Updated June 17, 2026
For rectangular footings and sidewalks: multiply length x width x depth (all in feet) and divide by 27 to get cubic yards. For round sonotubes (tube forms): multiply 3.14 x radius squared x depth (in feet) and divide by 27. Add 10 percent for waste.
Rectangular footing formula with worked example
A continuous footing that is 12 inches wide, 8 inches deep, and 40 feet long:
- Width: 12 in = 1 ft
- Depth: 8 in = 0.667 ft
- Volume: 1 x 0.667 x 40 = 26.7 cu ft
- Cubic yards: 26.7 / 27 = 0.99 yards, call it 1.1 yards with waste
Circular sonotube / post footing formula
| Tube diameter | Depth | Cu ft per footing | 80-lb bags per footing |
|---|---|---|---|
| 8 in | 24 in (2 ft) | 0.70 | 2 |
| 10 in | 24 in (2 ft) | 1.09 | 2 |
| 12 in | 36 in (3 ft) | 2.36 | 4 |
| 16 in | 36 in (3 ft) | 4.19 | 7 |
Radius is half the diameter in inches, then converted to feet. For a 12-inch tube (6-inch radius = 0.5 ft): 3.14 x 0.25 x depth.
Sidewalk estimating
A sidewalk is just a long, thin slab. A typical 4-ft wide sidewalk at 4 inches thick and 50 feet long: 4 x 50 x 0.333 = 66.6 cu ft / 27 = 2.47 yards. That is about 111 80-lb bags. For longer pours, ready-mix is almost always more economical. See how much concrete you need for the general slab formula.
How many bags for a 12 x 12 footing?
A 12 x 12 inch square footing (1 ft x 1 ft) at 12 inches (1 ft) deep is 1 cubic foot, or about 0.037 cubic yards. That is less than 2 80-lb bags per footing. If you have 10 footings, that is 10 cubic feet total, or roughly 17 bags. Run multiple footings through the Concrete Calculator to total them at once.
What is the 4:2:1 rule for concrete?
For footings specifically, the 4:2:1 sub-base rule applies when you need drainage beneath a grade beam or shallow footing: 4 inches of crushed stone, 2 inches of coarse sand, 1 inch of fine sand. Most frost-depth footings go deep enough that the sub-base detail is less critical than achieving the code-required depth below the frost line.
Footing depth requirements by frost zone
In cold climates, footings must extend below the local frost depth to prevent movement from freeze-thaw cycles. Frost depth varies widely: it is 12 inches or less in the deep South, 24 to 36 inches in the mid-Atlantic and Midwest, 42 to 48 inches in the upper Midwest, and up to 60 inches or more in northern Minnesota and Maine. Your local building department publishes the minimum frost depth for your county. A footing that does not reach below frost depth is not permitted and will heave every winter.
| Region | Typical frost depth | Minimum footing depth |
|---|---|---|
| Gulf Coast / South | 0 to 12 in | 12 in (structural minimum) |
| Mid-Atlantic / Southeast | 12 to 24 in | 18 to 24 in |
| Midwest (Kansas City, Chicago) | 24 to 40 in | 36 to 42 in |
| Northern US (Minneapolis, Maine) | 42 to 60 in | 48 to 60 in |
Worked example: deck footing for a 12 x 16 ft deck
A common 12 x 16 ft deck needs 6 post footings. Each footing uses a 12-inch sonotube at 42 inches deep (Midwest frost depth). Volume per footing: 3.14 x 0.25 sq ft x 3.5 ft = 2.75 cu ft. Six footings: 6 x 2.75 = 16.5 cu ft / 27 = 0.61 cubic yards total. With the 10 percent waste add: order 0.67 yards, or about 31 80-lb bags. For this quantity, most contractors use a half-yard ready-mix order or a rented mixer to avoid the labor of opening 31 bags and mixing each one by hand.
Common footing mistakes to avoid
- Undersized footing diameter: deck posts and pergola posts often need 12 to 16 inch diameter footings, not 8 inch, to spread the load over enough bearing area. Check your local span tables or structural plans.
- Not reaching frost depth: the single most common DIY footing error. Always verify the local frost depth with your building department before digging.
- Pouring in wet holes: concrete poured into a water-filled hole is diluted and weakened. Pump or bail the hole before pouring, or use a submersible pump for high water tables.
- Skipping the tube form: pouring concrete directly into an earthen hole (no form tube) in sandy or loose soil allows the sides to collapse into the wet concrete, creating voids. Use a sonotube or equivalent form.