Worked example: 20 × 20 ft at 4 inches
| Step | Math | Result |
|---|---|---|
| Area | 20 ft × 20 ft | 400 sq ft |
| Volume at 4 in | 400 sq ft × 4 in ÷ 12 | 133.3 ft³ |
| Add 10% extra | 133.3 × 1.10 | 146.7 ft³ |
| Cubic yards | 146.7 ÷ 27 | 5.43 yd³ |
| 80 lb bags | 146.7 ÷ 0.60, rounded up | 245 bags |
| 60 lb bags | 146.7 ÷ 0.45, rounded up | 326 bags |
| 40 lb bags | 146.7 ÷ 0.30, rounded up | 489 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 | 146.7 | 5.43 | 245 | 326 | 489 | 5.50 |
| 5 in | 183.3 | 6.79 | 306 | 408 | 612 | 7.00 |
| 6 in | 220.0 | 8.15 | 367 | 489 | 734 | 8.25 |
Bags vs ready-mix cost
| Thickness | 80 lb bags | 60 lb bags | 40 lb bags | Ready-mix | Cheaper (80 lb) |
|---|---|---|---|---|---|
| 4 in | $1,592 | $1,712 | $2,078 | $935 | Ready-mix |
| 5 in | $1,989 | $2,142 | $2,601 | $1,190 | Ready-mix |
| 6 in | $2,386 | $2,567 | $3,120 | $1,402 | Ready-mix |
For the 4 in slab, ready-mix is cheaper by about $658 and avoids mixing 245 bags (19,600 lb) by hand. At 5.50 yd³ you clear the typical 4 yd³ short-load threshold, so no short-load fee applies.
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 20 × 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 10.9 tons. Bags alone weigh 19,600 lb at 80 lb each.
- Gravel base: a 4 in compacted gravel base under this slab takes about 7.6 tons (5.4 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 16×16 slab , 20×24 slab , 12×20 slab, or the full slab chart. For other sizes, open the concrete calculator, or compare bags and ready-mix.