Footing Bearing Pressure Calculator

Calculator

Use the form to estimate bearing pressure under a rectangular footing.

Example data table

Sample runs to help you validate inputs and outputs.

Formula used

• Area: A = L × B
• Total load: P = (P_input × multiplier) + W_footing + W_soil
• Average pressure: q_avg = P / A
• Moment variation (rectangular footing): q = q_avg ± 6M_x/(L·B²) ± 6M_y/(B·L²)
• Extremes: q_max = q_avg + |6M_x/(L·B²)| + |6M_y/(B·L²)|, q_min = q_avg − those terms
• Eccentricity: e_x = M_y/P, e_y = M_x/P; kern limits are L/6 and B/6.

Bearing pressure for garden footings

Bearing pressure equals load divided by contact area. For a 40 kN post load on a 0.60 m × 0.60 m pad, area is 0.36 m² and qavg is about 111 kPa. Many compacted granular soils allow 100–200 kPa, while soft clays can be far lower.

Sizing with a load multiplier

Use the multiplier to reflect higher demand or conservatism. If the same 40 kN is multiplied by 1.25, the design load becomes 50 kN and qavg rises to about 139 kPa on the same pad. Increasing pad size from 0.60 m to 0.70 m lowers qavg by about 26% because area increases from 0.36 to 0.49 m².

Adding footing and soil cover weight

Self-weight matters for thick pads and heavy cover. A 0.20 m thick concrete pad with unit weight 24 kN/m³ adds 24 × 0.36 × 0.20 ≈ 1.73 kN. A 0.30 m soil cover at 18 kN/m³ adds 18 × 0.36 × 0.30 ≈ 1.94 kN. Together, that is about 3.67 kN extra load, or about 9% of a 40 kN post.

Moments and eccentric loading effects

Eccentric posts, wind, or bracing can create moments. The calculator applies rectangular footing distribution using q = qavg ± 6Mx/(L·B²) ± 6My/(B·L²). Example: with L=B=0.70 m and Mx=3 kN·m, the variation term is about 6×3/(0.70×0.70²) ≈ 52.5 kPa, raising qmax noticeably.

Interpreting qmax, qmin, and checks

Use qmax for capacity comparison because it represents the highest corner pressure. If qmin drops below 0, part of the footing may lift, indicating excessive eccentricity. Kern limits are L/6 and B/6; keeping ex and ey within these helps maintain compression over the full base. For imperial users, 2000 psf is 96 kPa; 1 kPa ≈ 20.9 psf. A 2.5 ft × 2.5 ft pad under 8500 lb gives qavg 1360 psf.

FAQs

1) What value should I use for allowable bearing?

Use a site-specific value from a geotechnical report when available. For small garden projects, local tables may suggest ranges like 100–200 kPa for compacted granular soils, but conditions vary widely with moisture, depth, and disturbance.

2) Why does qmin become negative?

Negative qmin means the linear pressure distribution predicts uplift at one corner. This typically occurs when eccentricity or moments are large relative to footing size. Increase L or B, reduce moments, or re-center the load to restore compression.

3) Should I include footing and soil cover weight?

Include them when the footing is thick, the soil cover is deep, or loads are small. They add to the total vertical load and increase bearing pressure. For many light garden posts, the increase may be only a few percent.

4) How do moments change the pressure results?

Moments create a pressure gradient across the base. The calculator adds terms 6Mx/(L·B²) and 6My/(B·L²) to qavg to estimate corner extremes. Even modest moments can raise qmax significantly on small pads.

5) What does the safety factor input do?

Safety factor computes the required allowable bearing as qmax × SF. It does not change qmax itself. Use it to back-calculate the soil capacity you would need to achieve a chosen margin of safety.

6) Is this calculator suitable for reinforced concrete design?

It estimates soil contact pressure only. Reinforcement, punching shear, settlement, frost depth, and code requirements still need separate checks. Use the results as an early sizing tool, and consult a qualified engineer for critical structures.