Worked example: 4 × 4 ft at 4 inches
| Step | Math | Result |
|---|---|---|
| Area | 4 ft × 4 ft | 16 sq ft |
| Volume at 4 in | 16 sq ft × 4 in ÷ 12 | 5.3 ft³ |
| Add 10% extra | 5.3 × 1.10 | 5.9 ft³ |
| Cubic yards | 5.9 ÷ 27 | 0.22 yd³ |
| 80 lb bags | 5.9 ÷ 0.60, rounded up | 10 bags |
| 60 lb bags | 5.9 ÷ 0.45, rounded up | 14 bags |
| 40 lb bags | 5.9 ÷ 0.30, rounded up | 20 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 | 5.9 | 0.22 | 10 | 14 | 20 | 1.00 |
| 5 in | 7.3 | 0.27 | 13 | 17 | 25 | 1.00 |
| 6 in | 8.8 | 0.33 | 15 | 20 | 30 | 1.00 |
Bags vs ready-mix cost
| Thickness | 80 lb bags | 60 lb bags | 40 lb bags | Ready-mix | Cheaper (80 lb) |
|---|---|---|---|---|---|
| 4 in | $65 | $74 | $85 | $320 | Bags |
| 5 in | $84 | $89 | $106 | $320 | Bags |
| 6 in | $98 | $105 | $128 | $320 | Bags |
For the 4 in slab, bags are cheaper by about $255, but you would mix and place 10 bags (800 lb) before the first batch sets. Your 1.00 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 4 × 4 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 0.4 tons. Bags alone weigh 800 lb at 80 lb each.
- Gravel base: a 4 in compacted gravel base under this slab takes about 0.3 tons (0.2 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 8×8 slab , 10×10 slab, or the full slab chart. For other sizes, open the concrete calculator, or compare bags and ready-mix.