Estimate soil bearing pressure from the SPT N-value, footing width and depth with the Bowles settlement correlation, or find the N-value a pressure needs.
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SPT To Bearing Capacity Formula
The formula below applies to the Custom conversion factor modes. The settlement-based modes use the Bowles fifth-edition SI correlation for granular soils, with engineer-corrected representative N55 rather than an uncorrected field count.
Variables:
- BC is the pressure estimate under the selected custom calibration (kN/m²), not an ultimate capacity or validated safe load.
- SPT is the representative N-value on the same correction and calibration basis as the custom factor (blows/30cm).
- CF is the Conversion Factor (kN/m²/blow)
In the custom modes, multiply N by a project-calibrated factor to obtain a pressure estimate in kPa (kN/m²); the factor is not a universal soil constant. The Bowles SI modes instead use q = 20N55 Kd(S/25)W for B ≤ 1.2 m, or q = 12.5N55[(B + 0.3)/B]² Kd(S/25)W for B > 1.2 m, where Kd = min(1 + 0.33D/B, 1.33), B and D are in meters, and S is in millimeters. The default S is 25 mm. W is an additional project-specific reduction, default 1, not an automatic groundwater correction. The approximate branches are not smoothed at 1.2 m.
| SPT N (blows/30cm) | Conversion Factor (kPa/blow) | Pressure estimate (kPa) |
|---|---|---|
| 5 | 50 | 250 |
| 10 | 50 | 500 |
| 15 | 50 | 750 |
| 20 | 50 | 1000 |
| 25 | 50 | 1250 |
| 30 | 50 | 1500 |
| 35 | 50 | 1750 |
| 40 | 50 | 2000 |
| 50 | 50 | 2500 |
| 60 | 50 | 3000 |
| 5 | 100 | 500 |
| 10 | 100 | 1000 |
| 15 | 100 | 1500 |
| 20 | 100 | 2000 |
| 25 | 100 | 2500 |
| 30 | 100 | 3000 |
| 35 | 100 | 3500 |
| 40 | 100 | 4000 |
| 50 | 100 | 5000 |
| 60 | 100 | 6000 |
| * BC = N × CF. Units: 1 kPa = 1 kN/m². The CF values 50 and 100 kPa/blow are illustrative arithmetic, not recommended design factors. Use a project-specific factor calibrated to the same N basis and soil conditions. | ||
What is SPT To Bearing Capacity?
SPT correlations use penetration resistance as one input to a preliminary soil-pressure estimate. Results depend on soil type, field-test corrections, representative depth range and the selected empirical model. In the Bowles modes, N55, N60 and N70 choices convert only the energy basis; they do not automatically apply equipment or overburden normalization. An inverse result is an equivalent design N, not a prediction of a measured field test. Numerical N bands are arithmetic intervals, not soil consistency or relative-density diagnoses. A geotechnical review must independently check settlement, shear resistance, groundwater, pressure convention and governing codes.
How to Calculate SPT To Bearing Capacity?
The following steps describe the custom-factor calculation. For a settlement-based estimate, select the relevant Bowles solve mode and enter the displayed design N or pressure, footing geometry and permissible settlement:
- First, select the value to solve for and determine N on the calibration basis required for the project-specific factor.
- Next, determine the Conversion Factor (CF) (kN/m²/blow).
- Next, gather the formula from above = BC = SPT * CF.
- Select Calculate for the pressure estimate or inverse result; review its assumptions and limitations.
- After inserting the variables and calculating the result, check your answer with the calculator above.
Example Problem :
Use the following variables as an example problem to test your knowledge.
Standard Penetration Test value (SPT) (blows/30cm) = 10
Conversion Factor (CF) (kN/m²/blow) = 5. In this arithmetic example, BC = 10 × 5 = 50 kPa. This illustrative factor is not a recommended design value.
