How to Calculate Concrete for Your Slab
Planning a new driveway, patio, or shed base? Accurately calculating the amount of concrete needed is the most critical step in your project. Ordering too little can result in catastrophic "cold joints" where new concrete meets curing concrete, while ordering too much is a waste of budget and requires difficult disposal.
The Concrete Volume Formula
Concrete is measured by volume, typically in Cubic Yards for truck deliveries or Cubic Feet for bagged premix. The basic formula involves three dimensions:
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Since slab thickness is usually measured in inches, you must convert it to feet first by dividing by 12. For example, a 4-inch slab is 0.33 feet thick (4 ÷ 12).
Converting to Cubic Yards
If you are ordering a ready-mix truck, you will need to order in Cubic Yards. To get this number, divide your total Cubic Feet by 27 (since there are 27 cubic feet in one cubic yard).
How Many Bags of Concrete Do I Need?
For smaller DIY projects, you will likely buy premixed bags from a hardware store. Here is the typical yield for standard pre-mix concrete:
| Bag Size | Yield (Approx.) | Bags per Cubic Yard |
|---|---|---|
| 80 lb Bag | 0.60 cubic feet | 45 bags |
| 60 lb Bag | 0.45 cubic feet | 60 bags |
Why Include a Waste Margin?
Professional contractors always calculate a "margin of safety" or waste percentage. We recommend adding 5% to 10% to your total volume. This accounts for:
- Spillage during transport.
- Uneven subgrade (dips in the ground) requiring more material.
- Compression of the subbase.
- Slight bowing of wooden forms.
Use the calculator above to automatically include this safety margin in your order to ensure you don't run dry halfway through the pour.