Calculator Input
Example Data Table
| Parameter | Example Value | Unit |
|---|---|---|
| Footing Length | 2.50 | m |
| Footing Width | 2.00 | m |
| Footing Thickness | 0.60 | m |
| Foundation Depth | 1.40 | m |
| Service Axial Load | 900 | kN |
| Moment About Width Axis | 60 | kN·m |
| Moment About Length Axis | 40 | kN·m |
| Uniform Surcharge | 15 | kPa |
| Soil Unit Weight | 18 | kN/m³ |
| Concrete Unit Weight | 24 | kN/m³ |
| Allowable Bearing Pressure | 280 | kPa |
| Gross Average Pressure | 209.40 | kPa |
| Maximum Pressure | 266.27 | kPa |
| Minimum Pressure | 152.53 | kPa |
| Factor of Safety | 1.052 | ratio |
Formula Used
This footing calculator uses rectangular area pressure relationships for service load checks.
1) Footing area
A = L × B
2) Footing self-weight
Wf = L × B × t × γc
3) Total vertical load
Ptotal = P + Wf
4) Gross average pressure
qavg = (Ptotal / A) + qs
5) Eccentricities
eL = Mw / Ptotal
eB = Ml / Ptotal
6) Corner pressure relation
q = qavg × (1 ± 6eB/B ± 6eL/L)
7) Net average pressure
qnet = qavg - (γsoil × Df)
8) Middle third check
Compression over the full base is typically expected when:
|eL| ≤ L/6 and |eB| ≤ B/6
How to Use This Calculator
- Enter footing length, width, thickness, and foundation depth.
- Add the service axial load acting on the footing.
- Enter moments about the width and length axes if eccentric loading exists.
- Provide surcharge, soil unit weight, concrete unit weight, and allowable bearing pressure.
- Click the calculate button to display results above the form.
- Review maximum and minimum pressures, corner values, utilization, and the Plotly graph.
- Download the result summary as CSV or PDF for reporting.
- Investigate warnings carefully when minimum pressure becomes negative or the allowable limit is exceeded.
FAQs
1) What does soil pressure mean here?
It is the contact stress between the footing base and supporting soil. The calculator estimates average, corner, maximum, and minimum values under service loading.
2) What is the difference between gross and net pressure?
Gross pressure includes the applied footing pressure at the base. Net pressure subtracts overburden from embedment depth, helping compare foundation demand with some design checks.
3) Why can the minimum pressure become negative?
A negative minimum value suggests uplift or loss of full contact caused by eccentric loading. That means the simple full-base linear distribution is no longer fully valid.
4) Why are moments included in the calculation?
Moments shift the resultant load away from the footing center. That increases pressure on one side and reduces it on the opposite side.
5) Can this be used with factored loads?
It can, but you must stay consistent with your chosen design method. Many bearing checks use service loads, while structural footing design may also require factored combinations.
6) Why is footing self-weight added?
The soil supports the footing and the supported structure together. Ignoring footing self-weight can understate the actual contact pressure.
7) Does this replace a geotechnical investigation?
No. It is a practical preliminary tool. Final bearing capacity, settlement, groundwater effects, and code compliance still need project-specific geotechnical and structural review.
8) What units should I use?
Use a consistent metric set. This page expects meters, kilonewtons, kilopascals, and kilonewtons per cubic meter throughout the entire calculation.