How many bags of concrete do I need?
Estimate bags for small slabs and pads, understand bag yield, and know when the calculator is not enough.
Concrete bag math is volume math. Length, width, and thickness tell you the cubic feet. The bag label tells you how much one bag yields.
1. Measure length and width
For a rectangular slab or pad, multiply length by width to get square feet.
2. Convert thickness to feet
Concrete thickness is often measured in inches. Divide inches by 12 before multiplying.
Example: 4 inches is 0.333 ft.
3. Calculate cubic feet
Length x width x thickness in feet = cubic feet of concrete.
4. Check bag yield
An 80 lb bag often yields about 0.60 cubic feet, but labels vary. Use the exact yield if you know the product.
5. Add overage
Small pours are stressful when you run short. A 5% to 10% overage is common for planning. For bigger or structural work, get better local guidance before ordering.
Running short during a pour is worse than having one extra bag. Concrete does not wait while you drive back to the store.