How Much Concrete for a 16 × 20 Slab?
A 16 × 20 ft slab is 320 sq ft. It needs 4.35 cubic yards of concrete at 4 inches thick or 6.52 yd³ at 6 inches, including 10% extra.
| Order | 4 in thick | 6 in thick |
|---|---|---|
| Cubic yards | 4.35 | 6.52 |
| 80-lb bags | 196 | 294 |
| 60-lb bags | 261 | 392 |
| 50-lb bags | 313 | 470 |
| 40-lb bags | 392 | 587 |
| Cubic meters | 3.32 | 4.98 |
Concrete needed
4.35 cubic yards
117.3 ft³ · 3.32 m³ · includes 10% extra
You need about 4.35 cubic yards of concrete, or 196 bags of 80 lb mix.
Bags of premix
- 39240 lb bags
- 31350 lb bags
- 26160 lb bags
- 19680 lb bags
- —20 kg bags
- —25 kg bags
- Before extra3.95 yd³ (106.7 ft³)
- Weight of bags15,680 lb
- Per hole—
- Ready-mix costAdd a price per yd³
- Bagged costAdd a price per bag
One 80 lb bag makes about 0.60 ft³ (manufacturer's published yield). Bags are rounded up.
At 4.35 yd³ that is 196 × 80 lb bags (15,680 lb) to mix. Most people order ready-mix at this size; ask suppliers for their minimum load and any short-load fee.
Estimate only, not a quote or engineering advice. Disclaimer
A 16 × 20 ft slab is a roomy patio or a small single-car garage or workshop floor.
16 × 20 slab at other thicknesses
| Thickness | yd³ exact | yd³ +10% | 80-lb bags | 60-lb bags |
|---|---|---|---|---|
| 3.5 in | 3.46 | 3.80 | 172 | 229 |
| 4 in | 3.95 | 4.35 | 196 | 261 |
| 5 in | 4.94 | 5.43 | 245 | 326 |
| 6 in | 5.93 | 6.52 | 294 | 392 |
| 8 in | 7.90 | 8.69 | 392 | 522 |
How it's worked out
For bags, the 117.3 ft³ (with extra) is divided by each bag's yield: 0.60 ft³ for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb, then rounded up. At 4 inches the slab itself weighs about 14,900 lb, using the 140 lb/ft³ unit weight QUIKRETE lists for its Concrete Mix.
Bags or a truck?
At 4.35 yd³ you would need 196 × 80-lb bags (about 15,680 lb) and a long session with a mixer and helpers. Most people order ready-mix at this size: call suppliers with the yardage and ask for their minimum load and any short-load fee.
For a slab this size, the number to give a ready-mix supplier is the yardage: 4.35 yd³ at 4 inches or 6.52 yd³ at 6 inches, with 10% extra already included. Some people order a little less than the full allowance and keep a few bags on hand to top off.
Don't forget the base: most slabs sit on a compacted layer of gravel, and thicker edges or footings around the perimeter add concrete. Use the footing calculator for a thickened edge.
Frequently asked questions
How many yards of concrete for a 16x20 slab?
3.95 cubic yards at 4 inches thick and 5.93 yd³ at 6 inches, before waste. With 10% extra, order 4.35 or 6.52 yd³.
How many bags of concrete for a 16x20 slab?
At 4 inches with 10% extra: 196 bags of 80-lb mix, 261 of 60 lb, 313 of 50 lb or 392 of 40 lb. At 6 inches: 294 bags of 80 lb or 392 of 60 lb.
How many cubic feet of concrete for a 16x20 slab?
106.7 ft³ at 4 inches (320 sq ft × 4 ÷ 12) and 160.0 ft³ at 6 inches, before waste.
How thick should a 16x20 slab be?
Four inches is usual for patios, walkways and shed pads. Garage floors, driveways and pads for heavy loads are commonly 5 to 6 inches. Follow local code or your plans for anything structural.
Other slab sizes
Related
Sources
- QUIKRETE: Concrete Mix No. 1101 data sheet (Revised Oct 2022; checked Nov 11, 2026)