Calculating concrete volume is straightforward once you understand the unit conversion. Concrete is ordered and sold by the cubic yard in the US — not by cubic feet. Here's the formula:
Example: A 10 ft × 10 ft patio at 4 inches thick = 10 × 10 × (4/12) = 33.33 cubic feet ÷ 27 = 1.23 cubic yards. With 10% overage: 1.35 cubic yards to order.
Bag equivalents: Ready-mix bags come in 40 lb, 60 lb, and 80 lb sizes. An 80 lb bag yields approximately 0.60 cubic feet. So 33.33 cubic feet ÷ 0.60 = 56 bags of 80 lb concrete. For small jobs under 1 cubic yard, bags are more practical. For larger pours, order ready-mix delivery.
Common slab sizes at standard thicknesses — cubic yards and bags needed:
| Slab Size (ft) | Thickness | Cubic Yards | 80 lb Bags | Use Case |
|---|---|---|---|---|
| 4 × 4 | 4 in | 0.20 | 9 | Small pad, AC unit base |
| 6 × 6 | 4 in | 0.44 | 20 | Garden path section |
| 10 × 10 | 4 in | 1.23 | 56 | Small patio |
| 12 × 12 | 4 in | 1.78 | 80 | Standard patio |
| 20 × 20 | 4 in | 4.94 | 222 | Large patio |
| 10 × 20 | 4 in | 2.47 | 111 | Single-car driveway section |
| 10 × 20 | 6 in | 3.70 | 167 | Driveway (heavier traffic) |
| 20 × 40 | 4 in | 9.88 | 444 | Two-car driveway |
| 24 × 24 | 6 in | 10.67 | 480 | Two-car garage floor |
| 4 × 50 | 4 in | 2.47 | 111 | Sidewalk (50 ft) |
Choosing the right slab thickness is critical for structural integrity and longevity. Using too little concrete leads to cracking; too much wastes money.
Rebar and wire mesh: For any slab over 4 inches or any surface subject to vehicle traffic, add #3 or #4 rebar on 18-inch centers, or 6×6 wire mesh. This doesn't reduce the concrete needed but dramatically increases crack resistance. Add 1 inch to your depth if pouring over compacted gravel base (always recommended).
Subgrade matters: Pour concrete on a compacted, uniform subgrade. Soft spots or tree roots beneath the slab are the #1 cause of cracking — not the concrete itself. Remove organic material, compact fill dirt or gravel, and ensure consistent depth throughout the pour area.
The decision between bagged concrete and ready-mix delivery depends on project size, access, and cost. Here's how to choose:
| Factor | Bagged Concrete | Ready-Mix Delivery |
|---|---|---|
| Best for | Under 1 cubic yard (under ~45 bags) | Over 1 cubic yard |
| Cost per yard | $150–$250 for materials + labor | $125–$200 delivered (minimum charge applies) |
| Minimum order | 1 bag | Typically 1 cubic yard minimum |
| Flexibility | Mix as needed, work at your pace | Must be placed within 90 minutes |
| Equipment needed | Wheelbarrow, mixer or hoe | Pump truck or direct chute access |
| Physical labor | High (each 80 lb bag) | Low (shovel to spread) |
| Quality consistency | Good if mixed properly | Excellent (plant-controlled) |
Bag sizes and yields:
Pro tip: Order 10% more than your calculation. Concrete can't be sent back, and running short mid-pour is a project-ruining problem. Leftover concrete can be used for small pads, post holes, or stepping stones.
Concrete strength is measured in PSI (pounds per square inch) — the compressive strength at 28 days after pouring. Choosing the right PSI for your project prevents both over-engineering (waste) and under-engineering (failure).
Most bagged concrete mixes sold at home improvement stores are 4,000–5,000 PSI when properly mixed and cured — higher than the minimum for most residential applications. The bag PSI rating assumes proper water-to-cement ratio. Adding extra water weakens concrete significantly: excess water increases workability but creates voids as it evaporates, reducing strength by up to 30%.
Water ratio is everything: Follow the bag instructions exactly. Typical water-to-cement ratio is 0.45–0.50 (about 3 quarts per 80 lb bag). Mix until no dry powder remains and the consistency is like thick oatmeal — not soup. A common DIY mistake is adding too much water to make mixing easier; this creates weak, porous concrete that cracks early.
Beyond flat slabs, concrete is used in many forms. Here's how to calculate volume for the most common project types:
Cylindrical footings (deck posts, fence posts): Volume = π × r² × depth. A 12-inch diameter hole (r = 0.5 ft) at 3 ft deep = 3.14159 × 0.25 × 3 = 2.36 cubic feet = 0.087 cubic yards. Plan on 4–5 bags of 80 lb concrete per post footing. Most deck builders use tube forms (Sonotubes) to keep the circular shape while pouring.
Steps and stairs: Calculate each step as a rectangle: depth × height × width. A typical outdoor step is 11 inches deep × 7 inches high × 4 feet wide = 0.917 ft × 0.583 ft × 4 ft = 2.14 cubic feet per step. Three steps = 6.42 cubic feet = 0.24 cubic yards + 10% = 0.26 cubic yards.
Wall footings: Length × width × depth. A continuous footing for a 40-ft wall, 16 inches wide, 8 inches deep = 40 × 1.33 × 0.67 = 35.7 cubic feet = 1.32 cubic yards. Add 15% for footings due to irregular subgrade.
Concrete curbing: Profile area × length. A typical curb cross-section is 6 in × 6 in (0.25 sq ft). At 100 linear feet: 0.25 × 100 = 25 cubic feet = 0.93 cubic yards. Curbing has complex profiles; always add 20% for decorative curb projects.
Always draw your project to scale before calculating. Breaking complex shapes into simple rectangles and cylinders makes calculations manageable. For irregular shapes, divide into sections and sum the volumes.
Pouring concrete correctly is only half the job. Proper curing is equally important for achieving design strength. Concrete doesn't "dry" — it cures through a chemical hydration process. Rushing or disrupting curing leads to weak, cracked concrete.
How to cure properly: Keep the slab moist for 7 days minimum. Methods include covering with wet burlap and plastic sheeting, using curing compound spray (applied immediately after finishing), or regular water misting every 4–6 hours. Don't let the surface dry out — it weakens the surface layer and causes surface cracking (crazing).
Temperature extremes: Below 50°F, hydration slows dramatically. Below 40°F, it nearly stops. Above 90°F, concrete loses moisture too fast. For hot-weather pours: wet the subgrade before pouring, add ice to mixing water, and cover immediately with wet burlap. For cold-weather pours: use hot water, avoid frozen aggregates, and insulate with blankets for at least 3 days.
A 10 ft × 10 ft slab at 4 inches thick requires 1.23 cubic yards of concrete. With 10% waste factor, order 1.35 yards. This equals approximately 56 bags of 80 lb ready-mix concrete if mixing by hand.
It depends on bag size: 45 bags of 80 lb concrete make 1 cubic yard (each 80 lb bag yields 0.60 cu ft; 27 ÷ 0.60 = 45). For 60 lb bags: 60 bags per yard. For 40 lb bags: 90 bags per yard. Ready-mix delivery is far more practical for anything over 1 yard.
Residential driveways should be a minimum of 4 inches thick, but 5–6 inches is recommended for regular passenger vehicle use. Areas where trucks, RVs, or heavy equipment will park should be 6–8 inches. Edges of driveways are often thickened to 8–12 inches for added strength.
You can, but it's not recommended. Compacted gravel base (4–6 inches of #57 gravel or crushed stone) provides drainage, prevents frost heave, and creates a stable subgrade. Pouring on undisturbed, well-compacted native soil is acceptable for small projects, but organic material (topsoil, clay) must be removed first.
Concrete reaches full design strength at 28 days. You can walk on it after 24–48 hours, but drive on it only after 7 days (and even then, at your own risk). For full load-bearing applications (parking heavy vehicles, placing heavy equipment), wait the full 28 days.