Concrete Calculator
Calculate exactly how much concrete you need for slabs, driveways, footings, and post holes.
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How to Use the Concrete Calculator
Planning a DIY concrete project? Getting the amount right is crucial. Too little and you're scrambling for bags while your pour hardens; too much and you've wasted money. Our Concrete Calculator helps you estimate the precise volume of concrete needed for slabs, footings, paths, or post holes.
Calculating for Slabs & Rectangles
For square or rectangular shapes like patios, driveways, or floor slabs, you simply need:
- Length: The longest side of the area.
- Width: The shorter side.
- Thickness (Depth): How deep the concrete will be poured (typically 4 inches for sidewalks).
Concrete Bag Estimates
If you are mixing by hand or using a small portable mixer, you'll likely buy pre-mixed bags (like Quikrete or Sakrete). Here are the typical yields:
| Bag Size | Yield (Cubic Feet) | Bags per Cubic Yard |
|---|---|---|
| 80 lb | ~0.60 | 45 |
| 60 lb | ~0.45 | 60 |
| 50 lb | ~0.375 | 72 |
Tip: Always buy 5-10% more than calculated to account for spillage, uneven ground, or waste.
Concrete Needed for Common Projects
Here are quick references for common residential concrete projects at 4 inches (10 cm) thick:
| Project | Size | Cubic Yards | 80lb Bags |
|---|---|---|---|
| Small Patio | 8×8 ft | 0.79 | 36 |
| Sidewalk | 3×20 ft | 0.74 | 34 |
| Shed Pad | 10×10 ft | 1.23 | 56 |
| Single Car Driveway | 10×20 ft | 2.47 | 111 |
| Two Car Driveway | 16×40 ft | 7.90 | 356 |
| Garage Floor | 20×20 ft | 4.94 | 222 |
Tips for Working with Concrete
- Order 5-10% extra — Uneven ground, spillage, and form bulging mean you always need more than the calculation shows.
- Use the right mix — Standard mix (2,500 PSI) is fine for sidewalks; use high-strength mix (5,000 PSI) for driveways and structural supports.
- Work quickly — Concrete begins to set within 30-45 minutes of mixing. Have all forms ready before you start.
- Cure properly — Keep freshly poured concrete moist for at least 7 days for maximum strength. Cover with plastic sheeting or use a curing compound.
- Check the weather — Avoid pouring concrete when temperatures are below 40°F (4°C) or above 90°F (32°C).
Ready-Mix vs Pre-Mixed Bags: Which to Choose?
For small projects under 1 cubic yard, pre-mixed bags (like Quikrete) are usually most economical. For projects over 1-2 cubic yards, ordering ready-mix concrete from a local batch plant is more efficient and ensures consistent quality. Most suppliers deliver in trucks that hold 8-10 cubic yards, with a minimum order of around 1 cubic yard.