Concrete Calculator
What Is a Concrete Calculator?
A concrete calculator tells you exactly how much concrete — in cubic yards, cubic feet, or pre-mix bags — you need for a slab, driveway, patio, footing, column, or curb. Ordering too little means a second delivery charge and a visible pour joint where concrete sets at different times. Ordering too much wastes $125–$175 per cubic yard for ready-mix concrete.
The core formula is Volume = Length × Width × Thickness, with thickness converted from inches to feet. A 10×10-foot patio at 4 inches thick = 10 × 10 × (4/12) = 33.3 cubic feet = 1.23 cubic yards.
Ready-mix concrete is ordered by the cubic yard. Pre-mixed bags (Quikrete, Sakrete) are sold by weight — an 80 lb bag yields about 0.60 cubic feet, and a 60 lb bag yields 0.45 cubic feet.
How to Use This Concrete Calculator
- Select the pour type: slab/patio, footing/foundation, column, or curb & gutter.
- Enter dimensions in feet (length, width) and inches (thickness). The calculator converts automatically.
- Choose your bag size (60 lb or 80 lb) to see how many pre-mix bags you need.
- The result includes a 10% waste buffer — always order at least this amount.
- For ready-mix orders, use the cubic yards figure to call your concrete supplier.
Worked Example: 20×30 Driveway at 6 Inches Thick
| Step | Calculation | Result |
|---|---|---|
| Cubic feet | 20 × 30 × (6/12) | 300 ft³ |
| Cubic yards | 300 ÷ 27 | 11.11 yd³ |
| Order amount (10% waste) | 11.11 × 1.10 | 12.2 yd³ |
| 80 lb bags alternative | 300 ft³ ÷ 0.60 × 1.10 | 550 bags (not practical — use ready-mix) |
For any project over 1 cubic yard, ready-mix delivery is faster and cheaper than hand-mixing bags.
Concrete Thickness Guidelines by Project
| Project | Minimum Thickness | Recommended |
|---|---|---|
| Walkway / sidewalk | 3 inches | 4 inches |
| Patio | 3.5 inches | 4 inches |
| Driveway (passenger cars) | 4 inches | 5–6 inches |
| Driveway (heavy trucks) | 6 inches | 8 inches |
| Garage floor | 4 inches | 6 inches |
| Foundation footing | 8 inches | 12+ inches |
Key Concrete Concepts
- Cubic yard (yd³): The standard unit for ready-mix orders. 1 cubic yard = 27 cubic feet = ~2 tons of wet concrete.
- PSI rating: 2,500 PSI for basic slabs; 3,000 PSI for driveways; 4,000+ PSI for structural work.
- Water-to-cement ratio: Less water = stronger concrete. Never add extra water at the pour to make mixing easier.
- Slump: Measures concrete workability. 3–4 inch slump for most residential pours.
- Curing time: Concrete reaches 70% strength in 7 days, full strength at 28 days. Keep it moist the first week.
Tips and Common Mistakes
- Always add 10% for waste: Uneven sub-bases, spills, and edging consume more concrete than you expect.
- Minimum order for ready-mix: Most plants require at least 1 cubic yard per delivery ($150–$250 minimum charge).
- Check sub-base depth: A 6-inch gravel base under a driveway affects overall excavation depth — factor this into your dig.
- Bags vs. ready-mix: Bags are cost-effective under 1 yd³. Above that, ready-mix saves time and usually money too.
- Freeze protection: Do not pour when temperatures will drop below 40°F within 24 hours without cold-weather protection.
Frequently Asked Questions About Concrete
How many bags of concrete do I need for a 10×10 slab?
At 4 inches thick: 10 × 10 × (4/12) = 33.3 ft³. With 10% waste = 36.7 ft³. Using 80 lb bags (0.60 ft³ each): 62 bags. At this scale, ready-mix is more practical.
How many cubic yards of concrete do I need per square foot?
It depends on thickness. At 4 inches: 1 ft² = 0.333 ft³ = 0.0123 yd³. Multiply your square footage by 0.0123 for a 4-inch slab estimate.
What does 1 cubic yard of concrete weigh?
About 3,900–4,050 pounds (roughly 2 tons) when wet. Weight varies slightly by mix design and aggregate type.
How long does concrete take to cure?
Walk on it after 24 hours. Drive on it after 7 days. Full structural strength at 28 days. Keep it damp the first week to prevent cracking.
How thick should a concrete driveway be?
At least 4 inches for passenger vehicles. Use 5–6 inches for better durability, especially in freeze-thaw climates. Heavier vehicle areas need 6–8 inches.
Can I mix concrete by hand for a large slab?
Hand-mixing is practical up to about 1 cubic yard. Above that, rent a concrete mixer or order ready-mix. Large slabs must be poured in a single continuous pour to avoid cold joints.
What PSI concrete should I use for a driveway?
Use 3,000 PSI as a minimum for driveways. In cold climates with freeze-thaw cycles, specify 4,000 PSI air-entrained concrete for better crack resistance.
How do I calculate concrete for a round column?
Volume = π × radius² × height. For a 12-inch diameter (6-inch radius), 8-foot column: 3.14159 × 0.5² × 8 = 6.28 ft³ = 0.23 yd³.