Concrete footing calculator: how big should a concrete footing be?

19 × 6 in footing

4.11 cubic yards of concrete, or 185 bags of 80 lb · Table R403.1(1) at 1,500 psf · for 2 stories over a basement, 140 ft of wall

Run your own numbers

A two-story light-frame house over a basement, on 1,500 psf clay with 25 psf of ground snow, needs a concrete wall footing of at least 19 × 6 in by IRC 2021 Table R403.1(1): 4.11 cubic yards for 140 ft.

A worked example

A 3 story house over a crawl space, 36 ft wide, under 50 psf of ground snow, on soil a report puts at 1,750 psf, with 112 ft of footing poured from 60 lb bags, needs a 20 × 7 in footing, 4.04 cubic yards. The 50 psf, 3 story crawl space row of Table R403.1(1) gives 20 × 6 in at 1,500 psf and 15 × 6 at 2,000. At 1,750 the width interpolates to 17.5 in, rounded up to 18. The house is 4 ft wider than the table's 32: 2 in wider and 1 in thicker, 20 × 7. 112 ft of it is 108.9 cu ft: 4.04 cubic yards, or 242 bags of 60 lb.

How big should a concrete footing be?

At least 12 in wide and 6 in thick under any house wall, by IRC 2021 Section R403.1.1. On 1,500 psf clay with 20 psf of roof load, a 1 story house on a slab stays at 12 × 6, a 2 story over a basement needs 19 × 6, and a 3 story over a basement under 70 psf of snow 24 × 8.

How thick should a concrete footing be?

6 in in nearly every row of Table R403.1(1). Only on 1,500 psf soil does it reach 7 in, for a 3 story basement at 20 or 30 psf, a 2 story basement at 50 or 70 psf and a 3 story crawl space at 70 psf, or 8 in, for a 3 story basement at 50 or 70 psf. A house wider than 32 ft adds 1 in for every 4 ft.

How deep should a concrete footing be?

Its bottom at least 12 in below undisturbed ground (IRC 2021 Section R403.1.4) and, where the ground freezes, below the frost line your jurisdiction sets in Table R301.2, unless the footing is protected from frost another way the code allows or sits on solid rock. The 6 in of the table is the footing's own thickness, not its depth.

What else it is called

Also searched as a concrete footing size calculator, a footing size chart or the footing size rule of thumb. The table assumes 9 ft walls, an 8 ft basement wall, a clear span roof and an interior bearing wall or beam at each floor. The footing projects at least 2 in past each face of the wall and no more than its own thickness (Section R403.1.1).

The full IRC footing table

IRC 2021 Table R403.1(1), minimum width × thickness in inches of concrete footings for light-frame construction, as printed. The table assumes a 32 ft wide building, 9 ft walls and an 8 ft basement wall; note a allows interpolating the width between soil columns, never extrapolating; notes c and d add or take 2 in of width and 1 in of thickness for every 4 ft of building width over or under 32 ft, never under 12 × 6 in (Section R403.1.1).

Where the numbers come from

• Footing size: 2021 International Residential Code, Table R403.1(1), Minimum Width and Thickness for Concrete Footings for Light-Frame Construction, printed in full below: width × thickness in inches by roof live or ground snow load, stories, slab, crawl space or basement, and soil bearing value from 1,500 to 4,000 psf. Note a permits interpolating the width between soil columns and forbids extrapolating; notes c and d add 2 in of width and 1 in of thickness for every 4 ft of building width over 32 ft, and allow the same off under 32 ft. Section R403.1.1 sets 12 in wide and 6 in thick as the floor. Read October 31, 2026, in the code text published by UpCodes, after a cell by cell match with the 2021 Seattle Residential Code, and checked again November 1, 2026. Read the code
• Soil: IRC 2021 Table R401.4.1, Presumptive Load-Bearing Values of Foundation Materials: clay and silt (CL, ML, MH, CH) 1,500 psf; sand, silty or clayey sand and silty or clayey gravel (SW, SP, SM, SC, GM, GC) 2,000 psf; sandy gravel and gravel (GW, GP) 3,000 psf; sedimentary and foliated rock 4,000 psf; crystalline bedrock 12,000 psf. Below 1,500 psf a soils investigation sets the value. Depth: Section R403.1.4 puts exterior footings at least 12 in below undisturbed ground, and R403.1.4.1 below the frost line of Table R301.2 unless otherwise protected. Read October 31, 2026. Read the code
• Bag yields: Quikrete Concrete Mix technical data sheet, document 1101. A 40 lb bag yields 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 and 90 lb 0.675. Read October 12, 2026. Data sheet (PDF)
• Choices this calculator makes where the code leaves room: an interpolated width is rounded up to the whole inch and takes the larger thickness of the two columns; over 32 ft a part of 4 ft counts as a full step, under 32 ft only full steps come off; over 4,000 psf it reads the 4,000 psf column. Concrete is length × width × thickness, 27 cu ft to the cubic yard, bags rounded up.

By Javier Miralles · Last updated November 1, 2026. How this site is made.

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