Pad Footing Bearing Pressure Calculator
Calculates the applied (serviceability) bearing pressure a rectangular pad footing imposes on the ground under axial load plus moment. Structural and geotechnical engineers use it to check that the maximum ground pressure stays below the allowable bearing capacity and that the footing does not lift off.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the axial load P (kN) and the footing plan length L (m) and width B (m).
- Enter an applied moment M (kN·m) if the load is eccentric (leave blank for a concentric load).
- Read q_max and q_min, the eccentricity e, and whether the resultant stays within the middle third.
How it works
The average pressure is P/(B·L) and the load eccentricity is e = M/P. While e is within the middle third (kern), e ≤ L/6, the whole base stays in compression and the pressure varies linearly: q = (P/(B·L))·(1 ± 6e/L). This is the standard combined axial-plus-bending stress distribution.
Once e exceeds L/6 the ground would have to carry tension, which it cannot, so contact becomes partial. From equilibrium of a triangular pressure block, q_max = 2P/[3·B·(L/2 − e)] with q_min = 0. Keeping e ≤ L/6 (no uplift) is normally required for spread footings.
Worked example
Eccentric pad footing. For P = 1000 kN, L = 3 m, B = 2 m, M = 200 kN·m: average = 1000/(2×3) = 166.7 kPa; e = 200/1000 = 0.2 m, which is ≤ L/6 = 0.5 m, so the base is fully in contact. q_max = 166.7×(1 + 6×0.2/3) = 233.3 kPa and q_min = 166.7×(1 − 0.4) = 100.0 kPa.
Common mistakes
- Confusing this applied pressure with bearing capacity — this is the demand (what the footing pushes down with), which must be compared against the allowable capacity.
- Using the linear (1 ± 6e/L) formula when e > L/6; beyond the kern the distribution is triangular over partial contact and q_min = 0.
- Forgetting to include the footing self-weight and any surcharge in P when checking gross pressure.
Frequently asked questions
What is the middle-third rule?
If the resultant vertical load acts within the middle third of the base (e ≤ L/6), the entire footing stays in compression with no uplift. This is the usual serviceability requirement for spread footings on soil.
What happens when e exceeds L/6?
The footing partially separates from the soil (the heel lifts). Bearing is then carried over a reduced length and q_max = 2P/[3·B·(L/2 − e)] with q_min = 0. Large eccentricity concentrates pressure and usually needs a bigger footing.
Is this an ultimate or serviceability check?
It gives the applied (serviceability) pressure. Compare q_max against the allowable bearing capacity — for example from a Terzaghi capacity check on the same soil.
Does moment about both axes matter?
This tool considers uniaxial bending about the length (L) direction. For biaxial moments the pressure distribution and the no-tension condition are more complex and need a two-way analysis.
Should I include the footing's own weight?
For the gross bearing pressure, yes — add the footing and backfill weight to P. For net checks you subtract the overburden that was there before construction.
Related tools
- Bearing Capacity (Terzaghi) Calculator
- Pile Capacity Calculator (end bearing + skin friction)
- Settlement (Consolidation) Calculator
- Soil Specific Gravity Calculator
- Permeability from Falling Head Test Calculator
- Shear Box Test Strength Parameters Calculator
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Tip: Enter any known values to calculate the remaining results.
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