16 × 20 ft slab: the short answer
Poured 4 inches thick (with 10% waste), a 16 × 20 ft slab needs about 117.3 cubic feet of concrete — roughly 196 × 80 lb bags, or about 4.35 cubic yards of ready-mix.
Slab size
Concrete for a 16 × 20 ft slab
A 16 × 20 ft slab poured 4 inches thick needs about 117.3 cubic feet of concrete — roughly 196 × 80 lb bags, or 4.35 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 16 × 20 ft slab is usually used for
A 16 × 20 ft slab is a large patio, garage pad, or workshop floor: 320 sq ft and about 4 cubic yards of concrete at 4 inches. At this scale the project is a real construction job that needs forms, reinforcement planning, access, and usually professional help or a well-prepared crew.
How to decide
Large slabs make the support structure part of the decision. A garage or workshop floor may need thicker edges, reinforcement, a vapor barrier, and control joints, while a patio can be simpler. Order concrete with a written volume including waste, and confirm the ready-mix plant can deliver the full pour in one day.
Checks before you pour
- Confirm the slab's use and local code requirements for thickness and reinforcement.
- Plan control joints and edge thickness before the pour, not after.
- Verify the ready-mix plant's delivery size and schedule for a single-day pour.
FAQ
How much concrete for a 16 × 20 ft slab?
At 4 inches thick with 10% waste, a 16 × 20 ft slab needs about 117.3 cubic feet — roughly 196 × 80 lb bags (or 261 × 60 lb), or 4.35 cubic yards of ready-mix.
Bags or ready-mix for a 16 × 20 ft slab?
At about 4.35 cubic yards, this slab is past the point where bags make sense — 196 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.