Concrete Slab Calculator
A concrete slab calculator tells you exactly how many cubic yards — or how many bags — you need before you order. Enter your slab's length, width, and thickness and we'll calculate the volume, add waste, and tell you whether to call a ready-mix truck or pick up bags at the hardware store.
How to use this calculator
- Enter your slab length and width in feet.
- Set the thickness — 4" for standard driveways and patios, 6" for heavy vehicle areas.
- Adjust the waste factor (10% is typical; add more for irregular shapes).
- Read your cubic yards and estimated bag count — use these to get supplier quotes.
How we calculate this
Slab volume = length × width × thickness, converted from cubic feet to cubic yards (divide by 27). A standard 4-inch slab under a 20 × 20 ft patio is 20 × 20 × (4/12) = 133.3 cubic feet = 4.94 cubic yards. We add 10% waste for spillage, uneven subgrade, and over-excavation — a standard industry allowance. Pours under 1 yard are shown in bags (80 lb bags yield 0.60 cu ft each); larger pours recommend ready-mix delivery.
What this calculator doesn't include
This calculator covers concrete volume only. It does not account for gravel base, rebar or wire mesh, forms, finishing labour, or curing compound. For a reinforced structural slab, consult a structural engineer for rebar sizing and spacing. Always verify your subgrade depth — an uneven base can add 15–20% to actual concrete used.
Example concrete estimates — Concrete Slab
Pre-calculated for common project sizes. Includes 10% waste factor.
| Dimensions | Thickness | Cubic Yards | With Waste | 80 lb Bags |
|---|---|---|---|---|
| 10 × 10 ft | 4" | 1.235 yd³ | 1.36 yd³ | 62 |
| 12 × 12 ft | 4" | 1.778 yd³ | 1.96 yd³ | 89 |
| 10 × 20 ft | 4" | 2.469 yd³ | 2.72 yd³ | 123 |
| 20 × 20 ft | 4" | 4.938 yd³ | 5.43 yd³ | 245 |
| 12 × 24 ft | 4" | 3.556 yd³ | 3.91 yd³ | 177 |
| 24 × 24 ft | 4" | 7.111 yd³ | 7.82 yd³ | 352 |
| 20 × 30 ft | 4" | 7.407 yd³ | 8.15 yd³ | 367 |
| 30 × 30 ft | 4" | 11.111 yd³ | 12.22 yd³ | 550 |
| 40 × 40 ft | 4" | 19.753 yd³ | 21.73 yd³ | 978 |
| 50 × 50 ft | 4" | 30.864 yd³ | 33.95 yd³ | 1528 |
All estimates include a 10% waste factor. Under 1 yd³ is typically practical with bagged mix; over 1 yd³ usually warrants a ready-mix truck.
Frequently asked questions
For a standard residential patio or walkway, 4 inches is the minimum. A driveway that will see passenger vehicles should be at least 4–5 inches; one that will see trucks or RVs should be 6 inches. Garage floors are typically 4–6 inches. Structural slabs and those over expansive soils may require 6–8 inches with rebar — consult an engineer.
A 10 × 10 ft slab at 4 inches thick is 33.3 cubic feet = 1.23 cubic yards. With 10% waste that's 1.36 yards or 61 bags of 80 lb concrete mix. At this volume, ready-mix delivery is often more economical than hauling and mixing bags — get quotes from both.
Concrete waste comes from several sources: spillage during pour, slightly uneven subgrade that adds depth, over-excavation at edges, and the small amount left in the truck drum. Ordering short means a cold joint if you need a second delivery — always over-order slightly. 10% is the industry standard for slabs; add 15% for complex shapes.
A standard ready-mix truck holds 8–10 cubic yards. Most suppliers have a minimum order of 1 yard, with short-load fees for small orders. For pours under about 1 yard it is usually cheaper to use bags. For very large pours, coordinate delivery time carefully — concrete begins setting within 60–90 minutes of mixing.
For residential flatwork (patios, walkways, driveways), 3,000–4,000 PSI is standard. Driveways and garage floors benefit from 4,000 PSI for durability. Structural elements like footings and columns typically require 3,000–4,000 PSI minimum. In freeze-thaw climates, specify air-entrained concrete (5–7% air) to resist cracking from freeze-thaw cycles.