Bearing Pressure Calculator for Footings
Calculate the actual bearing pressure a spread or continuous footing applies to soil, including eccentric loading with the kern (middle-third) check. Compare your result against an allowable pressure from a geotechnical report or an IBC 2024 Table 1806.2 presumptive value. For ultimate soil capacity from friction angle and cohesion, use the soil bearing capacity calculator instead.
📑 Bearing Pressure Calculator
Concentric & Eccentric Loading | Kern Check | IBC Presumptive Values
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View Chart →Find Your Presumptive Bearing Value
Look up the IBC 2024 Table 1806.2 value for your soil or rock class. Use these only when a geotechnical investigation has not been performed and your building official approves the presumptive value for your project.
| Material Class | Allowable Bearing (psf) | Lateral Bearing (psf/ft) |
|---|---|---|
| Crystalline bedrock | 12,000 | 1,200 |
| Sedimentary or foliated bedrock | 4,000 | 400 |
| Sandy gravel or gravel | 3,000 | 200 |
| Sand, silty sand, clayey sand | 2,000 | 150 |
| Clay, sandy clay, silty clay, silt | 1,500 | 100 |
Source: IBC 2024, Table 1806.2. Not applicable to mud, organic silt, organic clay, peat, or undocumented fill per §1806.2.
Calculation Method
Compute Base Area
Footing area A = L x B for a spread footing, or width B per linear foot for a continuous wall footing.
Check Eccentricity
Eccentricity e = M / P. Compare against the kern limit L/6 to determine if the base stays fully in contact with the soil.
Solve Pressure Distribution
If e is within the kern, use q = P/A ± M/S. If e exceeds L/6, use the reduced effective-area method for partial contact.
Compare to Allowable
Check maximum pressure against your allowable bearing value. Pressure below the allowable value passes; above it fails.
Applied Pressure Versus Soil Capacity
Bearing pressure and bearing capacity solve two different questions. Bearing pressure asks how hard your specific footing pushes on the soil beneath it, based on the load and the footing's own dimensions. Bearing capacity asks how much pressure the soil itself can take before it fails, based on soil strength parameters like friction angle and cohesion.
A safe foundation requires bearing pressure to stay at or below the allowable bearing capacity, which is the ultimate bearing capacity divided by a factor of safety, typically 2.5 to 3.0 in geotechnical practice. This calculator handles the pressure side of that comparison. For the capacity side, derived from Terzaghi-type equations using friction angle, cohesion, and unit weight, use the soil bearing capacity calculator.
Why Eccentric Loads Change the Picture
A perfectly centered column load produces uniform pressure across the entire footing base. Real footings often carry some moment, from an off-center column, an eccentric wall load, or overturning from wind or seismic force. That moment shifts the pressure distribution: one edge of the footing sees higher pressure, the opposite edge sees lower pressure, and if the eccentricity is large enough, one edge can theoretically go into tension.
Since soil cannot resist tension, engineers use the kern, or middle-third rule, to check whether the load stays within the zone that keeps the entire base in compression. Outside that zone, the design shifts to an effective-area method that assumes partial contact.
💡 Tip - Check Load Combinations Separately
A footing might pass a concentric gravity-only check but fail an eccentric check that includes wind or seismic overturning. Run this calculator once for your governing gravity case and again for your governing lateral case; the higher resulting pressure controls the footing design.
Example: Same Footing, With and Without Eccentricity
Case 1: Concentric Load
6 ft x 5 ft footing, P = 40 kips, M = 0
q = P/A = 40,000 / 30 = 1,333 psf (uniform)
With no moment, pressure is identical across the entire base. This is the baseline case most preliminary sizing assumes.
Case 2: Same Footing, Moderate Eccentricity
Same footing, P = 40 kips, M = 20 kip-ft (about the 6 ft axis)
S = BL²/6 = 5(6)²/6 = 30. q_max = 1,333 + 20,000/30 = 2,000 psf. q_min = 667 psf.
Adding a moderate moment raised peak pressure 50% above the concentric case, while the far edge dropped well below it.
Case 3: Same Footing, Larger Eccentricity
Same footing, P = 40 kips, M = 60 kip-ft
Leff = 3(3 - 1.5) = 4.5 ft. q_max = 2(40,000)/(5 x 4.5) = 3,556 psf
Once eccentricity exceeds the kern, peak pressure jumps sharply since the load is now resisted over a smaller effective area. This is a common source of footing failures in preliminary hand calculations that skip the kern check.
Common Bearing Pressure Mistakes
⚠ Errors That Change the Result
- Skipping the kern check entirely: Applying q = P/A ± M/S when e exceeds L/6 produces a negative (tension) pressure on one edge, which is not physically possible in soil. The effective-area method must be used instead.
- Confusing ultimate and allowable bearing capacity: Ultimate bearing capacity from Terzaghi-type equations must be divided by a factor of safety, typically 2.5 to 3.0, before comparing against applied pressure. Comparing applied pressure directly to ultimate capacity is unconservative.
- Using factored (LRFD) loads for a service-level bearing check: Bearing pressure checks against allowable soil pressure are almost always performed at service (ASD) load level, not factored LRFD load level, since geotechnical allowable values already carry their own safety margin.
- Forgetting footing self-weight and soil surcharge: The column or wall load alone is not the total load on the soil. Footing concrete weight and any soil backfill above the footing add to the bearing pressure and are often several percent of the total.
- Applying presumptive values without verification: IBC 2024 Table 1806.2 presumptive values require building official approval and do not apply to organic soil, peat, or undocumented fill. Using them without confirming soil class can significantly overstate actual capacity.
Using This Check for Foundation Design and Permits
Building departments reviewing foundation permits typically want to see the bearing pressure calculation compared directly against either a geotechnical report's allowable value or an approved presumptive value. Once your footing passes this check, related sizing continues with the concrete footing calculator for concrete volume and the foundation depth calculator for frost depth requirements.
For pressures that fail this check, common solutions are increasing footing width or length to reduce pressure, adding a footing thickness increase paired with more reinforcement, or combining adjacent footings. If settlement rather than bearing capacity governs, the foundation settlement calculator addresses that separate check. All final foundation sizing for permitted structural work must be verified by a licensed structural or geotechnical engineer per IBC 2024 Section 1604.
Frequently Asked Questions
For a concentric (axial-only) load, bearing pressure is q = P / A, where P is the total load and A is the footing base area. For an eccentric load with a moment, pressure varies across the base: q = P/A plus or minus M/S, where S is the section modulus of the footing base (S = BL squared / 6 for a rectangular footing).
Bearing pressure is the actual pressure a footing applies to the soil beneath it, calculated from the structural load and footing dimensions. Bearing capacity is the maximum pressure the soil can safely support before shear failure or excessive settlement, typically determined by a geotechnical investigation or Terzaghi-type bearing capacity equations. A safe design requires bearing pressure to stay below the allowable bearing capacity.
The kern, also called the middle third, is the central zone of a footing base where the resultant load can act while keeping the entire base in compression. For a rectangular footing of length L, the kern extends L/6 from the centroid in each direction. If the eccentricity e exceeds L/6, part of the footing base lifts off the soil, and the pressure distribution is no longer triangular across the full width.
When e is greater than L/6, the footing is assumed to lose contact with the soil over part of its base, since soil cannot resist tension. Design then uses an effective contact length, commonly Leff = 3 x (L/2 minus e), with the load resisted as a triangular pressure block over that reduced length. Maximum pressure in this condition is q_max = 2P / (B x Leff).
Per IBC 2024 Table 1806.2, presumptive allowable bearing values range from 1,500 psf for clay, sandy clay, and silty clay to 12,000 psf for crystalline bedrock, with sedimentary bedrock at 4,000 psf, sandy gravel or gravel at 3,000 psf, and sand or silty sand at 2,000 psf. These values apply only where a geotechnical investigation has not been performed and the building official approves their use.
IBC 2024 Section 1806.2 permits presumptive load-bearing values only when a geotechnical investigation has not been performed and the building official approves their use for the applicable soil class. Presumptive values do not apply to mud, organic silt, organic clay, peat, or undocumented fill. Many jurisdictions require an actual geotechnical report for anything beyond small, low-risk structures.
No. This calculator computes the bearing pressure your footing applies to the soil based on load and geometry you enter, and compares it to an allowable value you provide or a presumptive IBC value. Actual soil bearing capacity depends on site-specific soil conditions and must be determined by a geotechnical investigation or licensed engineer per IBC 2024 Section 1804.
Sources and Methodology
- IBC 2024, Table 1806.2 (Presumptive Load-Bearing Values) and Section 1806 (Presumptive Load-Bearing Values).
- IBC 2024, Section 1804 (Excavation, Grading and Fill) and Section 1604 (General Design Requirements).
- Eccentric footing pressure distribution (q = P/A ± M/S) and kern/middle-third rule: standard foundation engineering methodology, consistent with published spread footing design references.
- Partial-contact effective-area method (Leff = 3(L/2 - e)) for e > L/6: standard geotechnical/structural foundation design approach for footings with significant eccentricity.
- Terzaghi bearing capacity theory (referenced for context): Terzaghi, K. (1943), "Theoretical Soil Mechanics." Used in the companion soil bearing capacity calculator.
Last reviewed: September 2026. Reviewed by site author.
Disclaimer
This calculator provides estimates for planning purposes. For permitted structural work, foundations, multi-story construction, retaining walls over 4 feet, and commercial projects, calculations must be verified by a licensed structural engineer per IBC 2024 Section 1604. ConcreteCalculate.com is not liable for structural decisions made from these estimates.
Built by Muhammad Ramzan Babar, physics researcher (PhD candidate). Reviewed by site author.
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