Footing Bearing Pressure
Enter the vertical load and footing plan dimensions to find the average soil bearing pressure under the footing, and compare it against the allowable bearing pressure to see if the design is OK.
Check the Ground Pressure Under a Loaded Footing
Footing Bearing Pressure
Where:
- \(q\) — average soil bearing pressure under the footing (kPa, ksf, psi)
- \(P\) — total vertical load at the footing base, including self-weight and surcharge (kN, kip, lb)
- \(L\) — footing length, the longer plan dimension (m, ft)
- \(B\) — footing width, the shorter plan dimension (m, ft)
- \(A = L \times B\) — plan contact area between footing and soil
The formula divides the total vertical load by the plan area of the footing, giving the uniform pressure the footing transmits to the soil. It assumes the load acts through the centroid, so the pressure is evenly distributed.
Footing Bearing Pressure – Calculation Example
{example}
Example: a footing measuring 2.4 m × 1.8 m carries a total vertical load of 300 kN at its base. The allowable soil bearing pressure is 150 kPa.
\[ A = L \times B = 2.4 \times 1.8 = 4.32\ \text{m}^2 \]
\[ q = \frac{P}{A} = \frac{300}{4.32} = 69.44\ \text{kPa} \]
\[ \eta = \frac{q}{q_a} = \frac{69.44}{150} = 0.463 = 46.3\% \]
Result: the average bearing pressure is 69.44 kPa (≈ 1.45 ksf), which is only 46.3% of the 150 kPa allowable value — the check passes with a safety margin of about 2.16×.
{/example}
How to Use the Calculator
- Enter the Vertical load (P) — the total service load at the footing base, including the footing self-weight and any soil surcharge — and pick its unit (kN, kip or lb).
- Enter the Footing length (L), the longer plan dimension, and choose its unit (m, cm, ft or in).
- Enter the Footing width (B), the shorter plan dimension, with the same unit options.
- Optionally enter the Allowable bearing pressure (qₐ) in kPa, ksf, psi or MPa. Leave it empty or enter 0 if you only need the pressure value without a comparison.
- Read the results: the Average soil bearing pressure (shown in your chosen pressure unit plus a converted secondary unit) and, if an allowable value was given, the Comparison with allowable pressure with a utilization percentage and OK / EXCEEDS verdict.
What Is Bearing Pressure and Why It Matters
Bearing pressure is the contact pressure the base of a footing exerts on the soil beneath it. Every foundation, from a small pad footing under a column to a large strip footing under a wall, ultimately works by spreading the structural load over enough soil area that the ground can carry it without excessive settlement or shear failure. The average bearing pressure — total load divided by contact area — is the first and most fundamental number a designer checks.
The check itself is simple: the average pressure must not exceed the allowable bearing pressure, a value determined from geotechnical investigation, building codes, or conservative presumptive values for common soil types. Typical allowable values range from about 50–75 kPa for soft clays and loose sands up to 300 kPa or more for compact gravel, dense sand or weathered rock. If the calculated pressure exceeds the allowable value, the footing must be enlarged, the load reduced, or the soil improved (for example by compaction or replacement).
Interpreting the utilization ratio is equally important. A result of 46%, as in the example above, is comfortable but leaves room to shrink the footing if economy matters; a result above 100% fails the check outright. Many designers also aim to stay well below the allowable value to limit long-term settlement, since settlement often governs long before outright bearing failure does.
Assumptions, Limitations and Practical Tips
- Concentric load only. The calculator assumes the resultant load passes through the footing centroid, giving uniform pressure. For an eccentric load with eccentricity \(e\) measured from the centre, the maximum edge pressure becomes \(q_{max} = \frac{P}{A}(1 + \frac{6e}{B})\) and should also stay below the allowable value. Check that \(q_{min} = \frac{P}{A}(1 - \frac{6e}{B}) \geq 0\), otherwise part of the footing lifts off the soil.
- Gross vs. net pressure. Decide whether your load P includes the footing self-weight and the weight of soil above it. A gross pressure check includes these; a net check excludes the soil already present before excavation. Make sure P and the allowable value are consistent — both gross or both net.
- Service loads, not factored loads. Bearing pressure checks are normally performed with service (unfactored) loads against an allowable pressure. Factored loads belong to structural strength checks of the concrete, not the soil check.
- Verify the allowable value. Prescriptive allowable pressures from codes are conservative starting points; for real projects, use values from a geotechnical report that reflect the actual soil conditions, water table and settlement criteria.
- Horizontal loads and moments are not covered. This tool checks vertical equilibrium only. Wind uplift, overturning moments and sliding resistance require separate verification.
Once the footing size passes the bearing check, the structural design begins: sizing the concrete section for bending and punching shear and determining the reinforcement. To estimate how much steel the footing needs, try the Rebar Weight calculator, and to check bending stresses in the concrete section itself, the Section Modulus calculator is a useful companion. Load effects on the structure above can be checked with the Uniformly Loaded Simply Supported Beam calculator.
Frequently Asked Questions
How do I calculate footing bearing pressure by hand?
Multiply the footing length by its width to get the contact area, then divide the total vertical load by that area. For example, a 500 kN load on a 2 m × 2 m footing gives \(q = 500 / 4 = 125\ \text{kPa}\). Compare this with the allowable bearing pressure from your geotechnical report or code presumptive values. If the result exceeds the allowable value, increase the footing area or reduce the load.
What is a good soil bearing pressure value?
It depends entirely on the soil. Soft clay or loose sand may only allow 50–75 kPa, while firm clay and medium sand typically allow 100–200 kPa. Compact gravel, dense sand or weathered rock can support 300 kPa or more. As a rule of thumb, many designers keep the calculated pressure at roughly 50–75% of the allowable value to limit long-term settlement. Always confirm the allowable value with a geotechnical investigation for real projects.
How do I convert between kPa, ksf and psi?
1 ksf ≈ 47.88 kPa, and 1 psi ≈ 6.895 kPa. So a pressure of 100 kPa is about 2.09 ksf or 14.5 psi. In reverse, 1 kPa ≈ 0.02089 ksf ≈ 0.145 psi. Be careful mixing unit systems: a load in kips with dimensions in feet naturally gives pressure in ksf, while kN and metres give kPa. Keeping one system consistent throughout avoids most conversion errors.
What is the difference between bearing pressure and bearing capacity?
Bearing pressure is the demand — the actual contact stress the footing imposes on the soil, calculated as load divided by area. Bearing capacity is the capacity — the maximum pressure the soil can sustain before shear failure, usually divided by a safety factor (typically 2–3) to obtain the allowable bearing pressure used in design. The design check simply verifies that demand stays below this allowable capacity.
Can I use this calculator for a circular footing?
Yes, with an adjustment. Convert the circular footing to an equivalent rectangle: the area of a circle with diameter \(D\) is \(A = \pi D^2/4\), so enter \(L = B = D\sqrt{\pi}/2 \approx 0.886\,D\) to match the contact area exactly. Alternatively, compute the area by hand and divide the load by it — the average pressure result is identical for any footing shape with a concentric load.
What are the most common mistakes in bearing pressure calculations?
The biggest ones: forgetting to include the footing self-weight and soil surcharge in the load when a gross pressure check is required; mixing unit systems (kN with feet, or kip with metres); using factored loads instead of service loads; and ignoring load eccentricity, which raises the peak edge pressure well above the average. Also make sure the load and the allowable pressure are consistent — both gross or both net.
What utilization ratio should I aim for?
Anything at or below 100% passes the basic check, but a comfortable target is 60–80% of the allowable pressure. This leaves a buffer for load variations, minor eccentricities and long-term settlement, which often governs the design well before outright bearing failure. If your utilization is very low, say under 40%, the footing may be oversized and you could economize by shrinking its dimensions, subject to minimum-size and detailing rules in your code.