20 × 24 ft slab: the short answer
Poured 4 inches thick (with 10% waste), a 20 × 24 ft slab needs about 176.0 cubic feet of concrete — roughly 294 × 80 lb bags, or about 6.52 cubic yards of ready-mix.
Slab size
Concrete for a 20 × 24 ft slab
A 20 × 24 ft slab poured 4 inches thick needs about 176.0 cubic feet of concrete — roughly 294 × 80 lb bags, or 6.52 cubic yards of ready-mix (includes 10% waste). Change the thickness, bag size, or shape below.
Concrete needed
—Enter dimensions to estimate concrete.
| Bags (incl. waste) | — |
|---|---|
| Ready-mix (cubic yards) | — |
| Volume (cubic feet) | — |
| Estimated cost | — |
Shopping list
Run the calculator to prepare a list.
Estimate only.
Includes your waste allowance (concrete can't be reused). Bag yields are typical for standard
mixes; confirm on the bag. Past about 1 cubic yard, ready-mix delivery is usually cheaper and
easier than mixing bags.
Before you pour
Concrete can't be reused once mixed, so the 10% allowance matters. For anything structural, confirm thickness and reinforcement with your local code. Adjust thickness and bag size in the calculator for your exact pour.
Standard volume math at 4″ with 10% waste; 80 lb bag ≈ 0.6 cu ft. Estimates only.
What a 20 × 24 ft slab is usually used for
A 20 × 24 ft slab is a two-car garage floor or a large workshop slab: 480 sq ft and about 6 cubic yards of concrete at 4 inches. This is a significant pour that typically needs a formed, reinforced, professionally coordinated placement with a ready-mix truck and a crew.
How to decide
At roughly 6 cubic yards, the plan must cover formwork, subgrade, reinforcement, joints, finishing, and curing before any concrete is ordered. The volume number from the calculator is the start, not the spec: confirm the required thickness and any structural requirements with the local authority or an engineer, and order through the ready-mix supplier's own quantity check.
Checks before you pour
- Treat the volume as a planning estimate; confirm thickness and reinforcement requirements.
- Coordinate the pour crew and finishing plan before scheduling the truck.
- Verify access for a full ready-mix truck and plan where the chute can reach.
FAQ
How much concrete for a 20 × 24 ft slab?
At 4 inches thick with 10% waste, a 20 × 24 ft slab needs about 176.0 cubic feet — roughly 294 × 80 lb bags (or 392 × 60 lb), or 6.52 cubic yards of ready-mix.
Bags or ready-mix for a 20 × 24 ft slab?
At about 6.52 cubic yards, this slab is past the point where bags make sense — 294 bags is a lot of mixing. Ready-mix delivery is usually cheaper and lets you pour it all at once.
Sources & verification
Source links and page assumptions reviewed .
- QUIKRETE: quantity calculator — Manufacturer quantity calculator and bag-yield guidance for concrete mixes.
- QUIKRETE: concrete mix data sheet — States an 80 lb bag of concrete mix yields about 0.6 cu ft at recommended consistency.
Estimate only, based on standard volume math at 4″ thickness with 10% waste. Confirm bag yield on the package and consult a pro for structural work.