A 20x20 slab poured 4 inches thick needs 4.94 cubic yards of concrete (133.3 cubic feet: 20 ft × 20 ft × 4 in ÷ 12). With 10 percent extra you order 5.5 cubic yards of ready-mix, or buy 245 bags of 80 lb mix and mix it yourself.
A 20x20 slab, 400 square feet, is the footprint of a one-car-plus garage floor or a generous patio, and it is where ready-mix starts to beat bagged concrete on effort.
How much concrete for a 20x20 slab, by thickness
Volume is the area times the thickness in feet. The 20x20 slab covers 400 sq ft. Ordered volumes include 10 percent extra for uneven sub-base and spillage, the middle of the 4 to 10 percent NRMCA range, and ready-mix is sold in quarter yards.
| Build | Concrete, net | Order with 10% extra | Trucks | Weight |
|---|---|---|---|---|
| 4 in, plain | 4.94 cu yd (133.3 cu ft) | 5.5 cu yd | 1 (short load) | 20,533 lb |
| 4 in, wire mesh | 4.94 cu yd (133.3 cu ft) | 5.5 cu yd | 1 (short load) | 20,533 lb |
| 6 in, rebar | 7.41 cu yd (200 cu ft) | 8.25 cu yd | 1 (short load) | 30,800 lb |
The weight uses 140 lb per cubic foot, Quikrete's figure for the mix. A 6 inch slab is half again the concrete of a 4 inch one for the same footprint, which is worth spending on where a car or a hot tub will sit and not on a walkway.
How many bags of concrete for a 20x20 slab
Bag counts come from the yields on the manufacturers' data sheets: an 80 lb bag makes 0.60 cu ft, 60 lb 0.45, 50 lb 0.375 and 40 lb 0.30. They include the 10 percent allowance.
| Bag size | 4 in slab | 6 in slab |
|---|---|---|
| 80 lb | 245 bags | 367 bags |
| 60 lb | 326 bags | 489 bags |
| 50 lb | 392 bags | 587 bags |
| 40 lb | 489 bags | 734 bags |
245 bags of 80 lb is 9.8 tons of bagged mix to move and mix by hand, and a slab has to be poured before the first batch stiffens. Ready-mix is the practical route at this size. The concrete bag calculator converts any bag size and brand.
20x20 slab cost, three builds
Installed prices at US averages: ready-mix $110 to $150 per cu yd, delivery, fuel and a short-load fee below a full truck, placing and finishing $2 to $3 per sq ft (HomeAdvisor, June 2026), wire mesh $0.83 to $0.98 per sq ft (homewyse, September 2026) and rebar $2 to $3 per sq ft (This Old House, March 2026).
| Build | Materials only | Installed range | Typical per sq ft | Crew days |
|---|---|---|---|---|
| 4 in, plain | $900 to $1,250 | $1,700 to $2,450 (typical $2,100) | $5 | 1 |
| 4 in, wire mesh | $1,250 to $1,650 | $2,050 to $2,850 (typical $2,450) | $6 | 1 |
| 6 in, rebar | $2,100 to $3,050 | $2,900 to $4,250 (typical $3,600) | $9 | 1 |
A 5.5 cu yd order is well below the 8 to 12 yards a truck carries, so the estimate includes a short-load fee of about $280 ($220 to $330). Splitting a truck with a neighbor who is also pouring avoids it. The heavy build adds 31 bars of 20 ft No. 4 rebar in an 18 in grid, about 414 lb with laps and 10 percent spare; the rebar calculator lays out the grid.
Joints, thickness and permit
The 2024 IRC sets 3½ in as the minimum for a slab on ground (R506.1) over 4 in of granular base with a vapor retarder under floors. NRMCA advises control joints no farther apart than 24 times the thickness: 8 ft for a 4 in slab, 12 ft for a 6 in slab. On a 20x20 slab that means 2 joints each way at 4 in thick. Under IRC 2024 R105.2, sidewalks and driveways need no permit, and a free-standing patio slab is usually treated the same way. A slab that carries a building, such as a 20x20 garage or shed floor, is part of that building's permit.
Forming, placing and finishing is about 1 day for a two-person crew. Concrete reaches 2,500 psi at 7 days and 4,000 psi at 28 days (Quikrete data sheet), so keep vehicles off for the first week.
20x20 slab compared with other sizes
A 12x12 slab saves 3.16 cu yd less, $1,250 less installed. A 24x24 slab needs 2.17 cu yd more, $890 more installed.
Next steps for a 20x20 slab
- Change thickness, shape or add a thickened edge in the concrete slab calculator.
- Work out the rebar for the slab at your own spacing.
- Turn the yards into a truck order with the concrete yard calculator, and add pumping or finishes in the concrete cost calculator.
- Any other shape or a footing is in the concrete calculator. All size pages are grouped under cost calculators.