About the Footing Depth Calculator
What is the Footing Depth Calculator?
This calculator sizes an isolated footing and checks its footing depth. The area comes from the column load (plus self-weight) divided by the soil's safe bearing capacity.
The minimum depth uses Rankine's formula, which depends on the soil's unit weight and friction angle; IS 1080 requires at least 0.5 m.
How to use the Footing Depth Calculator
- Column Load (kN, unfactored) (sample: 600)
- Safe Bearing Capacity of Soil (kN/m²) (sample: 200)
- Soil Unit Weight (kN/m³) (sample: 18)
- Soil Friction Angle (°) (sample: 30)
- Footing & Backfill Self-Weight (% of load) (sample: 10)
- Press Calculate Footing Size & Depth (results also refresh as you type). The result card shows the Minimum Foundation Depth, along with Rankine depth, Footing area needed, Square footing side and Practical note.
Formula used
Minimum depth = (q ÷ γ) × ((1 − sin φ) ÷ (1 + sin φ))²
- q = safe bearing capacity (kN/m²)
- Footing area = load × (1 + self-weight %) ÷ q
For preliminary sizing only. Structural designs must be prepared and checked by a qualified structural engineer to IS 456, IS 800 and IS 875.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Column Load (kN, unfactored) | 600 |
| Safe Bearing Capacity of Soil (kN/m²) | 200 |
| Soil Unit Weight (kN/m³) | 18 |
| Soil Friction Angle (°) | 30 |
| Footing & Backfill Self-Weight (% of load) | 10 |
| Result | Value |
|---|---|
| Minimum Foundation Depth | 1.23 m |
| Rankine depth | 1.235 m |
| Footing area needed | 3.3 m² |
| Square footing side | 1.82 m |
| Practical note | IS 1080 requires at least 0.5 m; go below the topsoil, filled ground and seasonal moisture zone (often 1–1.5 m) |
Tips for using the Footing Depth Calculator
- Take footings below topsoil, filled ground and the zone of seasonal moisture change.
- Black cotton (expansive) soils need special foundations such as under-reamed piles.