residential slab commonly lands around $2,600 to $5,200 installed before unusual site prep, reinforcement, or decorative finishes.
How many yards are in a 20x20 slab at 4 inches?
A 20x20 slab at 4 inches is about 4.94 cubic yards before waste and about 5.5 cubic yards after adding a 10% ordering allowance.
How thick should a concrete slab be?
4 inches suits patios, sidewalks, and shed pads. Go to 5 inches for regular vehicle traffic and 6 inches where trucks, RVs, or heavy equipment will park. Thickness buys far more durability than a higher PSI mix does.
Does a concrete slab need rebar or wire mesh?
Neither prevents cracking; both hold cracks tight once they form. Wire mesh is common on light residential flatwork, while #4 rebar on an 18 to 24 inch grid is typical for driveways and garage slabs.
What PSI concrete should a slab use?
3000 PSI is the residential standard for patios and interior floors. 4000 PSI is worth the small premium on driveways, garages, and anything exposed to freeze-thaw cycles or de-icing salt.
Why is my slab quote higher than the calculator?
This tool estimates concrete, finish, and typical labour. Quotes also carry excavation, haul-away, hard access, thickened edges, vapour barrier, permits, and short-load fees, any of which can move the total substantially.