Concrete Calculator
Estimate the volume of concrete needed for a slab or footing, including the number of 80-pound bags required.
About this Calculator
Estimate the volume of concrete needed for a slab or footing, including the number of 80-pound bags required.
Formula & Calculations
Formula
Volume = Length × Width × DepthWhere:
- V=Volume of concrete required
- l=Length of the slab in feet
- w=Width of the slab in feet
- d=Depth of the slab in inches
Assumptions
- One 80 lb bag of concrete yields approximately 0.6 cubic feet.
- Measurements assume a uniform depth across the slab.
- Does not account for waste or over-excavation; consider adding 5-10% extra.
Calculation Examples
Example 1
Inputs:Length: 10 ft, Width: 10 ft, Depth: 4 in
Result:33.33 cu ft (1.23 cu yd, 0.94 cu m), ~56 bags
Volume = 10 × 10 × (4/12) = 33.33 cubic feet. Bags needed = 33.33 / 0.6 ≈ 56.
Example 2
Inputs:Length: 20 ft, Width: 15 ft, Depth: 6 in
Result:150 cu ft (5.56 cu yd, 4.25 cu m), ~250 bags
Volume = 20 × 15 × 0.5 = 150 cubic feet.
Frequently Asked Questions
How many 80 lb bags of concrete make a yard?
One cubic yard equals 27 cubic feet. An 80 lb bag yields about 0.6 cubic feet, so you need approximately 45 bags per cubic yard.
Should I order extra concrete?
Yes, it is recommended to order 5-10% extra to account for spillage, uneven subgrade, and slight depth variations.