Ground Bearing Calculator — Load vs Allowable Pressure

Ground bearing capacity is the maximum pressure the soil can support without shear failure or excessive settlement. This calculator uses the BS 8004 Table 1 presumed allowable bearing values — soft clay/silt ≤75, firm clay 75-150, stiff clay 150-300, very stiff/hard clay 300-600, loose sand <100, medium-dense sand 100-300, dense sand >300 and dense gravel >600 kN/m². These are presumed NET allowable values: the factor of safety against shear failure is already built in, so you do NOT divide them by a further 2.5-3.0. They do not cover settlement, which must be checked separately, and a site-specific geotechnical investigation should confirm the figure.

Foundation pressure = Total Load / Foundation Area. For example, a wall load of 50 kN/m on a 600mm wide strip foundation applies 50 / 0.6 = 83 kN/m² pressure. This must be less than the allowable bearing capacity of the soil. If it exceeds the capacity, widen the foundation or use a different foundation type (raft, piles). See our Foundation Calculator for sizing, or Raft Foundation Calculator and Pile Foundation Calculator for alternatives.

There are two routes. From a soil test you compute the ultimate bearing capacity and divide by a factor of safety (typically 2.5-3.0) to get the allowable value. For preliminary design you instead use the BS 8004 presumed allowable values by soil description — these are already factored for shear failure, so you must NOT divide them again by 2.5-3.0. This calculator uses the BS 8004 presumed (net allowable) figures; the optional Additional Safety Margin is an extra margin on top, defaulting to 1.0 (no extra). The net value is converted to a gross allowable by adding the overburden above foundation level (soil unit weight × depth), then compared with the applied pressure. A geotechnical investigation (trial pits, boreholes) determines the actual bearing capacity for your specific site.

If the applied pressure exceeds the allowable bearing capacity, the foundation will settle excessively or the soil will fail, causing structural damage. Solutions include: widening the foundation, deepening to a stronger stratum, using a raft foundation to spread loads, or piling through weak soil to a bearing layer below.

Yes, for any significant building project. While presumed bearing values (from tables) can be used for simple domestic work, a ground investigation (trial pits or boreholes with soil testing) is essential for accurate design. The investigation identifies soil type, bearing capacity, groundwater level, sulphate content and any contamination.