Bearing Pressure Calculator

Bearing Pressure Calculator finds the pressure a footing puts on the soil from its load and plan size. It also checks that pressure against a soil limit and sizes a footing to fit.

Bearing Pressure Calculator for Footings on Soil

The Bearing Pressure Calculator works out the pressure a footing puts on the ground and compares it with what the soil can carry. Enter the column load, the footing length and width, and an allowable soil pressure. A round footing takes a diameter in place of length and width.

With the default entries, a 50 kip column sits on a footing 5 ft long, 4 ft wide and 12 in thick. The bearing pressure is 2,650 psf. On soil rated for 3,000 psf that uses 88% of the allowable, with 350 psf to spare.

The cards in the Bearing Pressure Calculator go past the single pressure figure. They give the column load that would reach the limit and the smallest footing that works. They also show where a moment starts to lift one edge, and the same pressure in psi, tons per square foot and kPa.

Bearing Pressure Formula

Bearing pressure is the total downward load divided by the area in contact with the soil. In the Bearing Pressure Calculator the total load is the column load plus the weight of the footing itself.

$$q=\frac{P+W}{A}$$

P is the column load, W is the footing weight and A is the plan area. For the example, the area is 5 × 4, or 20 ft². The column adds 50,000 ÷ 20, or 2,500 psf. The footing is 1 ft of concrete at 150 pcf, which adds 150 psf. Together they make 2,650 psf.

Footing weight is easy to forget and it is not small. Every inch of normal weight concrete adds 12.5 psf, so a 24 in thick footing uses 300 psf of the allowable before the column load arrives. Set the thickness to zero under Moment, Footing Weight and Soil Value to see the pressure from the column alone.

IBC Allowable Soil Pressure by Class

The soil list in the Bearing Pressure Calculator loads the presumptive load-bearing values from Table 1806.2 of the International Building Code. These are the pressures the code allows when no soils report is available. The table shows each class with the result for the example footing.

The same footing that passes on gravel fails on sand and on clay. Open This Footing on Each IBC Soil Class under the cards to see that comparison for your own footing. The code values are presumptive, so a geotechnical report governs wherever one exists.

Sizing a Footing From the Load

The second card in the Bearing Pressure Calculator turns the check around and gives the smallest footing for the load. Part of the allowable pressure is already used by the footing's own weight, so that part is taken off first.

$$A_{req}=\frac{P}{q_a-{\gamma }_{c}t}$$

Here qa is the allowable pressure, γc is the concrete unit weight and t is the footing thickness. For 50 kips on 3,000 psf soil with a 12 in footing, the area is 50,000 ÷ 2,850, or 17.54 ft². That is a 4.19 ft square or a 4.73 ft round footing.

Weaker soil changes the answer quickly. On sand at 2,000 psf the same column needs 27.03 ft², a 5.20 ft square. On clay at 1,500 psf it needs 37.04 ft², a 6.09 ft square. The first card gives the opposite figure, which is the 57.0 kips the 5 by 4 ft footing can carry on 3,000 psf soil.

Bearing Pressure With a Moment

Wind, a sign post or an off-center column puts a moment on the footing. The pressure is then higher at one edge and lower at the other. Enter the moment and the Bearing Pressure Calculator reports the peak pressure at the heavy edge.

$$q_{max}=\frac{P+W}{A}+\frac{6M}{B{L}^2}$$

M is the moment, L is the footing length in the direction of the moment and B is the width. A 25 kip-ft moment on the example footing adds 1,500 psf at one edge. The peak rises to 4,150 psf and the light edge falls to 1,150 psf. That is over the 3,000 psf allowable, even though the average is unchanged.

The moment acts like moving the load off center by a distance e, the moment divided by the total load. While e stays within one sixth of the length, the whole footing bears on the soil. That zone is the kern, also called the middle third.

$$e=\frac{M}{P+W}$$

For the 5 ft example the kern limit is 10 in, and uplift starts at 44.17 kip-ft. Past that point one edge lifts off and the soil carries the load on a shorter length. The pressure then follows a triangle.

$$q_{max}=\frac{2(P+W)}{3B(L/2-e)}$$

At 50 kip-ft the load sits 11.32 in off center. Only 4.67 ft of the 5 ft length stays in contact and the peak pressure is 5,675 psf. On a round footing the kern is one eighth of the diameter, and the tool solves pressure only inside that limit.

Allowable or Ultimate Bearing Capacity

A soils report may give an ultimate bearing capacity, which is the pressure at which the soil fails. It is not the number to load a footing to. The allowable pressure is the ultimate value divided by a safety factor.

Set Soil Value Entered Is to Ultimate and choose a safety factor of 2, 2.5 or 3. The Bearing Pressure Calculator divides the entry before it runs the check. An ultimate capacity of 9,000 psf with a factor of 3 is treated as 3,000 psf allowable.

Ground Bearing Pressure in Other Units

Soils reports, plate tests and metric drawings use different units for the same pressure. The last card in the Bearing Pressure Calculator converts the result. The example 2,650 psf is 18.40 psi, 1.325 tons per square foot and 126.9 kPa. Switching Units to metric takes loads in kN and sizes in meters.

Ground bearing pressure under a crane outrigger, a track or a mat is a different check, because the load comes from equipment, not a column. The Ground Pressure Calculator works from machine load and contact area for that job.

Bearing Pressure FAQ

What is a safe bearing pressure for soil?

Without a soils report, the IBC presumptive values are the usual starting point. They are 1,500 psf for clay and silt, 2,000 psf for sand and 3,000 psf for gravel. Rock is 4,000 psf or 12,000 psf depending on the type.

Those figures are for undisturbed natural ground. Fill, organic soil and soft clay can be far weaker, and a building official can require an investigation where the soil is in doubt. A tested site often allows more than the table.

Which load goes into the column load field?

Use the service load in the Bearing Pressure Calculator, meaning dead load plus live load without strength design load factors. The IBC ties its presumptive values to allowable stress design load combinations, so factored loads would overstate the pressure.

Add up everything the column delivers to the footing, including floor, roof and wall loads above it. The Beam Load Calculator gives the reaction a beam sends down to its support, which is often the largest part.