How to Calculate Concrete for Common Projects
Calculating concrete comes down to one straightforward formula:
Cubic Yards = (Length × Width × Depth in inches ÷ 12) ÷ 27
For example, a 20×20-foot slab at 4 inches deep works out to: (20 × 20 × 0.333) ÷ 27 = 4.94 cubic yards. With 10% waste added, you'd order 5.44 cubic yards.
The formula is the same whether you're pouring a patio, driveway, sidewalk, or foundation footer — only the numbers change.
Concrete Thickness by Project Type
| Project | Recommended Depth | Notes |
|---|---|---|
| Walkway / Sidewalk | 4 inches | Light foot traffic |
| Patio Slab | 4 inches | Add reinforcement mesh for larger patios |
| Garage Floor | 4–5 inches | 5" for heavier vehicles |
| Driveway | 5–6 inches | 6" if supporting large trucks or RVs |
| Foundation Footer | 8–12 inches | Varies by frost depth and load |
| Fence Post Footing | 12 inches | Diameter = 3× post width |
Always Order 10% Extra — Here's Why
Concrete calculations are based on perfect conditions — flat ground, uniform depth, zero spillage. Real jobsites are different. Ground is rarely perfectly level, forms flex under pressure, and some concrete always gets wasted at the chute. Industry standard is to add 10% to every order.
This is especially critical with ready-mix: a short load means you're waiting for another truck (if one is even available), the first pour starts to set, and the cold joint becomes a structural and aesthetic weak point. Running out is far more expensive than ordering a little extra.
For irregularly shaped slabs, calculate the area as if it were a rectangle bounding the shape, then subtract obvious voids. Err on the side of more.
Ready-Mix vs. Bagged Concrete: Which to Use
Ready-Mix Concrete
Pros
- ✓ Best for jobs over 1 cubic yard
- ✓ Consistent mix quality
- ✓ Faster pour on large projects
- ✓ Less physical labor
Cons
- — Minimum order fees
- — Short time window to place and finish
- — Requires truck access to site
Bagged Concrete Mix
Pros
- ✓ Great for small jobs under 1 CY
- ✓ No minimum order, no delivery
- ✓ Work at your own pace
- ✓ Easy to store leftover bags
Cons
- — Labor-intensive for large jobs
- — Manual mixing is inconsistent
- — Cold joints if batches aren't matched
Tips for a Successful Concrete Pour
Prep Your Subgrade
Compact the base material and remove soft spots. A stable, compacted subgrade prevents cracking and settling over time.
Set Forms Precisely
Double-check all dimensions before ordering. Your forms define the final slab — getting them level and square saves headaches later.
Control Joints Are Not Optional
For slabs over 10 feet in any direction, cut control joints every 8–10 feet to direct cracking. Concrete will crack — joints tell it where.
Cure Properly
Keep concrete moist for at least 7 days after pouring using plastic sheeting or a curing compound. Proper curing dramatically increases long-term strength.
Check Your Local Frost Depth
Footings must extend below the frost line in your area. A slab that heaves every winter is a liability — check local building codes before pouring.
Frequently Asked Questions
How do I calculate how much concrete I need?+
Multiply Length (ft) × Width (ft) × Depth (ft), then divide by 27 to convert cubic feet to cubic yards. Always add 10% extra for waste and spillage.
How many 60 lb bags of concrete make a cubic yard?+
A 60 lb bag of concrete covers approximately 0.45 cubic feet. Since 1 cubic yard equals 27 cubic feet, you need about 60 bags per cubic yard.
How many 80 lb bags of concrete make a cubic yard?+
An 80 lb bag covers approximately 0.60 cubic feet. You need about 45 bags to fill one cubic yard of concrete.
When should I use ready-mix concrete vs bags?+
Use ready-mix for projects requiring more than 1 cubic yard — it's faster and more consistent. For smaller jobs under 1 CY (like a fence post or small pad), pre-mixed bags are cost-effective and easy to handle.
How thick should a concrete slab be?+
Most residential slabs are 4 inches thick. Driveways that will support vehicle weight should be 5–6 inches. Footings are typically 8–12 inches depending on local frost depths and load requirements.
How much does a cubic yard of concrete cost?+
Ready-mix concrete typically costs $120–$180 per cubic yard depending on your region, mix design, and current material prices. Add delivery fees and finishing labor for total project cost.