Presumptive Bearing Values — Reference
Allowable foundation pressures by soil class per IBC Table 1806.2, for use where the code permits design without a site-specific geotechnical investigation. Confirm local amendments — several states modify this table.
IBC Table 1806.2 — Presumptive Load-Bearing Values
| Class of material | Vertical bearing (psf) | Lateral bearing (psf/ft depth) | Lateral sliding |
|---|---|---|---|
| 1. Crystalline bedrock | 12,000 | 1,200 | μ = 0.70 |
| 2. Sedimentary & foliated rock | 4,000 | 400 | μ = 0.35 |
| 3. Sandy gravel and/or gravel (GW, GP) | 3,000 | 200 | μ = 0.35 |
| 4. Sand, silty sand, clayey sand, silty gravel, clayey gravel (SW, SP, SM, SC, GM, GC) | 2,000 | 150 | μ = 0.25 |
| 5. Clay, sandy clay, silty clay, clayey silt, silt, sandy silt (CL, ML, MH, CH) | 1,500 | 100 | cohesion 130 psf |
Sliding: friction coefficient μ applies to dead load for classes 1–4; for class 5, cohesion times contact area, not to exceed one-half the dead load. Mud, organic silt, organic clays, peat, and unprepared fill have no presumptive capacity — investigation required (IBC 1806.2 footnotes).
Quick Screening Numbers
| Situation | Working number |
|---|---|
| Continuous wall footing, 1,500 psf soil, 8-ft wall | ≈ 16″–24″ wide |
| Column on 2,000 psf soil, 40 kip service load | ≈ 4.5 ft × 4.5 ft |
| Minimum footing depth (frost, typical mid-Atlantic/Southeast) | 12″–36″ per local code |
Screening only — size from actual loads with the bearing capacity tool or the geotech report's allowable pressure.
Sources: 2021 International Building Code, §1806 and Table 1806.2 (ICC). Soil classifications per ASTM D2487 (USCS). Check state/local amendments — the table is frequently modified.
Worked example: column footing on presumptive values
40-kip service column load on Class 4 soil (2,000 psf), footing 1.5 ft thick with 1.5 ft of soil cover
The screening table’s 4.5-ft footing ignores the weight of the footing and the backfill. On a 40-kip column that weight is about 20% of the load, enough to push bearing over the allowable value. For sliding on the same Class 4 soil, μ = 0.25 times the dead load gives the resistance. With 30 kip of dead load that is 7.5 kip. On Class 5 clay, cohesion of 130 psf on a 4.5-ft footing gives only 2.6 kip. Clay footings that carry lateral load usually need a key or passive resistance.
Common mistakes
- Comparing service loads with an allowable pressure but including load factors. IBC presumptive values are allowable stresses for service (unfactored) loads.
- Classifying the soil by eye. The table depends on USCS group. A “sandy” soil that tests as ML (sandy silt) rather than SM (silty sand) drops from Class 4 to Class 5 and loses 25% of its bearing.
- Using presumptive values on fill. IBC gives no presumptive value for unprepared fill, organic soil or mud. Fill needs a compaction specification and testing before any bearing value applies.
- Forgetting the frost depth. The bearing value applies at the footing depth required by the local code, which in much of the country is deeper than 12 in.
When this card doesn’t apply
IBC 1806 values are permitted only where the building official does not require a geotechnical investigation. Expansive soils, collapsible soils, high groundwater, slopes and structures on previously filled sites all trigger IBC 1803 investigation requirements. Settlement is not checked by presumptive values at all. Retaining walls, dams and heavily loaded or vibrating foundations need site-specific geotechnical design. For residential work, IRC Table R401.4.1 has its own, similar presumptive values.
Related cheat sheets and tools
Pair with the unit weights card for dead-load takeoffs, the Terzaghi bearing capacity factors (Nc, Nq, Nγ) table behind the computed value, the Terzaghi bearing capacity tool for computed capacity, and the Rankine earth pressure tool for wall design.