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How Much Concrete for a 20×24 Slab at 6 Inches?
A 20 ft × 24 ft slab 6 inches thick needs this much concrete — cubic yards and bag counts, calculated for you. Here is the answer, plus a calculator you can adjust if your measurements are slightly different.
Answer: 8.89 cubic yards
How it changes with thickness
Same area, different thickness:
| Thickness | 4 | 5 | 6 |
|---|---|---|---|
| Concrete needed | 5.93 cubic yards | 7.41 cubic yards | 8.89 cubic yards |
| 80 lb bags | 267 bags | 334 bags | 400 bags |
What a 20×24 slab at 6 inches actually involves
At 8.89 yd³ this is a serious pour that deserves a plan. You want a crew of at least six, a definite answer on where the trucks park, and a pump if access is tight or reach exceeds the chute. Splitting a slab this size into two pours leaves a cold joint at the seam, so place it in one continuous run if you possibly can.
The slab is 480 ft² and, at 6 inches thick, takes 240.00 ft³ of concrete — 8.89 yd³, weighing about 17.8 tons. Everything below is worked out from those figures.
Full material list
| Item | Quantity | Notes |
|---|---|---|
| Concrete | 8.89 yd³ | 400 × 80 lb bags, or 534 × 60 lb bags |
| Gravel base | 7.11 yd³ | order loose for 4 in compacted; see the gravel calculator |
| Form lumber | 97 lin ft | 2×4 or 2×6, includes 10% waste |
| Form stakes | 22 | roughly one every 4 ft of perimeter |
| Welded wire mesh | 11 sheets | 5×10 ft sheets of 6×6-W1.4 mesh, 10% overlap |
| Rebar | 700 lin ft | #4 (1/2 in) on a 18 in grid, tied and on chairs |
| Vapor barrier | 528 ft² | about 53 lin ft of 10 ft wide roll |
| Control joints | 2 joints | 44 lin ft of saw cut |
Control joints and cracking
Control joints stop a slab cracking wherever it likes. The conservative rule is to space them at twice the slab thickness in inches — here that means every 12 ft, which divides this slab into 2 × 2 panels of roughly 10.0 × 12.0 ft. That is 2 joints, about 44 linear ft of cutting. Get them in while the concrete is still green, typically 6 to 12 hours after finishing, before random cracks get there first.
Reinforcement at 6 inches
At 6 in, welded wire mesh is no longer adequate — this slab is thick enough to carry loads, and mesh just stretches once a crack opens. The usual reinforcement is #4 (1/2 in) rebar on a 18 in grid: about 17 bars running the 20 ft direction and 15 running the 24 ft direction, roughly 700 linear ft of steel. Tie every intersection and set the grid on chairs so the bars finish at mid-depth with 2 to 3 in of cover. Bars lying on the ground do almost nothing.
Bagged concrete or a ready-mix truck?
| Option | Quantity | Estimated cost |
|---|---|---|
| Bagged 80 lb | 400 bags | $2,400 |
| Ready-mix delivered | 8.89 yd³ | $1,511 |
On these assumptions ready-mix is cheaper ($1,511 vs $2,400) as well as far less work. It only wins if the truck can actually reach the site and you can place the whole load without stopping. If access is a problem, bagged at $2,400 is the practical fallback and lets you pour in stages.
Costs assume $6 per 80 lb bag, $170 per cubic yard delivered, and a $90 short-load surcharge below 5 yd³. Ready-mix pricing moves with region, season and fuel, so treat these as a starting point for calling suppliers rather than a quote.
Pour day, in order
- Excavate to 10 in below finished grade: 6 in of slab plus 4 in of base.
- Compact the base in two lifts and proof-roll it with a loaded wheelbarrow to find soft spots before you cover them.
- Set and generously stake the forms. On a pour this size the forms take real pressure, so brace every run and back up outside corners.
- Lay the vapor barrier and reinforce on chairs. Have a pump or a second chute plan if reach exceeds 18 ft.
- Book the trucks as a staggered sequence so one arrives as the last is finishing. Gaps create cold joints you cannot undo.
- Brief the crew on roles: two on the chute, two raking, one screeding, one floating. Nobody improvises on the day.
- Place continuously and keep a wet edge throughout. If you must stop, stop at a planned joint, never mid-panel.
- Cut or tool the control joints while the concrete is still green, aiming for 6 to 12 hours after finishing. You need about 44 linear ft of cutting.
- Cover and keep damp for 7 days, and keep vehicles off it for 28.
Local codes cover frost depth, reinforcement and permits, and they vary from place to place. Check with your local building department before you dig or pour.
Common questions
How many bags of concrete for a 20×24 slab at 6 inches?
400 bags of 80 lb mix, or 534 bags of 60 lb mix. A 20×24 slab at 6 in is 240.00 ft³, which is 8.89 yd³. An 80 lb bag yields about 0.60 ft³ and a 60 lb bag about 0.45 ft³, so divide the volume by the yield and round up.
How much does a 20×24 concrete slab cost?
Roughly $2,400 in 80 lb bags, or about $1,511 delivered as ready-mix. Material only — it excludes excavation, base, reinforcement and labour.
How thick should a 20×24 slab be?
Four inches for a patio, shed floor or footpath. Five to six inches if vehicles will drive on it. This page is worked through at 6 inches, so switch the calculator above to compare thicknesses.
How much gravel goes under a 20×24 slab?
About 7.11 yd³ of loose crushed base for a 4 in compacted layer, covering 480 ft². Order it loose rather than already compacted — the material settles by roughly a fifth under a plate compactor.
Other sizes
- How Much Concrete for a 10×10 Slab at 4 Inches?
- How Much Concrete for a 10×10 Slab at 6 Inches?
- How Much Concrete for a 12×12 Slab at 4 Inches?
- How Much Concrete for a 12×12 Slab at 6 Inches?
- How Much Concrete for a 10×12 Slab at 4 Inches?
- How Much Concrete for a 10×12 Slab at 6 Inches?
- How Much Concrete for a 12×16 Slab at 4 Inches?
- How Much Concrete for a 12×16 Slab at 6 Inches?
- How Much Concrete for a 16×16 Slab at 4 Inches?
- How Much Concrete for a 16×16 Slab at 6 Inches?