Concrete Bag Calculator
Quickly determine how many bags of concrete do I need for your slab or post project. Supports 40lb, 50lb, 60lb, and 80lb bags for accurate project planning.
Project Type: Slab / Pad
How this calculator works
The concrete bag calculator is designed to simplify the math for small-to-medium construction projects. Instead of dealing with complex formulas, you simply enter the dimensions of your slab, patio, or walkway.
- Volume Calculation: First, the tool calculates the total volume of your project by multiplying length × width × thickness.
- Unit Conversion: It converts your dimensions (feet, inches, meters) into a unified cubic volume (cubic feet or cubic meters).
- Yield Division: The total volume is divided by the specific yield of the chosen bag size (e.g., an 80lb bag yields approx 0.60 cubic feet).
- Rounding: The result is rounded up to the next whole number because you cannot buy partial bags.
This concrete mix calculator bags tool works perfectly for brands like Quikrete or Sakrete, which follow standard density ratios.
Concrete bag sizes explained
Understanding bag yield is critical when asking "how many bags of concrete do I need". Here is a breakdown of standard pre-mixed concrete sizes:
- 80 lb Bag: The most common size for larger jobs. Yields approximately 0.60 cubic feet. Best value for volume.
- 60 lb Bag: Yields approximately 0.45 cubic feet. Easier to handle than 80lb bags but requires more bags for the same volume.
- 50 lb Bag: Yields approximately 0.375 cubic feet. Often used for fast-setting mixes.
- 40 lb Bag: Yields approximately 0.30 cubic feet. Ideal for small repairs or setting a single post.
Examples
Here are a few real-world scenarios using our quikrete concrete calculator logic:
- Patio Slab (10x10 ft): For a 10ft x 10ft patio that is 4 inches thick, you need about 3.33 cubic yards of volume. Using 80lb bags, you would need approximately 56 bags.
- Walkway (3x20 ft): A sidewalk 3 feet wide, 20 feet long, and 4 inches thick requires about 34 bags of 80lb mix.
- Post Hole: A single fence post hole (approx 12 inches wide and 3 feet deep) typically takes 2 to 3 bags of 50lb fast-setting concrete.
If you are working on a massive project requiring a truck delivery, please check our main Concrete Calculator to estimate cubic yards directly.
? Frequently Asked Questions
One cubic yard of concrete is equal to 27 cubic feet. Since an 80lb bag yields 0.60 cubic feet, you need 45 bags to make one cubic yard.
Yes. We strongly recommend buying 5-10% more than the calculator output. Spillage, uneven ground, and form bowing can increase the required volume.
Absolutely. Standard pre-mix concrete density is roughly 145 lbs per cubic foot, which applies to Sakrete, Quikrete, and generic hardware store brands.
If you can lift them, 80lb bags usually offer the best price per cubic foot. However, 60lb bags are significantly easier to move and mix by hand.