Concrete Slab Calculator
Enter your slab's length, width and thickness below — the concrete slab calculator updates cubic yards, bag counts and estimated cost instantly, right on this page.
Concrete needed
2.96 yd³
80.0 ft³
Estimated cost
$489
at $165 per yd³
Approx. weight
10,640 lb
≈ 5.3 tons
Bagged mix
134
80-lb bags
178
60-lb bags
267
40-lb bags
Before you order: add about 10% for uneven subgrade and spillage — that's 3.26 yd³ or 147 × 80-lb bags.
Formula
Slab volume is simply length × width × thickness, with thickness converted from inches to feet (divide by 12):
volume (ft³) = length (ft) × width (ft) × [thickness (in) ÷ 12]
Then convert to the units you actually order:
- Cubic yards = cubic feet ÷ 27
- Bags = cubic feet ÷ bag yield — 0.60 ft³ for an 80-lb bag, 0.45 ft³ for 60 lb, 0.30 ft³ for 40 lb (round up to whole bags)
- Estimated cost = cubic yards × price per yard
| Constant | Value |
|---|---|
| 1 cubic yard | 27 ft³ |
| 80-lb bag yield | 0.60 ft³ (45 bags per yd³) |
| 60-lb bag yield | 0.45 ft³ (60 bags per yd³) |
| 40-lb bag yield | 0.30 ft³ (90 bags per yd³) |
| Wet concrete density | ≈ 133 lb/ft³ (2,130 kg/m³) |
Example
- A patio measures 12 ft × 20 ft, poured 4 inches thick: 4 ÷ 12 = 0.333 ft.
- Volume: 12 × 20 × 0.333 = 80 ft³.
- Cubic yards: 80 ÷ 27 = 2.96 yd³ — order about 3.3 yd³ with the 10% allowance.
- Bags: 80 ÷ 0.60 = 133.3 → 134 × 80-lb bags (147 with waste).
- Cost at $165 per yard: 2.96 × 165 ≈ $489.
Frequently Asked Questions
How many bags of concrete do I need for a 12×20 slab?
A 12 × 20 ft slab at 4 inches thick is 80 ft³, or about 2.96 cubic yards. That takes 134 of the 80-lb bags (or 178 of the 60-lb bags) — and roughly 147 × 80-lb bags once you add the 10% waste allowance. At that size, a ready-mix truck is usually the better buy.
How thick should a concrete slab be?
Four inches is the standard for patios, walkways and shed floors. Driveways and anything carrying vehicles should be five to six inches, with thicker edges or reinforcement where trucks or RVs will park.
How much does one cubic yard of concrete cover?
One cubic yard is 27 ft³. Poured 4 inches thick, it covers 81 ft² — a 9 × 9 ft slab. At 6 inches it covers 54 ft², and at 3 inches, 108 ft².
Is it cheaper to use bags or order ready-mix?
For pours under about one cubic yard (roughly 45 × 80-lb bags), bagged mix is cheaper and easier to handle. Above that, ready-mix delivery wins on price and effort — hand-mixing 45+ bags per yard is a full day of work, and many suppliers charge short-load fees for small orders.
Does this calculator include the gravel base or rebar?
No — it estimates the concrete slab itself. Plan a compacted gravel base (typically 4 inches) and reinforcement such as rebar or wire mesh separately; neither affects the concrete volume, but both affect your total material budget.