Footing Calculator
A Strip Footing Is Measured in a Different Order Than a Slab
Unlike a slab, where length and width are both large and depth is comparatively thin, a continuous strip footing has two small cross-sectional dimensions — width and depth, both usually under two feet — running along a long total wall length. Getting the volume right means treating it as a long, narrow bar rather than a flat plate.
The Formula
Footing width and depth are entered in inches (or centimeters), since a footing's cross-section is typically sized in small units, while total wall length is entered in feet (or meters). The calculator converts width and depth to feet before multiplying by the total length to get cubic feet, divides by 27 for cubic yards, and applies a waste allowance — 10% by default — for trench irregularities and over-excavation.
Where This Calculation Matters
- Continuous perimeter footings — the footing running under a foundation wall's full perimeter is priced and poured as one continuous volume, even though it's really many linear feet of a small, repeated cross-section.
- Frost depth requirements — footing depth is often driven by the local frost line rather than structural load alone, which is why depth varies so much by region even for similar-sized structures.
- Trench pours vs. formed pours — footings poured directly into an excavated trench use the trench walls as the form, so the actual depth and width can vary more from the design figures than a formed pour would.
Worked Example
A 16 in wide, 8 in deep footing running 120 ft, with the default 10% waste allowance:
| Step | Calculation | Result |
|---|---|---|
| Volume | (16 ÷ 12) × (8 ÷ 12) × 120 | 106.67 cu ft |
| In cubic yards | 106.67 ÷ 27 | 3.95 cu yd |
| With 10% waste | 3.95 × 1.10 | 4.35 cu yd |
How to Use This Calculator
- Select Imperial (ft / in) or Metric (m / cm) units.
- Enter the Footing Width and Footing Depth.
- Enter the Total Wall Length the footing runs along.
- Optionally adjust the Waste % — defaults to 10%.
- Select Calculate to see the total concrete volume needed.
Related Calculations
Building the foundation wall on top of this footing? The Foundation Calculator covers the slab or wall above it, and the Rebar Calculator sizes the reinforcing steel that typically runs through both.