How this calculator works
A strip footing is a long rectangular beam of concrete. The calculator converts width and depth from inches to feet, multiplies by the total length, adds waste and converts to cubic yards and bags.
Volume (cu ft) = Length (ft) × Width (in)/12 × Depth (in)/12 × (1 + Waste%/100)
Cubic yards = Volume ÷ 27
Assumptions and tips
- Bag yields follow the manufacturer’s published figures: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 50 lb ≈ 0.375 cu ft, 40 lb ≈ 0.30 cu ft.
- Bag counts are always rounded up to whole bags.
- The waste allowance covers spillage, uneven subgrade and over-excavation. 5–10% is common; use more for irregular forms.
- Footing size, depth below frost line and reinforcement are set by your plans and local building code — this tool only estimates material.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Total footing length | 40 ft |
| Footing width | 16 in |
| Footing depth (thickness) | 8 in |
| Waste allowance | 10 % |
| Bag size | 80 lb bag (0.60 cu ft) |
| Result | Value |
|---|---|
| Concrete needed | 1.45 cubic yards |
| 80 lb bags | 66 bags |
| Volume (with waste) | 39.1 cu ft |
| Concrete per foot of footing | 0.889 cu ft/ft |
Step by step:
- Cross-section = (16 ÷ 12) × (8 ÷ 12) = 0.889 sq ft
- Volume = 0.889 × 40 ft × 1.1 = 39.11 cu ft
- Cubic yards = 39.11 ÷ 27 = 1.449
- Bags = 39.11 ÷ 0.6 → 66
Concrete per 10 ft of footing
No waste added — multiply by your total footing length ÷ 10.
| Width × depth | Concrete needed | 80 lb bags |
|---|---|---|
| 12 × 6 in | 0.19 cubic yards | 9 bags |
| 16 × 8 in | 0.33 cubic yards | 15 bags |
| 20 × 10 in | 0.51 cubic yards | 24 bags |
| 24 × 12 in | 0.74 cubic yards | 34 bags |
Common mistakes to avoid
- Leaving out the short runs at corners and steps when adding up total length.
- Assuming the trench is exactly to size — trenches dug by machine are often wider.
Frequently asked questions
How deep should a footing be?
Footings must usually sit below the local frost line and on undisturbed soil. Your building department sets the minimum depth for your area.
Does the calculator include stepped footings?
Not directly. Calculate each section separately (or add the extra volume of the steps) and add the results together.