Ordering concrete is one project where guessing wrong is expensive in both directions. Order too little and you get a cold joint — a visible, weak seam where an old pour meets a new one. Order too much and you're paying for wasted material, since ready-mix trucks don't take returns. The good news: the math is simple once you know the formula.
The Concrete Volume Formula
Concrete volume comes down to three numbers: length, width, and thickness. The formula is:
Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)
Since thickness is usually given in inches, convert it first by dividing by 12. Then convert cubic feet to cubic yards — the unit concrete is sold in — by dividing by 27.
Example: A 10×10 ft patio at 4 inches thick. Thickness in feet = 4 ÷ 12 = 0.33 ft. Volume = 10 × 10 × 0.33 = 33 cubic feet. Cubic yards = 33 ÷ 27 = 1.2 cubic yards.
Standard Thickness by Project Type
| Project | Standard Thickness | Notes |
|---|---|---|
| Sidewalk / walkway | 4 inches | Foot traffic only |
| Patio | 4 inches | Add rebar or mesh for stability |
| Driveway (cars) | 4–6 inches | 6 inches for regular vehicle traffic |
| Driveway (RV/trucks) | 6–8 inches | Heavier loads need more thickness and reinforcement |
| Shed foundation | 4 inches | Check local code for load requirements |
| Fence post footing | N/A — see below | Calculated by hole volume, not slab area |
Calculating Footings for Posts
Post footings use a different formula since they're cylindrical, not rectangular. For a round hole:
Volume (cubic feet) = π × radius² × depth
A typical fence post footing is 10 inches (0.83 ft) in diameter — so radius is 0.42 ft — and 24 inches (2 ft) deep. Volume = 3.14 × 0.42² × 2 = 1.1 cubic feet per hole. For 10 posts, that's 11 cubic feet, or about 0.4 cubic yards — roughly six 60-lb bags.
Bags vs. Ready-Mix: The 1-Yard Rule
For projects under about 1 cubic yard, bagged concrete mixed by hand or with a mixer is usually more practical — most ready-mix trucks have a 1-yard minimum delivery charge that makes small orders expensive per yard. For anything larger, ready-mix delivery is faster, more consistent, and often cheaper overall.
| Bag Size | Yield | Bags per Cubic Yard |
|---|---|---|
| 40 lb bag | 0.30 cu ft | ~90 bags |
| 60 lb bag | 0.45 cu ft | ~60 bags |
| 80 lb bag | 0.60 cu ft | ~45 bags |
Quick Reference: Concrete Needed by Slab Size
| Slab Size | Thickness | Cubic Yards | 80 lb Bags |
|---|---|---|---|
| 4×4 ft | 4 in | 0.2 cu yd | ~9 bags |
| 10×10 ft | 4 in | 1.2 cu yd | ~55 bags |
| 12×16 ft | 4 in | 2.4 cu yd | ~107 bags |
| 20×20 ft | 4 in | 5.0 cu yd | Order ready-mix |
| 20×40 ft (driveway) | 6 in | 14.8 cu yd | Order ready-mix |
Always Add a Waste Buffer
Add 5–10% extra to your calculated volume to account for uneven excavation, spillage, and subgrade irregularities. Running short mid-pour is a serious problem — concrete starts setting within 30–45 minutes, and a delayed second batch creates a visible, weak cold joint where the two pours meet.
Frequently Asked Questions
How much concrete do I need for a 10×10 patio? At 4 inches thick, a 10×10 ft patio needs about 1.2 cubic yards, or roughly 55 bags of 80 lb pre-mix. Round up to account for waste.
How many bags of concrete do I need for one fence post? A standard 10-inch diameter, 24-inch deep footing needs about 1.1 cubic feet, which is roughly two 60-lb bags per post.
Should I order ready-mix or use bags? Under about 1 cubic yard, bags are usually more practical since most ready-mix trucks have a 1-yard delivery minimum. Above that, ready-mix is typically faster and more cost-effective.
How thick should a concrete slab be? 4 inches is standard for patios, walkways, and sheds. Driveways carrying regular vehicle traffic should be at least 6 inches thick.