Concrete Footing Calculator
Estimate rectangular strip-footing quantities from dimensions.
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What this tool does
Estimate rectangular strip-footing quantities from dimensions. Bag yields are typical; verify the bag label. Reinforcement, bearing and foundation design are not included.
How to use the Concrete Footing Calculator
- Enter or select length in ft.
- Enter or select width in ft.
- Enter or select concrete thickness in in.
- Enter or select waste allowance (%).
- Enter or select bag weight.
- Read the calculated result; change any measurement to compare alternatives.
Formula
cu ft=length×width×depth/12×(1+waste/100); cu yd=cu ft/27; bags=ceil(cu ft/bag yield)
- length
- Length (ft, before metric conversion)
- width
- Width (ft, before metric conversion)
- depth
- Concrete thickness (in, before metric conversion)
- waste
- Waste allowance (%)
- bag
- Bag weight
Bag yields are typical; verify the bag label. Reinforcement, bearing and foundation design are not included.
Worked example
For concrete footing calculator, the following measurements illustrate the exact method: Length: 20; Width: 10; Concrete thickness: 4; Waste allowance (%): 10; Bag weight: 0.3.
Inputs
- Length20 ft
- Width10 ft
- Concrete thickness4 in
- Waste allowance (%)10
- Bag weight40 lb (0.3 cu ft yield)
Result
- Concrete volume2.72 cu yd
- Volume in cubic feet73.33 cu ft
- Bags245
Typical concrete bag yield
| Bag weight (lb) | Yield (cu ft) |
|---|---|
| 40 | 0.3 |
| 60 | 0.45 |
| 80 | 0.6 |
Results explained
- Concrete volume
- Concrete volume from the formula above, expressed in yd3 before metric conversion. Bag yields are typical; verify the bag label. Reinforcement, bearing and foundation design are not included.
- Volume in cubic feet
- Volume in cubic feet from the formula above, expressed in cuft before metric conversion. Bag yields are typical; verify the bag label. Reinforcement, bearing and foundation design are not included.
- Bags
- Bags from the formula above. Bag yields are typical; verify the bag label. Reinforcement, bearing and foundation design are not included.
Frequently asked questions
Multiply length and width by depth converted to feet, add waste, then divide by 27 or bag yield.
Use the concrete dimensions, not the excavation dimensions.
Use 5–10% for simple layouts and 10–15% for diagonal cuts or complicated outlines; the allowance is editable.
Split the outline into rectangles or triangles and calculate each section separately. Do not count overlapping sections twice.
This is a purchasing estimate, not a structural design. Confirm measurements and local requirements before buying or building.