Worked example: 30 × 30 ft at 4 inches
| Step | Math | Result |
|---|---|---|
| Area | 30 ft × 30 ft | 900 sq ft |
| Volume at 4 in | 900 sq ft × 4 in ÷ 12 | 300.0 ft³ |
| Add 10% extra | 300.0 × 1.10 | 330.0 ft³ |
| Cubic yards | 330.0 ÷ 27 | 12.22 yd³ |
| 80 lb bags | 330.0 ÷ 0.60, rounded up | 550 bags |
| 60 lb bags | 330.0 ÷ 0.45, rounded up | 734 bags |
| 40 lb bags | 330.0 ÷ 0.30, rounded up | 1100 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 | 330.0 | 12.22 | 550 | 734 | 1100 | 12.25 |
| 5 in | 412.5 | 15.28 | 688 | 917 | 1375 | 15.50 |
| 6 in | 495.0 | 18.33 | 825 | 1100 | 1650 | 18.50 |
Bags vs ready-mix cost
| Thickness | 80 lb bags | 60 lb bags | 40 lb bags | Ready-mix | Cheaper (80 lb) |
|---|---|---|---|---|---|
| 4 in | $3,575 | $3,854 | $4,675 | $2,082 | Ready-mix |
| 5 in | $4,472 | $4,814 | $5,844 | $2,635 | Ready-mix |
| 6 in | $5,362 | $5,775 | $7,012 | $3,145 | Ready-mix |
For the 4 in slab, ready-mix is cheaper by about $1,492 and avoids mixing 550 bags (44,000 lb) by hand. At 12.25 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 30 × 30 ft
- Truck loads: about 24.4 tons of concrete is more than one standard mixer carries (up to roughly 10 yd³), so expect 2 loads, and have the crew and finishers ready for a continuous pour.
- Weight: the 4 in pour weighs about 24.4 tons. Bags alone weigh 44,000 lb at 80 lb each.
- Gravel base: a 4 in compacted gravel base under this slab takes about 17.1 tons (12.2 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 24×24 slab , 20×24 slab, or the full slab chart. For other sizes, open the concrete calculator, or compare bags and ready-mix.