By ToolzHive
About this tool
Calculate the concrete needed for slabs, footings, columns and steps, plus bags required.
How to use Concrete Calculator
- Choose units and the type of project.
- Enter the dimensions and how many you are pouring.
- Read the volume and the number of 40, 60, 80 lb or 25 kg bags.
Worked example
A 10 × 10 ft patio slab 4 in thick needs about 1.36 cubic yards with 10% waste, which is about 61 bags of 80 lb premix.
Concrete slab volume formula
Multiply length × width × depth in one consistent unit. Feet give cubic feet; divide by 27 for cubic yards. Convert an inch depth to feet by dividing by 12 before multiplying.
Example: a 20 ft × 10 ft slab, 4 inches deep
20 × 10 × (4 ÷ 12) = 66.67 cubic feet, or approximately 2.47 cubic yards. An illustrative 10% allowance brings that to approximately 2.72 cubic yards.
Bag counts depend on yield
Use the mixed-concrete yield printed on your chosen product. Divide the required volume by the yield per bag and round up. Bag weight alone does not determine coverage.
Quantity estimate, not structural design
This estimates material volume. It does not specify slab thickness, strength, reinforcement or suitability for a structure. Follow the project specification and verify the measurements before ordering.
Bags or ready-mix truck?
Bags suit small jobs under about 1 cubic yard. For larger pours, ready-mix is usually cheaper and faster.
How much extra should I order?
Add 5–10% for spillage, uneven ground and forms that bow.
Put this guide into practice
- Concrete Calculator — Calculate the concrete needed for slabs, footings, columns and steps, plus bags required.