Footing Calculator

Check a concrete footing under a column, post or wall: enter its size, the load on it and the soil, and see whether the soil can take it and the bars it needs. Add uplift, a moment or sideways shear from wind or an earthquake, and it checks that the footing won't lift, tip or slide. Pro adds the bar details and a PDF report.

The footing

Its width; 3.5 for a 4x4, 5.5 for a 6x6
The way the moment and shear push
From the top of the footing up to grade

Loads

Down, at the top of the footing
Including snow
At strength level, as ASCE 7 gives it
Like a hold-down pulling up; 0 if none
From a post or wall fixed to the footing
Sideways, at the top of the footing

The soil

1,500 without a soils report (IBC 1806.2)
Usually 110 to 125
0.25 for sand, 0.35 for gravel (IBC 1806.2)
On the side of the footing below 1 ft; 0 to leave out

The footing works with #4 at 9 in each way, using 76% of what it can take (limited by soil pressure).

1,138 psf 14,000 lb Grade 4 ft 12 in 4 ft by 4 ft Plan

The footing in section and plan, to scale: the soil pressure under it from the worst allowable stress combination, and its bottom bars.

The soil

Soil pressure1,138 psf under D + L, 1,500 psf allowed 76%

The footing weighs 2,400 lb and the soil over it 1,800 lb, both counted as dead load.

The concrete

#4 at 9 in each way, 3 in clear of the bottom.

Bending along the length16.2 kip-in per ft (factored) at the face 13%
Shear along the lengthd = 8.75 in from the face; the footing takes 18.8 kips 20%
Bending across the width16.2 kip-in per ft (factored) at the face 14%
Shear across the widthd = 8.25 in from the face; the footing takes 18.1 kips 22%
Punching shear around the column15.7 kips (factored) around the column, d = 8.50 in, with the moment's share 14%
Bearing under the column19.2 kips (factored) on 477 kips 4%

Bar details Pro

Where the bars go and how they end, the band of bars under the column of a rectangular footing, top bars where the footing is pulled up, dowels into a concrete pier, the bar development checks, and a PDF report with every calculation.

  • The bottom of the footing is 2′ below grade. Set it below the frost line and on undisturbed soil.

Soil checked under allowable stress loads (ASCE 7 2.4) and the concrete under factored loads (ASCE 7 2.3), with 3,000 psi concrete and grade 60 bars to ACI 318-19.

How the calculator checks a footing

The soil is checked with allowable stress loads, and the concrete with factored loads, the way the building code does it (ASCE 7 2.3 and 2.4). The footing and the soil on top of it count as dead load.

  • Soil pressure: the most pressure under D + L, D + 0.6W, D + 0.75L + 0.75(0.6W) and 0.6D + 0.6W (0.7E for an earthquake), against the allowable. With a moment the pressure is higher on one side; once the load is more than a sixth of the footing off center, part of the footing lifts off the soil and the pressure is a triangle under the rest.
  • Uplift, overturning and sliding: under 0.6D + 0.6W (or 0.7E), the dead load must hold the footing down against the uplift and keep it from tipping, and friction under it plus the soil's passive pressure on its side must stop it sliding, leaving out the top 1 ft.
  • Bending: the soil pushing up on the footing past the face of the column or wall, as a cantilever, against bottom bars to the American Concrete Institute's ACI 318-19: at least 0.0018 of the section, spaced to keep cracks narrow, and long enough to develop, straight or with hooks, in the length past the face.
  • Rectangular footings: more of the short bars go under the column, 2 / (β + 1) of them in a band as wide as the footing's short side (ACI 13.3.3.3).
  • Shear: one-way shear d from the face of the column or wall, and two-way (punching) shear d/2 around the column, with the share of the column's moment that the shear carries (ACI 8.4.4.2).
  • Plain concrete: when the footing sticks out too little past the column or wall for bars to develop, it's checked as plain concrete, with 2 in of its thickness left out (ACI 14.5). Most house footings are plain this way.
  • Uplift on the concrete: when the loads pull the column up, the footing hangs from it by its own weight and the soil on it, bending the top. Plain concrete is checked first, then top bars.
  • Bearing under the column: 0.65 × 0.85 f′c A1, up to twice that for the footing around it (ACI 22.8.3.2).

Not checked: settlement, frost depth, footings on fill or expansive clay, footings next to a slope, and the post base, anchor or hold-down that ties the column to the footing. For sizes by load, see the footing size chart; for a square footing under a concrete column, the footing designer works through it step by step.

Footing questions

How big should a footing be?

Big enough that the load, plus the footing and the soil on it, spread over its area, is no more than the soil's allowable pressure. 1,500 psf is the code's value without a soils report (IBC Table 1806.2), so a 10,000 lb post load needs about 7 sq ft, or a footing about 2 ft 8 in square.

How thick should a footing be?

Thick enough for shear, usually 10 to 12 in for houses and decks, and at least as thick as it sticks out past the column or wall if it's plain concrete. Thicker footings also give bars the room to develop.

Does a footing need rebar?

Not always. A narrow footing that sticks out only a few inches past the wall can be plain concrete, and the International Residential Code (IRC) allows plain footings for most houses. Wider footings, footings under heavy columns and footings pulled up by a hold-down need bars.

Why does a footing with a moment get longer?

A moment pushes the soil pressure to one side. A longer footing in that direction keeps the load near the middle, so the pressure stays below the allowable and the footing doesn't tip. That's why a footing under a shear wall's hold-down or a flagpole is often rectangular, long the way the wind pushes.