Concrete Calculator
Calculate how much concrete you need for slabs, footings, patios, driveways, and garage floors. See results in cubic yards and bag counts for all bag sizes, with a buying recommendation to help you decide between bags and ready-mix delivery.
Worked Examples
Example 1: Small Patio (bags vs ready-mix)
A homeowner is pouring an 8 × 10 ft patio slab at standard 4-inch thickness.
- 1 Patio dimensions: 8 ft × 10 ft × 4″ thick
- 2 Volume: 8 × 10 × (4/12) = 26.7 cu ft
- 3 Add 10% overage: 26.7 × 1.10 = 29.3 cu ft = 1.1 cu yd
- 4 Bags needed: 98 bags of 40-lb, 78 bags of 50-lb, 66 bags of 60-lb, or 49 bags of 80-lb
- 5 Buying recommendation: Compare bags vs ready-mix at this volume
Example 2: Driveway (ready-mix delivery)
A homeowner is pouring a 12 × 20 ft driveway apron at 5-inch thickness (driveway preset).
- 1 Driveway dimensions: 12 ft × 20 ft × 5″ thick (driveway preset)
- 2 Volume: 12 × 20 × (5/12) = 100 cu ft
- 3 Add 10% overage: 100 × 1.10 = 110 cu ft = 4.1 cu yd
- 4 Bags needed: 367 bags of 40-lb or 184 bags of 80-lb
- 5 Buying recommendation: Ready-mix truck delivery recommended — mixing 184 bags by hand is impractical
How Concrete Volume Is Calculated
Volume: Length × Width × (Thickness ÷ 12) gives the volume in cubic feet. Dividing thickness by 12 converts inches to feet so all measurements use the same unit.
Cubic yards: Divide cubic feet by 27. Ready-mix trucks sell by the cubic yard, so this is what you order.
Bag count: Each bag has a yield in cubic feet: 40-lb = 0.30, 50-lb = 0.375, 60-lb = 0.45, 80-lb = 0.60 cu ft. Divide total volume by yield and round up — you can't buy half a bag.
Buying recommendation: Based on the adjusted volume, the calculator suggests whether bags are practical (≤ 0.5 cu yd), worth comparing (0.5-1.0 cu yd), or whether ready-mix delivery is more practical (> 1.0 cu yd). These thresholds come from common industry practice and community feedback.
Gravel base: Most slabs need a compacted gravel base underneath (typically 4 inches). Use our gravel calculator to estimate the base material needed for your slab area.
Recommended Slab Thickness
| Application | Thickness | Notes |
|---|---|---|
| Sidewalk / Walkway | 4″ | Standard for pedestrian traffic |
| Patio | 4″ | Adequate for furniture and foot traffic |
| Shed Pad / Utility Pad | 4″ | Standard for lightweight structures |
| Driveway | 5–6″ | Must support vehicle weight; check local codes |
| Garage Floor | 4–6″ | 4″ minimum; 6″ for heavy vehicles or equipment |