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Concrete Footing Calculator: Size and Volume
This Is Different From a Slab
A slab calculation is flat and rectangular. Footings are deeper, narrower, and sized to the load they carry and the soil they sit on.
Continuous Footings
A continuous footing is a strip of concrete under a foundation wall. The 2021 IRC sizes them in Tables R403.1(1) through R403.1(3): light-frame, light-frame with brick veneer, and masonry or concrete construction. Each table goes by ground snow load (or roof live load), number of stories, foundation type, and the soil’s load-bearing value.
No matter what the table says, R403.1.1 sets a floor: 12 inches wide and 6 inches thick. The footing has to project at least 2 inches past the wall on each side, and no more than its own thickness.
A Few Lookups (Light-Frame, 30 psf Ground Snow)
From Table R403.1(1), width x thickness in inches:
| House | 1,500 psf soil | 2,000 psf soil | 2,500 psf+ soil |
|---|---|---|---|
| 1 story, crawl space | 13 x 6 | 12 x 6 | 12 x 6 |
| 1 story, basement | 16 x 6 | 12 x 6 | 12 x 6 |
| 2 story, crawl space | 16 x 6 | 12 x 6 | 12 x 6 |
| 2 story, basement | 19 x 6 | 14 x 6 | 12 x 6 |
| 3 story, basement | 22 x 7 | 16 x 6 | 13 x 6 (2,500) |
Heavier snow, brick veneer, and masonry walls all push those numbers up. Without a soils report, Table R401.4.1 gives presumptive soil values; many clay soils are 1,500 psf, and your building official may set a value for your area.
Volume for Continuous Footings
Width (ft) x Thickness (ft) x Total Length (ft) / 27 = Cubic Yards
A 16” wide, 8” thick footing around a 30 x 40 ft house:
Perimeter: (30 + 40) x 2 = 140 linear feet
1.333 ft x 0.667 ft x 140 ft = 124.4 cubic feet
124.4 / 27 = 4.61 cubic yards
Add 10%: 5.07 yards. Trenches rarely come out perfectly square, so many crews order 5.5.
Frost Depth
IRC R403.1.4.1 requires footings to extend below the frost line, with exceptions such as frost-protected shallow foundations and some small structures. The depth comes from your local building department (it’s in the climatic and geographic design criteria of Table R301.2), not from a national number. It ranges from zero in parts of the South to several feet in the northern states.
Call 811 before you dig.
Pier Footings (Sonotubes)
Decks, post-frame buildings, and some additions sit on round piers, usually formed with a cardboard tube.
Volume per Pier
Pi x (radius in feet)^2 x depth (ft) = cubic feet
| Tube | 36” deep | 42” deep | 48” deep |
|---|---|---|---|
| 8” | 1.05 cu ft | 1.22 cu ft | 1.40 cu ft |
| 10” | 1.64 cu ft | 1.91 cu ft | 2.18 cu ft |
| 12” | 2.36 cu ft | 2.75 cu ft | 3.14 cu ft |
| 16” | 4.19 cu ft | 4.89 cu ft | 5.59 cu ft |
| 20” | 6.54 cu ft | 7.64 cu ft | 8.73 cu ft |
An 80-pound bag of concrete mix yields about 0.6 cubic feet. A 12” tube 42” deep is 2.75 / 0.6 = 4.6 bags. Call it 5, and a little more if the hole is wider than the tube at the bottom.
Deck footing sizes come from IRC Table R507.3.1, by the area each post supports and the soil’s bearing value. Pier diameter is a structural number, not a preference.
Deck Footing Example
A freestanding 12 x 16 ft deck with two beam lines, four posts per line, so 8 piers. Frost depth 36 inches, 12” tubes:
8 piers x 2.36 cu ft = 18.9 cu ft
18.9 / 0.6 = 31.4 bags. Add 10% and buy 35.
A deck attached to the house with a ledger needs only the outer beam line: 4 piers, about 18 bags.
Belled Footings
Some builders flare the bottom of the hole wider than the tube. The wider base adds bearing area and resists frost heave. A bell 20” across and 8” tall under a 12” tube adds roughly 1 cubic foot, close to 2 extra bags per pier.
Stepped Footings
On sloped sites, footings step down the hill. R403.1.5 requires the top of the footing to be level and allows the bottom to slope up to 1 in 10; steeper sites get stepped footings. Each step adds formwork and concrete, so measure each section rather than estimating from a flat-site number.
Reinforcement
The IRC doesn’t require rebar in most footings outside high seismic areas. In Seismic Design Categories D0, D1, and D2, R403.1.3 and Figure R403.1.3 set minimum reinforcement. Many builders run two #4 bars in every footing anyway. If your plans call for laps, follow them; the IRC’s own lap table for concrete walls calls for 30 inches on Grade 60 #4 bar. See the rebar spacing guide for the material math.
Retaining walls that hold back more than 4 feet of soil are engineered, footing included.
Running the Numbers
Trussly has calculators for continuous footings and piers. Enter the dimensions and it gives cubic yards or bag counts with a 10% overage. The footing size itself comes from your plans or the IRC tables above; the calculator does the volume.