Isolated Footing Size Calculator by Column Load
An isolated spread footing transfers a single column's load to the soil through a concrete pad wider than the column itself. Sizing it correctly prevents settlement, punching shear failure, and structural damage — while avoiding costly oversizing. The governing formula is straightforward: Required Area (m²) = Column Load (kN) ÷ Allowable Soil Bearing Capacity (kPa). From the area, the square side length is its square root. This calculator applies those two steps instantly and lets you cross-check the actual bearing pressure after rounding up the side.
Footing area (m²) = Column load (kN) ÷ Allowable soil bearing capacity (kPa). For a 200 kN column on 150 kPa soil: area = 1.33 m², giving a square footing of 1.20 × 1.20 m (calculated side 1.15 m, rounded up to the nearest 5 cm).
When to use this calculator
- Size a pad footing for a reinforced concrete column carrying 200 kN on medium-density sand
- Check whether existing footings can handle increased column loads from a building addition
- Estimate concrete volume and excavation quantities before bidding on a multi-footing project
- Learning or teaching foundation design — step-by-step numeric verification
Worked example: two-storey residential column
- Column load: 200 kN (two-storey structure, dead + live load)
- Allowable soil bearing capacity: 150 kPa (dense sand)
- Required area: A = 200 / 150 = 1.33 m²
- Square side: L = √1.33 = 1.155 m → round up to 1.20 m
- Verification: actual pressure = 200 / (1.20²) = 138.9 kPa < 150 kPa ✓
How it works
3 min readHow to Calculate Isolated Footing Size
The two-step sizing procedure follows standard geotechnical practice (ACI 318, Eurocode 7, AS 2159):
// Step 1 — Required area
A (m²) = P (kN) / q_allow (kPa)
// Step 2 — Square side length
L (m) = √A → round UP to nearest 5 cm
// Verification of actual pressure
q_actual (kPa) = P / (L × L) → must be ≤ q_allow
// Numeric example
P = 200 kN ; q_allow = 150 kPa
A = 200 / 150 = 1.333 m²
L = √1.333 = 1.155 m → round up to 1.20 m
q_actual = 200 / (1.20²) = 138.9 kPa ✓> Units note: 1 kPa = 1 kN/m², so the formula gives area directly in m². Loads given in kips must be converted (1 kip = 4.448 kN); bearing capacity given in ksf must be converted (1 ksf ≈ 47.88 kPa).
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Footing Size Reference Table
Square footing dimensions for q_allow = 150 kPa (typical dense sand / stiff clay):
| Column Load (kN) | Load (kips) | Required Area (m²) | Calculated Side (m) | Adopted Side (m) | Footing |
|---|---|---|---|---|---|
| 50 | 11.2 | 0.33 | 0.58 | 0.60 | 0.60 × 0.60 |
| 100 | 22.5 | 0.67 | 0.82 | 0.85 | 0.85 × 0.85 |
| 150 | 33.7 | 1.00 | 1.00 | 1.00 | 1.00 × 1.00 |
| 200 | 45.0 | 1.33 | 1.15 | 1.20 | 1.20 × 1.20 |
| 300 | 67.4 | 2.00 | 1.41 | 1.45 | 1.45 × 1.45 |
| 400 | 89.9 | 2.67 | 1.63 | 1.65 | 1.65 × 1.65 |
| 500 | 112.4 | 3.33 | 1.83 | 1.85 | 1.85 × 1.85 |
| 600 | 134.9 | 4.00 | 2.00 | 2.00 | 2.00 × 2.00 |
| 800 | 179.8 | 5.33 | 2.31 | 2.35 | 2.35 × 2.35 |
| 1000 | 224.8 | 6.67 | 2.58 | 2.60 | 2.60 × 2.60 |
> For other bearing capacity values, use the calculator above.
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Typical Allowable Bearing Capacity Values
| Soil Type | q_allow (kPa) | q_allow (tsf) | Notes |
|---|---|---|---|
| Solid bedrock | 600 – 3,000 | 6.3 – 31.4 | Verify with site investigation |
| Dense gravel | 300 – 600 | 3.1 – 6.3 | Very favorable |
| Dense sand | 150 – 300 | 1.6 – 3.1 | Common in many regions |
| Loose sand | 50 – 150 | 0.5 – 1.6 | Varies significantly with moisture |
| Stiff clay | 100 – 200 | 1.0 – 2.1 | Moisture-sensitive |
| Soft clay | 25 – 100 | 0.3 – 1.0 | Require SPT or CPT data |
| Organic soil / fill | < 25 | < 0.3 | Not suitable without improvement |
> ⚠️ These are reference values only. A geotechnical investigation report is required for any permitted construction.
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Three Design Scenarios
Scenario 1 — Light residential (soft soil)
Scenario 2 — Industrial column (dense sand)
Scenario 3 — Multi-storey on weak soil
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Common Sizing Mistakes
1. Omitting footing self-weight. The load on soil includes the column load plus the concrete pad and backfill above it. Add 5–10% of column load as a quick estimate.
2. Not rounding up. If the calculated side is 1.13 m and you build 1.10 m, actual pressure exceeds the allowable. Always round up.
3. Using table values without site verification. Soil bearing capacity varies enormously — even within the same site. Always obtain a geotechnical report for permitted work.
4. Ignoring the water table. If the water table is within B/2 of the footing base (B = footing width), granular soil bearing capacity can drop by up to 50%.
Frequently asked questions
What is the formula for sizing an isolated footing?
A = P / q_allow, where A is the required footing area (m²), P is the column load (kN), and q_allow is the allowable soil bearing capacity (kPa). For a square footing, side length L = √A. Example: 200 kN ÷ 150 kPa = 1.33 m², square side 1.15 m, rounded up to 1.20 m.
What is a typical allowable soil bearing capacity?
Typical values: dense sand/gravel: 150–300 kPa, stiff clay: 100–200 kPa, soft clay: 25–100 kPa, rock: 600+ kPa. These are reference values. The actual value must come from a geotechnical investigation report for your specific site.
Should I round up the calculated footing size?
Yes — always round the side length up to the nearest 5 cm (metric) or 1 inch (imperial). If the calculation gives 1.15 m and you build 1.10 m, the actual soil pressure exceeds the allowable. After rounding, verify: actual pressure = P / L² must still be ≤ q_allow.
How deep should an isolated footing be placed?
At a minimum, below the frost line in your region, plus enough depth to reach competent soil. Typical minimums: 0.5 m in frost-free climates, 0.8–1.2 m in cold climates. In expansive clays, footings may need to go 1.0–1.5 m deep to escape the zone of seasonal moisture variation.
How do I convert column load from tons to kN?
Use 1 metric ton (tonne) = 9.807 kN (often rounded to 10 kN for quick estimates). For US short tons: 1 short ton = 8.896 kN. For kips: 1 kip = 4.448 kN. For bearing capacity: 1 ton/ft² (tsf) = 95.76 kPa; 1 kip/ft² (ksf) = 47.88 kPa.
How much concrete does a 1.20 × 1.20 m footing need?
Multiply length × width × depth. A 1.20 × 1.20 × 0.30 m footing requires 0.43 m³ of concrete. For a typical f'c = 21 MPa (3000 psi) mix, that's roughly 1,030 kg of concrete — not including formwork or rebar.
How does the water table affect footing design?
When the water table rises within B/2 below the footing base (B = footing width), effective stress in granular soils drops and bearing capacity can be reduced by up to 50%. Always obtain the seasonal high water table depth from your site investigation report and use the corrected bearing capacity in the formula.
What is the difference between an isolated footing and a strip footing?
An isolated (pad) footing supports a single column; it is square or rectangular in plan. A strip (continuous) footing runs beneath a load-bearing wall or a row of closely spaced columns. Strip footings distribute load over a longer linear area; pad footings are more efficient when columns are widely spaced.
Can I use this calculator for rectangular footings?
Yes. The calculator gives you the required area (A = P / q_allow). For a rectangular footing, pick dimensions L × B such that L × B ≥ A. For example, if A = 2.00 m² and space limits the width to 1.00 m, the required length is 2.00 / 1.00 = 2.00 m. The square solution is the most material-efficient.
Is a geotechnical report required for a building permit?
In most jurisdictions, yes — for any structure with a foundation. A licensed geotechnical engineer must characterize the soil and specify allowable bearing capacity. Using table values without a site-specific report is technically risky and may constitute professional liability for the engineer of record.