A 30 × 40 ft slab 5 inches thick needs 18.52 cubic yards of concrete — 917 bags of 80-lb mix (or 1,223 × 60-lb), including 10% extra.
| Bag | Bags | Est. cost |
|---|---|---|
| 40 lb · 0.3 ft³ | 1,834 | — |
| 50 lb · 0.375 ft³ | 1,467 | — |
| 60 lb · 0.45 ft³ | 1,223 | $5,932 |
| 80 lb · 0.6 ft³ | 917 | $5,438 |
At these prices a truck becomes cheaper from about 3.25 yd³ (147 bags). Over 1–2 yd³, a truck also saves hours of mixing: plan on 3–4 minutes per bag in a mixer.
Typical U.S. big-box prices checked October 2026. Tap any price to enter your store's.
| Thickness | Cubic yards | 80-lb bags | 60-lb bags | Bag cost | Ready-mix |
|---|---|---|---|---|---|
| 3.5 in (2×4) | 12.96 | 642 | 856 | $3,807 | $2,855 |
| 4 in | 14.81 | 734 | 978 | $4,353 | $3,195 |
| 5 in | 18.52 | 917 | 1,223 | $5,438 | $4,070 |
| 6 in | 22.22 | 1,100 | 1,467 | $6,523 | $4,750 |
30 ft × 40 ft = 1,200 sq ft. Multiply by the thickness in feet (5 ÷ 12 = 0.417 ft) to get 500 cubic feet, and divide by 27 for 18.52 cubic yards.
Adding 10% for uneven ground and spillage gives 550 cu ft. Quikrete and Sakrete both rate an 80-lb bag at 0.6 cu ft, so 550 ÷ 0.6 = 916.7, rounded up to 917 bags.
Ready-mix is priced at about $170 per yard for 4,000 psi plus delivery, fuel and — under 4 yards — a short-load fee. Plants sell in quarter-yard steps, so the order is 20.5 yd³.
The full method, material data and tips are on the main concrete slab calculator page.