Concrete is unforgiving to estimate by eye. Order too little and you get a cold joint where one pour meets the next; order too much and you have paid for a wheelbarrow of waste. The good news: the math is simple once you work in volume.
Always estimate in volume
Concrete is sold by volume, not area. Every estimate starts with three numbers — length, width, and thickness. Multiply them for cubic feet, then divide by 27 for cubic yards, the unit ready-mix trucks use. Our concrete calculator does this and also converts the result into 80-lb bags for small jobs.
The formula, step by step
Say you are pouring a 10 ft × 10 ft patio at 4 inches thick. Convert the thickness to feet (4 ÷ 12 = 0.33 ft), then multiply: 10 × 10 × 0.33 ≈ 33 cubic feet. Divide by 27 and you need about 1.2 cubic yards.
Bags or ready-mix?
An 80-lb bag yields roughly 0.6 cubic feet, so a cubic yard takes about 45 bags. Mixing 45 bags by hand is a brutal day. As a rule of thumb, anything over about one cubic yard is cheaper, faster, and stronger as ready-mix — and most suppliers have a one-yard minimum anyway.
How much extra to order
Add 5–10% to your calculated volume. Subgrades are never perfectly level, forms bow slightly, and some concrete is always lost to spillage. Running short mid-pour is far more expensive than a little leftover.
Common slab thicknesses
- Walkways and patios: 4 inches.
- Driveways and parking pads: 5–6 inches.
- Footings: follow local code and load — often 8 inches or more.
Get the volume right, round up, and you will pour with confidence. When you are ready, plug your numbers into the concrete calculator, and size the base underneath with the gravel calculator.