| Parameter | Value |
|---|
This tool helps you turn structural loads into practical answers. It estimates vertical load on the footing, average contact pressure on the soil and a rough settlement so you can compare strip footings, slabs and spread footings. Use it for fast feasibility checks before you call a geotech or run full code calculations.
Table of Contents
Who this is for and when to use it
Engineers and designers use the calculator for early-stage checks. Contractors and estimators use it to size a footing concept. If you need to choose between a slab and a strip footing, or to check whether a pad needs widening, this is the quick sanity check before deeper analysis.
What you enter and what it means
| Field | Meaning | US unit example |
|---|---|---|
| Foundation type | Strip footing, slab, spread footing or pile cap | — |
| Width B | Footing width across the loaded line | feet |
| Length L | Length of the footing segment considered | feet |
| Thickness t | Thickness of the slab or footing | feet or inches |
| Permanent line load g | Dead load per linear foot for strip footings | lb/ft |
| Live line load q | Live load per linear foot for strip footings | lb/ft |
| Permanent area load g | Dead load per square foot for slabs | psf |
| Live area load q | Live load per square foot for slabs | psf |
| Snow | Snow load on the slab in pounds per square foot | psf |
| Column sum | Total concentrated column load arriving at the footing | kips or lbf |
| Allowable soil pressure qadm | Allowable bearing pressure from geotech | psi |
| Modulus E | Soil elastic modulus for rough settlement | psi |
Core formulas and units
Area of the footing A equals B times L
A = B × L
We convert masses and metric inputs to US weight units before summation. To find the total vertical load V we sum dead loads, live loads, snow and concentrated loads and add the footing self-weight. For strip and slab inputs the formulas below show the logic.
- Strip footing where g and q are per linear foot: V equals g plus q times length plus snow times area plus column loads plus self weight
- Slab where g and q are per square foot: V equals g plus q times area plus snow times area plus column loads plus self weight
- Spread footing where g and q are total weights: V equals sum of those totals plus snow times area plus column loads plus self weight
Self-weight of a concrete footing uses typical concrete weight 150 pounds per cubic foot. Multiply that by thickness in feet and by area in square feet to get footing weight in pounds.
Average contact pressure qact equals vertical load divided by contact area. Express qact in psf and in psi as needed. Safety factor FS equals allowable bearing pressure divided by qact.
US example 1 — strip footing converted from a metric case
Inputs in US units
- Footing width B = 3.28 feet
- Length L = 32.81 feet
- Thickness t = 1.31 feet which is about 15.7 inches
- Dead line load g = 671 lb/ft
- Live line load q = 335.5 lb/ft
- Snow = 10.24 psf
- Column sum = 0
- Allowable soil pressure qadm = 21.76 psi
- Soil modulus E ≈ 3626 psi
Steps and computed numbers
- Area A = B × L = 3.28 × 32.81 ≈ 107.64 square feet
- Convert line loads to kips: g per foot times length equals 671 × 32.81 ≈ 22,020 pounds or 22.02 kips
- Live line load gives about 11.01 kips
- Snow load over the footing area equals 10.24 psf × 107.64 ft² ≈ 1,102 pounds or 1.10 kips
- Self-weight of concrete equals 150 lb/ft³ × t × A = 150 × 1.31 × 107.64 ≈ 21,178 pounds or 21.18 kips
- Total vertical load V ≈ 22.02 + 11.01 + 1.10 + 21.18 = 55.31 kips
- Average contact pressure qact = total load per area = 55.31 kip / 107.64 ft² ≈ 0.514 kip/ft² which is about 514 psf or 3.57 psi
- Factor of safety FS = qadm / qact = 21.76 psi / 3.57 psi ≈ 6.1
- Rough settlement estimate converts to about 0.0009 meters which equals roughly 0.035 inches. This is a ballpark number and not a replacement for a proper settlement analysis
US example 2 — slab converted to US units
Inputs
- Slab footprint 13.12 feet by 19.69 feet giving area A ≈ 258.6 square feet
- Thickness t = 0.98 foot equals about 11.8 inches
- Dead area load g = 409.8 psf
- Live area load q = 204.9 psf
- Snow = 20.48 psf
- Allowable soil pressure qadm = 29.01 psi
- Soil modulus E ≈ 4351 psi
Summary of calculation
- Self-weight equals 150 lb/ft³ × 0.984 ft × 258.6 ft² ≈ 38,164 pounds or 38.16 kips
- Dead load contribution equals 409.8 psf × 258.6 ft² ≈ 106.0 kips
- Live load contribution equals 204.9 psf × 258.6 ft² ≈ 53.0 kips
- Snow adds about 5.30 kips
- Total V ≈ 202.5 kips which is roughly 202,500 pounds
- Average contact pressure qact ≈ 202,500 lb / 258.6 ft² ≈ 783 psf which equals about 5.44 psi
- FS = 29.01 psi / 5.44 psi ≈ 5.3
- Estimated settlement from the simple formula is about 0.0046 meters which equals about 0.18 inches
Soil bearing reference values
| Soil type | Typical allowable bearing pressure | Notes |
|---|---|---|
| Dense sand | 2.2 to 4.4 ksf which is about 150 to 300 kPa or 2.2 to 4.4 ksf | Good bearing, small settlement |
| Medium sand | 1.5 to 3.2 ksf | Often use 1.8 to 2.5 ksf without tests |
| Loose sand | 0.9 to 1.9 ksf | May need compaction |
| Silt | 1.3 to 3.0 ksf | Very moisture sensitive |
| Clay stiff | 1.7 to 3.6 ksf | Good strength but settlement may occur |
| Soft clay, organic | < 0.7 ksf | Usually requires remediation or piles |
Material densities and typical loads
| Material | Typical value | Unit | Note |
|---|---|---|---|
| Normal concrete | 150 | lb/ft³ | About 150 pounds per cubic foot |
| Masonry | 112 to 125 | lb/ft³ | Depends on brick type |
| Dry sand | 94 to 106 | lb/ft³ | Compacted sand near upper bound |
| Gravel | 100 to 119 | lb/ft³ | Depends on gradation and compaction |
| Water | 62.4 | lb/ft³ | Useful for hydrostatic load checks |
| Steel | 490 | lb/ft³ | Use for structural weights |
| Conversion factor | 1 kg = 2.20462 lb | — | For converting metric inputs |
Limitations and best practice
This calculator gives first-order estimates. For permit documents and final design you must obtain a geotechnical report and perform code-based calculations. Settlement here is computed with a simple elastic estimate. Real soils show nonlinear behavior and time-dependent consolidation which requires lab tests and layered analysis. Pile foundations and heavily loaded footings require their own methods.
How to use the interface
- Pick the foundation type
- Enter geometry B, L and thickness in feet or inches
- Enter dead and live loads in the appropriate units, linear or area
- Enter column loads that land on this footing in kips
- Enter allowable bearing pressure qadm from geotech in psi
- Click calculate and review qact, FS and settlement
Use this calculator as a fast, practical check to compare footing options and detect potential bearing problems early. Always verify unit consistency, prefer geotech values for allowable pressures and treat the settlement output as an orientation rather than a design result. When the numbers are close to limits, call a geotechnical engineer to avoid surprises.
Suggested reading
- Foundation Engineering — B.M. Das
- Principles of Foundation Engineering — B.M. Das
- Soil Mechanics in Engineering Practice — Karl Terzaghi and Ralph B. Peck
- Foundation Analysis and Design — Joseph E. Bowles
Markus Fletcher — Structural Design Specialist
Expert in structural integrity, 3D modeling, and applied mathematics. Markus focuses on creating precise tools for construction professionals and DIY engineers.
