Introduction
Every foundation – a house footing, a garage slab, a deck pier – transfers the building’s weight into the ground. The soil’s allowable bearing pressure is the load each square foot (or square meter) of soil can safely carry, and it sets how wide your footings must be. Dense gravel may carry several thousand pounds per square foot; soft saturated clay may carry only a fraction of that. Guess wrong, and the result is settlement, cracking, or worse.
The Soil Bearing Capacity Calculator runs the sizing math: enter the footing load and the soil’s allowable bearing pressure, and it returns the required footing width or area in imperial and metric units. This guide covers the terminology, the formulas, typical planning ranges by soil type, and a full worked example. Final foundation design always belongs to a licensed geotechnical or structural engineer working from a site-specific soil report.
What Soil Bearing Capacity Is and Where It’s Used
Soil bearing capacity is how much load the ground can support before failing by shear (giving way) or settling excessively. Engineers distinguish the ultimate capacity (the failure load – never used directly for design) from the allowable bearing pressure (ultimate divided by a factor of safety, typically 2.5 to 3, and limited for settlement). You always design with the allowable value.
The core principle is one sentence: footing width = load divided by allowable bearing pressure. The people who need this math include homeowners sizing deck footings or small additions, builders planning footing sizes for quotes, and owner-builders comparing soil conditions across lots. Load estimation is covered in the estimating guide; testing options are compared in the DIY vs pro guide.
Key Terms and Variables
Load
The weight the footing must carry: pounds per linear foot (lb/ft) for a wall footing, or total pounds (lb) for a column footing. Metric: kilonewtons per meter (kN/m) or kilonewtons (kN). Structural loads should come from a structural engineer for anything beyond a small accessory structure.
Allowable bearing pressure (q_a)
The soil’s safe load-carrying capacity in pounds per square foot (psf) or kilopascals (kPa). It comes from a geotechnical report (site-specific testing) or, for preliminary planning, conservative code values. It already includes a factor of safety – never divide by a safety factor on top of it.
Footing width
For a strip footing: width = load per foot / allowable bearing pressure. For a square pad footing: area = total load / allowable pressure, then side = square root of the area.
Soil type
The classification of the bearing layer – gravel, sand, silt, clay, rock, or mixtures. Soil type suggests a planning range for allowable pressure (see the material guide), but it never replaces testing: two clays in the same neighborhood can differ several-fold.
Typical Planning Ranges by Soil Type
Broadly cited planning ranges for allowable bearing pressure, similar to the presumptive values in residential building codes. Use these for early budgeting and comparison only – never as design values. Actual capacity varies with moisture, density, depth, and groundwater; only a site-specific geotechnical report gives a defensible number.
| Soil / rock type | Typical planning range (psf) | Typical planning range (kPa) |
|---|---|---|
| Crystalline bedrock | 8,000 – 12,000+ | 380 – 575+ |
| Dense sandy gravel, gravel | 2,500 – 3,000 | 120 – 144 |
| Compact sand, silty sand | 1,500 – 2,500 | 72 – 120 |
| Firm clay, sandy clay, silt | 1,000 – 1,500 | 48 – 72 |
| Soft clay, loose silt, organic soil | 500 – 1,000 | 24 – 48 |
The Formulas Involved
The calculator runs a short sequence; the full derivation is in the formula-explained guide.
Formula 1: Strip footing width
Width = Load per unit length / Allowable bearing pressure. Example: 4,000 lb/ft / 1,500 psf = 2.67 ft. Metric: 58.4 kN/m / 71.8 kPa = 0.81 m.
Formula 2: Square pad footing
Area = Total load / Allowable bearing pressure, then Side = square root of Area. Example: 30,000 lb / 2,000 psf = 15 ft2, side = 3.87 ft. Metric: 133.4 kN / 95.8 kPa = 1.39 m2, side = 1.18 m.
Formula 3: Unit conversions
1 psf = 0.04788 kPa (so 1,500 psf = 71.8 kPa). 1 lb/ft = 0.014594 kN/m (so 4,000 lb/ft = 58.4 kN/m). 1 ft = 0.3048 m. Run every calculation in both systems as a cross-check.
Formula 4: Round up
Always round the computed width up to a practical dimension – 2.67 ft becomes 2 ft 8 in (0.81 m) at minimum. Rounding down is designing for failure.
Worked Example: From Load and Soil to Footing Width
A bearing-wall strip footing carrying 4,000 lb/ft on soil with an allowable bearing pressure of 1,500 psf:
Step 1 – Divide. 4,000 / 1,500 = 2.667 ft.
Step 2 – Round up. 2.667 ft = 32 in, so use a 2 ft 8 in footing minimum (the engineer may specify 3 ft 0 in).
Step 3 – Cross-check in metric. 4,000 lb/ft = 58.4 kN/m; 1,500 psf = 71.8 kPa. 58.4 / 71.8 = 0.813 m, and 2.667 x 0.3048 = 0.813 m – the systems agree.
Step 4 – Reality check. Is the 1,500 psf from a geotechnical report or a planning table? If it’s a planning value, this is a budget number, not a construction dimension – the report and the structural engineer set the final width.
Common Variations
- Strip footings under walls use load per linear foot – the standard case above.
- Square pad footings under columns use total point load plus the square-root step.
- Layered soils – a firm crust over soft clay – require checking the weaker layer below, not just the bearing surface. Engineer territory.
- Groundwater near footing depth reduces effective capacity; table values assume normal moisture conditions.
Mistakes That Distort Results
- Using ultimate instead of allowable pressure – designing to the failure load with zero margin.
- Applying the safety factor twice – dividing an already-allowable value again, massively oversizing everything.
- Unit mixing – pounds per foot with kilopascals, or kips entered as pounds (off by 1,000x). Keep one system per calculation.
- Sizing to topsoil or fill – footings bear on undisturbed native soil below the organic zone, not on the surface layer.
- Rounding down – a computed 2.67-ft width built as 2 ft 6 in is under-designed by definition.
The full catalogue, with fixes, is in the common mistakes guide.
Related Guides
- Soil Bearing Capacity FAQ – quick answers to the most asked questions.
- Formula Explained – the full derivation from Terzaghi to footing width.
- Material (Soil Type) Guide – identifying soils and their typical ranges.
- Cost Guide – soil testing, reports, and what footing size costs.
- Pro Tips – field judgment from geotechnical practice.
- Site Checklist – everything to verify before footings are designed.
Frequently Asked Questions
How do you calculate footing width from soil bearing capacity?
Width = load per linear foot / allowable bearing pressure. Example: 4,000 lb/ft on 1,500-psf soil needs 2.67 ft of width. For a column: area = total load / allowable pressure, then side = square root of the area.
What is the difference between bearing capacity and allowable bearing pressure?
Bearing capacity (ultimate) is the soil’s failure load. Allowable bearing pressure is the ultimate divided by a factor of safety (typically 2.5-3) and limited for settlement. Always design with the allowable value.
Can I use a table value instead of a soil test?
For early planning and budgeting, yes – code presumptive values are deliberately conservative. For final design, most jurisdictions require a geotechnical report or an engineer’s evaluation. Tables never replace testing for the actual build.
How do I convert psf to kPa?
Multiply psf by 0.04788: 1,500 psf = 71.8 kPa, 2,000 psf = 95.8 kPa, 3,000 psf = 143.6 kPa.
Should footing width be rounded up or down?
Always up. Construction tolerances only shrink effective dimensions, so rounding down under-designs the footing.
Get Your Number
Footing sizing is simple division built on two hard-won numbers: the real load and the real soil capacity. The Soil Bearing Capacity Calculator does the division – your job is trustworthy inputs, starting with a soil test or a conservative code value and ending with a licensed engineer’s review. Run the number, sanity-check it against the worked example above, and get the geotechnical report before anyone digs.
Disclaimer: Calculator results are planning estimates based on the values you enter. Soil capacity varies by site, moisture, depth, and loading conditions – verify with your local building code and a site-specific geotechnical report, and consult a licensed geotechnical or structural engineer for foundation design.
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