Ordering concrete is one of the most stressful parts of any construction project. Too little means a cold joint and a weak slab. Too much means wasted money and disposal fees. Here’s how to get it right.
Why Accurate Concrete Estimation Matters
Concrete is typically sold by the cubic yard (in the US) or cubic meter (elsewhere). A single cubic yard of concrete weighs about 4,000 pounds, so you can’t just eyeball it. Accurate estimation:
- Prevents costly over-ordering
- Avoids structural weak points from cold joints
- Saves trips to the ready-mix plant
- Keeps your project on schedule
How to Calculate Concrete Volume
The basic formula is simple: length × width × height (or depth). Our concrete calculator handles the math instantly, but here’s how it works:
For a Slab
Measure the length, width, and thickness in feet:
Volume (cubic feet) = Length × Width × Thickness
Then convert to cubic yards: divide by 27.
Example: A 10 ft × 12 ft slab that’s 4 inches thick (0.33 ft): 10 × 12 × 0.33 = 39.6 cubic feet ÷ 27 = 1.47 cubic yards
For a Column
Use the cylinder formula:
Volume = π × r² × height
Where r is the radius (half the diameter).
For a Footing
Footings are typically rectangular or trapezoidal. For rectangular footings, use the same length × width × depth formula as a slab.
How Many Concrete Bags?
If you’re mixing your own concrete with bagged mix, here’s what you need:
| Bag Size | Cubic Feet Per Bag | Bags Per Cubic Yard |
|---|---|---|
| 40 lb (18 kg) | 0.30 | 90 |
| 60 lb (27 kg) | 0.45 | 60 |
| 80 lb (36 kg) | 0.60 | 45 |
Our concrete calculator automatically converts between cubic yards, cubic feet, and bags so you know exactly what to buy.
Common Concrete Thicknesses
| Project | Typical Thickness |
|---|---|
| Sidewalk | 4 inches |
| Patio | 4 inches |
| Driveway | 5-6 inches |
| Garage floor | 4 inches |
| Foundation wall | 8-12 inches |
| Column footing | 12-24 inches |
Tips for Ordering Concrete
- Always order 10% extra — to account for spillage, uneven subgrade, and slight depth variations
- Consider reinforcing — Use our rebar calculator to plan reinforcement
- Check weather — Don’t pour concrete in rain or extreme heat
- Have helpers ready — Concrete sets in 30-60 minutes; you need to work fast
- Order at the right time — Ready-mix trucks charge for waiting time
Summary
Getting your concrete estimate right saves money, time, and headaches. Use our free concrete calculator to get accurate yardage, bag counts, and cost estimates for any project.