Whether you're pouring a patio, setting fence posts, or building a foundation, getting your concrete estimate right the first time saves money and prevents wasted trips to the hardware store. This guide walks through exactly how to calculate concrete for…
Whether you’re pouring a patio, setting fence posts, or building a foundation, getting your concrete estimate right the first time saves money and prevents wasted trips to the hardware store. This guide walks through exactly how to calculate concrete for the most common residential projects.
How Concrete Is Measured
Concrete is measured in cubic yards for large pours and in bags for smaller jobs. One cubic yard equals 27 cubic feet. An 80-pound bag of concrete yields approximately 0.60 cubic feet — so you need about 45 bags to fill one cubic yard.
For quick calculations, use our free concrete calculator to get instant results for slabs, footings, and post holes.
Calculating Concrete for a Slab
The formula is straightforward: Length × Width × Thickness = Volume. Convert your thickness to feet (divide inches by 12) before multiplying.
Example: A 10-foot × 12-foot patio at 4 inches thick:
10 × 12 × (4/12) = 10 × 12 × 0.333 = 40 cubic feet
40 ÷ 27 = 1.48 cubic yards
Always add 10% for waste: 1.48 × 1.10 = 1.63 cubic yards
Standard Concrete Slab Thicknesses
- 3–4 inches: Sidewalks, patios, and non-load-bearing slabs
- 4–6 inches: Driveways and areas with foot traffic
- 6+ inches: Heavy vehicle traffic, RV pads, and structural applications
Calculating Concrete for Post Holes
Post holes are cylinders. The formula is: π × radius² × depth = volume per hole.
Example: Setting 6 fence posts with 10-inch diameter holes at 30 inches deep:
π × (5/12)² × (30/12) = 3.14159 × 0.1736 × 2.5 = 1.363 cubic feet per hole
6 holes × 1.363 = 8.18 cubic feet total
8.18 ÷ 0.6 = 14 bags of 80 lb concrete (plus one extra for waste)
Tips to Avoid Running Short
Always order or buy 10–15% more than your calculation. Subgrades are rarely perfectly flat, forms may not be perfectly straight, and some concrete will be lost to spillage. Returning unused bags is easy; making an emergency hardware run when your pour is half-done is not.
Mixing Your Own vs. Ready-Mix
For jobs under 1 cubic yard, bagged concrete is convenient and cost-effective. For larger pours (over 1 yard), ready-mix delivered by a concrete truck is typically cheaper per yard and results in a more consistent mix. Ready-mix has a 90-minute working window after the truck departs, so have your forms ready before it arrives.
Concrete Curing Time
Concrete reaches 70% of its rated strength in 7 days and nearly full strength in 28 days. Protect fresh concrete from freezing temperatures (below 50°F) for at least 7 days. In hot weather, mist the surface with water to prevent rapid drying, which causes cracking.
Conclusion
Accurate concrete estimation prevents wasted money and mid-project supply runs. Use the slab formula for flat pours, the cylinder formula for post holes, and always add a 10% waste buffer. For complex projects with multiple shapes, calculate each section separately and add them together for your total order.