Our perfect concrete footing size calculator is designed to determine the appropriate dimensions of concrete footings using SI units (meters, kilonewtons, kilopascals) to ensure accurate calculations for safe and stable foundations.
- Load conversion: 1 kN ≈ 224.81 lbs: A 10 kN load would be approximately 2,248.1 lbs
- Soil bearing capacity: Often expressed in kilopascals (kPa): 100 kPa ≈ 2,088.54 psf (pounds per square foot)
- Dimension conversion: 1 meter = 3.28084 feet: A footing length of 2 meters would be about 6.56 feet
Concrete Footing Size Calculator
| Load (kN) | Soil Bearing Capacity (kPa) | Footing Length (m) | Footing Width (m) | Calculated Area (m²) | Bearing Pressure (kPa) | Result |
|---|---|---|---|---|---|---|
| 250 | 180 | 2.0 | 2.0 | 4.00 | 62.5 | Pass |
| 400 | 200 | 2.5 | 2.5 | 6.25 | 64.0 | Pass |
| 600 | 250 | 3.0 | 3.0 | 9.00 | 66.7 | Pass |
| 800 | 300 | 3.5 | 3.5 | 12.25 | 65.3 | Pass |
| 1000 | 350 | 4.0 | 4.0 | 16.00 | 62.5 | Pass |
Concrete Footing Sizes Chart
| Load (kN) | Soil Bearing Capacity (kPa) | Min. Length (m) | Min. Width (m) | Area (m²) |
|---|---|---|---|---|
| 100 | 150 | 1.0 | 0.8 | 0.8 |
| 200 | 200 | 1.2 | 1.0 | 1.2 |
| 300 | 250 | 1.4 | 1.2 | 1.68 |
| 400 | 300 | 1.6 | 1.4 | 2.24 |
| 500 | 350 | 1.8 | 1.6 | 2.88 |
Concrete Footing Size Formula
The formula for calculating concrete footing size involves two main steps:
Calculating the Footing Area (A):
A = L × WWhere:
- L = Length of the footing (in meters)
- W = Width of the footing (in meters)
Calculating the Bearing Pressure (P):
P = Load / AWhere:
- Load = Applied load on the footing (in kilonewtons, kN)
- A = Footing area (calculated as L × W)
Let’s consider a scenario where we have:
- Load = 200 kN
- Footing length (L) = 2 meters
- Footing width (W) = 2 meters
- Soil bearing capacity = 150 kPa
- Calculate the footing area: A = L × W = 2 m × 2 m = 4 m²
- Calculate the bearing pressure: P = Load / A = 200 kN / 4 m² = 50 kPa
- Compare the bearing pressure to the soil bearing capacity: 50 kPa < 150 kPa
The footing size is sufficient because the calculated bearing pressure (50 kPa) is less than the soil bearing capacity (150 kPa).
How to Calculate Concrete Footings?
Follow these systematic steps:
- Gather Input Data:
- Total structural load (kN)
- Soil bearing capacity (kPa)
- Site constraints
- Calculate Required Area: Required Area = Total Load ÷ Soil Bearing Capacity Example: Load = 300 kN Soil Capacity = 250 kPa Area = 300 ÷ 250 = 1.2 m²
- Determine Dimensions: For rectangular footing: Choose L and W where L × W ≥ Required Area Example: L = 1.2 m, W = 1.0 m 1.2 × 1.0 = 1.2 m² ✓
Suppose we have:
- Load = 300 kN
- Soil bearing capacity = 200 kPa
- Initial proposed dimensions: 2.5 m × 2.5 m
- Calculate area: A = 2.5 m × 2.5 m = 6.25 m²
- Calculate bearing pressure: P = 300 kN / 6.25 m² = 48 kPa
- Compare: 48 kPa < 200 kPa
The proposed footing size is adequate as the bearing pressure is well below the soil bearing capacity.
What is the rule of thumb for concrete footing size
The footing width should be approximately three times the width of the wall it supports.
Modern rule of thumb based on SI units:
- Minimum width = 0.6 meters
- Length-to-width ratio ≤ 2:1
- Depth = Width ÷ 3
For example……..
For 300 kN load:
Minimum width = 0.6 m
Length = 1.2 m (2:1 ratio)
Depth = 0.6 ÷ 3 = 0.2 mFor example, a 200 mm wide wall would require a 600 mm wide footing.
Consider a 250 mm wide wall:
Rule of thumb: 250 mm × 3 = 750 mm wide footing
Length: Typically matches the width for square footings, so 750 mm × 750 mm
Thickness: Generally 1/3 to 1/2 of the width, so between 250 mm to 375 mm
What is the ratio of concrete to footings?
A common ratio is:
1 : 2 : 3
This represents:
- 1 part cement
- 2 parts sand
- 3 parts gravel
For a footing measuring 1 m × 1 m × 0.3 m (0.3 cubic meters):
Calculate material quantities:
- Cement: 0.3 × 1 = 0.3 m³
- Sand: 0.3 × 2 = 0.6 m³
- Gravel: 0.3 × 3 = 0.9 m³
Convert to weight (approximate):
- Cement: 0.3 m³ × 1500 kg/m³ = 450 kg
- Sand: 0.6 m³ × 1600 kg/m³ = 960 kg
- Gravel: 0.9 m³ × 1700 kg/m³ = 1530 kg
This ratio ensures a strong, durable footing capable of supporting significant loads.
References
- American Concrete Institute. (2021). Building Code Requirements for Structural Concrete (ACI 318-19). https://www.concrete.org/store/productdetail.aspx?ItemID=318U19
- International Code Council. (2021). International Building Code. https://codes.iccsafe.org/content/IBC2021P1
- Concrete Network. (2022). Concrete Footing Fundamentals. https://www.concretenetwork.com/concrete/footing_fundamentals/
Related Tools:
