Concrete for a 20×20 slab
A 20×20 ft slab at 4″ thick needs about 245 × 80 lb bags of concrete mix (with 10% waste) — or 5.5 cubic yards of ready-mix. Estimated materials cost ≈ $908 (ready-mix).
| Volume | 133.33 ft³ = 4.94 cu yd |
|---|---|
| 80 lb bags (no waste) | 223 |
| 80 lb bags (10% waste) | 245 |
| 60 lb bags (10% waste) | 326 |
| Ready-mix | 5.5 cu yd |
| Recommended | Order ready-mix |
Adjust it
Change the size, thickness, bag size, or waste % below:
How much concrete do I need?
| Bags (with overage) | |
|---|---|
| Bags (no overage) | |
| Ready-mix (delivered) | |
| Estimated materials cost |
Estimate for standard concrete mix (e.g. Quikrete #1101 / Sakrete High-Strength). Bag counts round up and include your overage %. Cost is an estimate (as of June 2026, US national average) — confirm bag price and ready-mix delivery (often a 1 yd³ minimum + short-load fees) with your supplier. Not a substitute for a structural engineer on load-bearing work.
The math: volume = 20 × 20 × (4 ÷ 12) = 133.33 ft³, then ÷ 27 = 4.94 cubic yards. An 80 lb bag yields 0.60 ft³. See the full concrete calculator for footings, columns, and other shapes.
Other slab sizes
Last updated: 2026-06-28.
Bag yields from the
QUIKRETE #1101
data sheet. Cost figures are US-national-average estimates and vary by region and supplier.