Ordering the right amount of concrete is essential for any project — too little means a disrupted pour and cold joints, too much means wasted money. This guide shows you how to calculate concrete for the most common project types.
The Basic Formula
Volume = Length × Width × Depth (all in the same unit)
For concrete, the standard unit is cubic yards (in the US) or cubic meters.
Cubic Yards = (Length ft × Width ft × Depth ft) ÷ 27
For instant calculations, use our Concrete Calculator.
Calculating by Project Type
Slabs (Driveways, Patios, Garage Floors) Slabs are the simplest calculation: 1. Measure length and width in feet 2. Determine thickness (typically 4" for patios, 6" for driveways) 3. Convert thickness to feet (4" = 0.333 ft) 4. Multiply: L × W × Depth ÷ 27
Example: 20' × 10' patio at 4" thick = 20 × 10 × 0.333 ÷ 27 = 2.47 cubic yards
Footings Footings are typically rectangular or trapezoidal in cross-section: 1. Calculate the cross-sectional area 2. Multiply by the total linear feet of footing 3. Divide by 27
Example: 12" wide × 6" deep footing running 120 linear feet = 1 × 0.5 × 120 ÷ 27 = 2.22 cubic yards
Columns and Posts For round columns, use the cylinder formula: **Volume = π × r² × h**
Example: 12" diameter × 4' tall column = 3.14159 × 0.5² × 4 ÷ 27 = 0.116 cubic yards per column
Stairs Calculate each step as a rectangular section and sum them: Each step = Width × Tread Depth × Rise Height Or use our [Stair Calculator](/construction/stair-calculator/) for precise dimensions.
How Many Bags Do You Need?
If you are using pre-mixed bags instead of ready-mix trucks:
| Bag Size | Yields |
|---|---|
| 40 lb bag | 0.011 cubic yards |
| 60 lb bag | 0.017 cubic yards |
| 80 lb bag | 0.022 cubic yards |
Bags needed = Total cubic yards ÷ Yield per bag
Always Add a Waste Factor
Always order 10% more than your calculated amount to account for: - Uneven subgrade - Spillage - Forms that are slightly off - Finishing waste
For irregular shapes, add 15% extra.
Tips for Ordering
- Ready-mix trucks deliver concrete in cubic yards (minimum order is usually 1 yard)
- Short load fees apply for orders under 5-10 yards
- Concrete sets quickly — have enough help to pour and finish before it starts hardening
- Check the weather — avoid pouring in extreme heat, cold, or rain
Calculate your exact needs instantly with our Concrete Calculator, which handles slabs, footings, columns, and provides both cubic yard and bag estimates.