Worked example: 12 × 20 ft at 4 inches
| Step | Math | Result |
|---|---|---|
| Area | 12 ft × 20 ft | 240 sq ft |
| Volume at 4 in | 240 sq ft × 4 in ÷ 12 | 80.0 ft³ |
| Add 10% extra | 80.0 × 1.10 | 88.0 ft³ |
| Cubic yards | 88.0 ÷ 27 | 3.26 yd³ |
| 80 lb bags | 88.0 ÷ 0.60, rounded up | 147 bags |
| 60 lb bags | 88.0 ÷ 0.45, rounded up | 196 bags |
| 40 lb bags | 88.0 ÷ 0.30, rounded up | 294 bags |
Yards and bags at 4, 5 and 6 inches
| Thickness | Cubic feet (+10%) | Cubic yards (+10%) | 80 lb bags | 60 lb bags | 40 lb bags | Ready-mix order (yd³) |
|---|---|---|---|---|---|---|
| 4 in | 88.0 | 3.26 | 147 | 196 | 294 | 3.50 |
| 5 in | 110.0 | 4.07 | 184 | 245 | 367 | 4.25 |
| 6 in | 132.0 | 4.89 | 220 | 294 | 440 | 5.00 |
Bags vs ready-mix cost
| Thickness | 80 lb bags | 60 lb bags | 40 lb bags | Ready-mix | Cheaper (80 lb) |
|---|---|---|---|---|---|
| 4 in | $956 | $1,029 | $1,250 | $745 | Ready-mix |
| 5 in | $1,196 | $1,286 | $1,560 | $722 | Ready-mix |
| 6 in | $1,430 | $1,544 | $1,870 | $850 | Ready-mix |
For the 4 in slab, ready-mix is cheaper by about $210 and avoids mixing 147 bags (11,760 lb) by hand. Your 3.50 yd³ ready-mix order is under the typical 4 yd³ minimum, so the plant adds a short-load fee (about $150 here). That fee is already inside the ready-mix cost above.
Prices used: 80 lb bag $6.50, 60 lb $5.25, 40 lb $4.25; ready-mix $170/yd³ with a $150 short-load fee under 4 yd³ (orders are rounded up to ¼ yd³, 1 yd³ minimum). Local prices vary a lot; change them in the calculator.
Planning notes for 12 × 20 ft
- Truck access: one truck holds this pour in a single load. Chutes reach about 15–20 ft; farther than that you need wheelbarrows or a pump.
- Weight: the 4 in pour weighs about 6.5 tons. Bags alone weigh 11,760 lb at 80 lb each.
- Gravel base: a 4 in compacted gravel base under this slab takes about 4.6 tons (3.3 yd³) with 10% extra. See gravel for a driveway.
- Thickness: 4 in suits patios, sheds and walks; 5–6 in is common for vehicle loads. See how thick a slab should be.
Nearby sizes
Also see 12×16 slab , 16×16 slab , 12×12 slab, or the full slab chart. For other sizes, open the concrete calculator, or compare bags and ready-mix.