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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. The math is a little more involved.
Continuous Footings
A continuous footing is a strip of concrete running under a foundation wall. IRC Table R403.1 gives minimum widths based on the number of stories and the soil bearing capacity.
IRC Minimum Footing Width (inches)
| Soil Bearing (psf) | 1 Story | 2 Story | 3 Story |
|---|---|---|---|
| 1,500 | 16” | 21” | 32” |
| 2,000 | 12” | 16” | 24” |
| 2,500 | 10” | 12” | 19” |
| 3,000 | 8” | 10” | 16” |
| 4,000+ | 6” | 8” | 12” |
Most residential construction assumes 2,000 psf soil bearing unless a soils report says otherwise. A two-story house on 2,000 psf soil: 16” wide footing.
Minimum thickness: 6 inches, and not less than the projection beyond the foundation wall. If the foundation wall is 8 inches wide and the footing is 16 inches wide, the projection is 4 inches on each side. The footing must be at least 6 inches thick (since 6 > 4). A wider footing projection requires a thicker footing.
Volume for Continuous Footings
Width (ft) x Thickness (ft) x Total Length (ft) / 27 = Cubic Yards
A 16” wide, 8” thick footing running the perimeter of a 30 x 40 ft house:
Perimeter: (30 + 40) x 2 = 140 linear feet
1.333 ft wide x 0.667 ft thick x 140 ft = 124.4 cubic feet
124.4 / 27 = 4.61 cubic yards
Add 10% waste: 5.07 yards. Order 5.5.
That’s a separate pour from the foundation walls and the slab. Three pours for a full basement: footings, walls, floor slab.
Frost Depth
IRC R403.1.4.1: footings must extend below the frost line unless the building is heated and the footing is protected. The frost depth varies by location:
- Southern US (zones 1-3): 0-12 inches. Some areas have no frost requirement.
- Mid-Atlantic: 24-36 inches
- Midwest/Northeast: 36-48 inches
- Northern Minnesota, Montana: 60-72 inches
A 48-inch frost depth with a 16” wide, 8” thick footing means your trench is 4 feet deep. That’s a lot of digging. A backhoe is $250-400/day rental. Worth it.
Check your local building department for the exact frost depth. It’s set by the jurisdiction, not the IRC directly. Some cities within the same state have different requirements based on microclimates.
Pier Footings (Sonotubes)
Decks, post-frame buildings, and some additions use pier footings instead of continuous footings. A pier is a round column of concrete, usually formed with a Sonotube (cardboard tube).
Volume per Pier
Pi x (Radius in feet)^2 x Depth (ft) = Cubic Feet per Pier
Common Sonotube sizes:
| Tube Diameter | Depth 36” | Depth 42” | Depth 48” |
|---|---|---|---|
| 8” | 0.23 cu ft | 0.27 cu ft | 0.31 cu ft |
| 10” | 0.36 cu ft | 0.42 cu ft | 0.48 cu ft |
| 12” | 0.52 cu ft | 0.61 cu ft | 0.70 cu ft |
| 16” | 0.93 cu ft | 1.08 cu ft | 1.23 cu ft |
| 20” | 1.45 cu ft | 1.69 cu ft | 1.94 cu ft |
An 80-pound bag of concrete makes 0.6 cubic feet. So a 12” Sonotube, 42” deep: 0.61 / 0.6 = 1.02 bags. Call it 2 bags to account for the bell-bottom at the base (the hole is always wider at the bottom than the tube).
Deck Footing Example
A 12 x 16 ft deck with piers at 6-foot spacing in both directions, frost depth 36 inches, using 12” Sonotubes.
Posts along the beam lines:
- Two beam lines at 0 and 12 feet from the house
- Each beam needs a pier at 0, 6, and 12 feet along the 16-foot width (the last one at 16 feet)
- That’s 4 piers per beam line if posts at 0, ~5.3, ~10.6, and 16 feet
- Actually: 16 / 6 = 2.67, so 3 spaces, 4 posts per line
- Two lines: 8 piers total
(The ledger side attaches to the house and doesn’t need piers, so it’s really one free-standing beam line with 4 piers, not two. But let’s assume a freestanding deck for this example.)
8 piers x 0.52 cu ft each = 4.16 cu ft. That’s about 7 bags of concrete.
At 2 bags per hole to be safe: 16 bags. At $6 each: $96.
Plus 8 Sonotubes at $8-15 each depending on diameter and length: $64-120.
Footing With a Wider Base (Bell Bottom)
Some deck builders dig the hole wider at the bottom than the Sonotube diameter. This creates a bell-shaped footing that resists frost heave better than a straight cylinder, because the soil can’t grip the wider base and push it up.
The extra volume from a 12” tube with a 20” bell bottom (roughly 6” of extra diameter for the bottom 8”):
It adds maybe 0.2 cubic feet per pier. Not enough to change your bag count per hole, but multiplied across 8 piers it’s an extra 1.6 cu ft (3 bags).
Stepped Footings
On a sloped site, continuous footings step down the hill. IRC R403.1.5 requires that each step be no taller than 2 feet and that the horizontal and vertical dimensions of the step are in full units of block or brick (for masonry foundation walls).
Stepped footings use more concrete because the step sections are essentially short walls within the footing. For a rough estimate on a sloping site, add 15-20% to the flat-site volume.
Reinforcement
IRC R403.1.3: footings for buildings in Seismic Design Category D0, D1, or D2 require at least two #4 rebar running continuously along the footing. Many builders run two #4 bars in all footings regardless of seismic zone because it’s cheap insurance.
Two #4 bars x 140 feet of footing = 280 linear feet of rebar. #4 rebar comes in 20-foot sticks. That’s 14 sticks with 40 bars worth of lap splices (24 bar diameters minimum, or about 12 inches for #4). See the rebar spacing guide for more on reinforcement math.
Running the Numbers
SiteCalc has calculators for both continuous footings and pier footings. Enter the dimensions and it gives you cubic yards (or bag count for piers), including waste factor. The code references feature shows the IRC table citations for minimum footing width by story count and soil type. For a full project estimate, the guided projects feature chains the footing calculator with the slab, framing, and cost calculators into one material list.