Written by Awais Khadim · Updated
How to calculate concrete for a slab
Multiply length by width to get the area, multiply by thickness to get volume, then add a waste allowance and convert to cubic yards.
area (sq ft) = length × width
volume (cu ft) = area × thickness (in) ÷ 12
with waste = volume × (1 + waste% ÷ 100)
cubic yards = with waste ÷ 27
80 lb bags = with waste ÷ 0.60 (round up)
60 lb bags = with waste ÷ 0.45 (round up)
total cost = yards × price per yd³ + area × labor per sq ftWorked example: 10 × 10 ft patio, 4 in thick
- Area: 10 × 10 = 100 sq ft
- Volume: 100 × 4 ÷ 12 = 33.33 cu ft
- With 10% waste: 33.33 × 1.10 = 36.67 cu ft
- Cubic yards: 36.67 ÷ 27 = 1.36 yd³
- Bags: 62 × 80 lb or 82 × 60 lb
- Cost: 1.36 yd³ × $135 + 100 sq ft × $3 = $483.33
Concrete slab cost by size
Installed cost for a 4-inch slab. Concrete volumes include 10% waste.
| Slab size | Area | Concrete (10% waste) | Installed cost |
|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 1.36 yd³ | $600 – $1,200 |
| 10 × 12 ft | 120 sq ft | 1.63 yd³ | $720 – $1,440 * |
| 12 × 12 ft | 144 sq ft | 1.96 yd³ | $860 – $1,700 |
| 20 × 20 ft | 400 sq ft | 5.43 yd³ | $2,400 – $4,800 |
| 24 × 24 ft | 576 sq ft | 7.82 yd³ | $3,460 – $6,910 * |
Source: HomeGuide (Feb 2026). * 10 × 12 and 24 × 24 calculated from HomeGuide's $6–$12 per sq ft rate.
How many bags of concrete do you need?
To find how many bags of concrete you need, divide the slab volume in cubic feet by the bag yield and round up. An 80 lb bag yields about 0.60 cubic feet and a 60 lb bag about 0.45 cubic feet (Quikrete). A 10 × 10 ft slab 4 inches thick with 10% extra needs 62 bags of 80 lb mix.
| Slab size | Volume (with waste) | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 × 10 ft, 4 in | 36.7 cu ft | 62 | 82 |
| 10 × 12 ft, 4 in | 44 cu ft | 74 | 98 |
| 12 × 12 ft, 4 in | 52.8 cu ft | 88 | 118 |
| 20 × 20 ft, 4 in | 146.7 cu ft | 245 | 326 |
| 24 × 24 ft, 4 in | 211.2 cu ft | 352 | 470 |
See the full 10x10 bag chart →
What affects the cost of a concrete slab
- Thickness: going from 4 to 6 inches adds 50% more concrete. HomeGuide puts 4-inch slabs at $6–$8/sq ft and 6-inch slabs at $8–$10/sq ft installed.
- Site prep: grading costs about $0.40–$2.00/sq ft, and a gravel base about $1–$3/sq ft (HomeGuide).
- Reinforcement: wire mesh about $0.20–$0.30/sq ft (Inch Calculator); rebar about $0.50–$3/sq ft depending on spacing (Angi, Inch Calculator).
- Finish: stamped concrete runs about $8–$19/sq ft installed; sealing adds $1–$3/sq ft (HomeGuide).
- Delivery: orders under 10 yd³ often add a short-load fee of about $53/yd³ (HomeGuide).
- Access: pumping or wheelbarrowing concrete to a backyard adds time and equipment cost.
- Location: labor rates vary by state and city.
Frequently asked questions
How many cubic yards of concrete do I need for a 10×10 slab?
A 10×10 ft slab at 4 inches thick is 33.3 cubic feet, or about 1.23 cubic yards. With 10% extra for waste and spillage, plan on about 1.36 cubic yards.
How many 80 lb bags of concrete are in a yard?
One 80 lb bag yields about 0.60 cubic feet, so a cubic yard (27 cubic feet) takes about 45 bags. A 60 lb bag yields about 0.45 cubic feet, so you need about 60 bags per yard.
When should I order ready-mix instead of bags?
For anything over about 1 cubic yard (roughly 45 bags of 80 lb mix), ready-mix is usually easier and often cheaper. Bags make sense for small pads, posts and repairs.
How thick should a driveway slab be?
Most residential driveways use 4 to 6 inches of concrete. Go thicker (5–6 in) if heavy vehicles such as trucks or RVs will park on it. Angi lists 4–8 inches for driveways.
How much extra concrete should I order?
Extra concrete covers uneven subgrade, spillage and form flex. This calculator uses 10% by default; you can change it with the waste slider.
Sources
- Quikrete — Concrete Mix No. 1101 Data Sheet (bag yields)
- HomeGuide — How Much Does a Concrete Slab Cost? (Feb 3, 2026)
- HomeGuide — How Much Does Concrete Cost Per Yard? (Jan 6, 2026)
- Angi — How Much Does a New Concrete Slab Cost? (updated Apr 16, 2026)
- Inch Calculator — Cost to Install a Concrete Slab