A continuous footing 16 inches wide and 8 inches thick holds 0.89 cubic feet of concrete per running foot, so 120 feet of it, the perimeter of a 30 × 30 ft building, takes 106.7 cubic feet or 3.95 cubic yards. Put a 4 ft foundation wall 8 inches thick on top and the job grows to 426.7 cubic feet: 17.5 cubic yards to order with a 10% allowance, two mixer trucks.
How footing and foundation concrete is calculated
Every footing and wall is a long box. Multiply the total run by the cross-section:
Cubic feet = total length (ft) × width (in) ÷ 12 × thickness (in) ÷ 12
A foundation wall is the same with its height and thickness: length × height (ft) × thickness (in) ÷ 12. Square pads under piers or posts are length × width × thickness, times how many there are. The calculator adds every section into one order and draws the cross-section of the one you are editing.
For the example:
- Footing: 120 × 16 ÷ 12 × 8 ÷ 12 = 106.67 cu ft
- Wall: 120 × 4 × 8 ÷ 12 = 320 cu ft
- Total 426.67 cu ft, 15.80 cu yd; with 10% extra 17.38 cu yd, ordered as 17.5
At 10 cubic yards per truck that is one full truck and a second carrying 7.5 yards.
Measuring the footing length
Add up every run. For a rectangular building, the centerline perimeter is close enough: 2 × (length + width). Count interior footings under bearing walls separately, and add each run once, so corners are not counted twice. A footing that steps down a slope is still the sum of its lengths; add the extra concrete in the risers as short wall sections if they are formed.
IRC minimum footing size and depth
The 2024 International Residential Code sets the floor for footings; the calculator checks your entries against it:
| Rule | Minimum | Section |
|---|---|---|
| Footing width | 12 in | R403.1.1 |
| Footing thickness | 6 in | R403.1.1 |
| Projection past the wall | 2 in, and not more than the footing thickness | R403.1.1 |
| Depth below undisturbed ground | 12 in | R403.1.4 |
| Rebar cover where cast against earth | 3 in | R403.1.3 |
These are minimums, not a design. The width actually required depends on the soil and the load (IRC Table R403.1), and the bottom must sit below the local frost line, which your building department publishes. The calculator's depth check only tests the 12 inch minimum.
Concrete footing calculator with footings and a slab
A "slab with footings" can mean two things. If the slab and its edge are poured together, pick "Slab with footings" and turn on the thickened edge: the calculator adds the deeper band around the perimeter. If the footings are poured first and the slab later, enter the footing, then add the slab as its own section to see the whole job; order each pour separately, since each will need its own truck. The concrete slab calculator adds an installed price for the slab.
Footing pads
Pads under posts and piers default to 24 × 24 inches and 12 inches thick, 4 cubic feet each. Eight of them with 10% extra need 35.2 cubic feet, 59 bags of 80 lb mix. For round piers poured in a form tube, the sonotube concrete calculator does the cylinder math and subtracts the post; for deck piers specifically, see the deck footing calculator.
Concrete per 10 feet of footing
| Footing | Per 10 ft | Per 100 ft |
|---|---|---|
| 12 × 6 in (IRC minimum) | 5 cu ft | 1.85 cu yd |
| 16 × 8 in | 8.9 cu ft | 3.29 cu yd |
| 20 × 10 in | 13.9 cu ft | 5.14 cu yd |
| 24 × 12 in | 20 cu ft | 7.41 cu yd |
Plan volumes, before waste. Footings are dug into soil rather than formed, so trenches run wide; a larger allowance than 10% is common in loose ground.
Next steps
Size the rebar for the footing: the IRC's seismic provisions call for a No. 4 bar near the bottom, and the rebar calculator handles laps and bar counts. Turn the total into a truck order with the concrete yard calculator, price it in the concrete cost calculator, or build the wall in block instead with the cinder block calculator. The concrete calculator takes every other shape.